a preliminary design of a land yacht with an auxiliary
TRANSCRIPT
A Preliminary Design of a Land Yacht with an Auxiliary
Electric Motor
Sérgio Manuel Felícia Ramires
Thesis to obtain the Master of Science Degree in
Mechanical Engineering
Supervisor: Prof. Miguel António Lopes de Matos Neves
Examination Committee
Chairperson: Prof. João Orlando Marques Gameiro Folgado
Supervisor: Prof. Miguel António Lopes de Matos Neves
Member of the Committee: Prof. Eduardo Joaquim Anjos de Matos Almas
November 2016
ii
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Acknowledgements
This work and the academic years that preceded it were only possible due to the help and support of
people to whom I must declare my gratitude.
I would like to thank first and foremost my supervisor in this thesis, Professor Miguel Matos Neves, for
suggesting this challenge and for the constant availability, support and help offered throughout the
elaboration of this work.
It is also important to recognize the contribution of the different professors and colleagues from whom I
have learned in the past few years and thus helped me reach my goals.
Finally, I would like to thank my parents are brothers for the support in all these years and in particular
in the past months.
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Abstract
The main goal of this work is to study the integration of an auxiliary electric propulsion system in a land
sailing vehicle destined to recreation. By making the vehicle a hybrid it is expected to make it more
appealing to potential users.
Initially, it is exposed the relevance of this thematic, being followed by a market and patent research
with the objective of knowing the existent options in terms of components that may be explored in this
project, namely in what concerns the electric motors, batteries and controllers. Based on the information
gathered different concepts of vehicles were generated. Utilizing a set of selection methods these
concepts were reduced to a single one to be studied in detail.
In a second stage it is done the preliminary structural design of the vehicle, namely the static analysis
of the chassis in different load situations, the study of the natural modes of vibration and the analysis of
the bolted joints. Finally, it is made the study of the electric circuit to be implemented.
In conclusion, the implementation of electric motors in this type of vehicles is a current topic that begins
to be explored by the main brand in the sector, and although this integration brings along some
challenges it also allows to benefit from advantages, one of the most relevant being the possibility of
using the vehicle under a broader set of atmospheric conditions, smaller sail, etc.
Keywords:
Land Sailing
Electric Propulsion System
Concept Generation and Selection
Mechanical Design
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Resumo
Este trabalho tem como principal objetivo o estudo da integração de um sistema de propulsão elétrica
auxiliar num veículo à vela terrestre de recreação. Ao tornar o veículo híbrido é esperado que este se
torne mais apelativo para os potenciais utilizadores.
Inicialmente é feita a exposição da relevância deste tema, seguindo-se uma pesquisa de mercado e
patentes que visa fazer um levantamento das opções de componentes já existentes e que podem ser
explorados para utilização no projeto, nomeadamente ao nível de motores elétricos, baterias e
controladores. Com base na informação recolhida, foram gerados diferentes conceitos de veículos que
permitissem responder ao objetivo do trabalho, e utilizando um conjunto de métodos de seleção estes
conceitos foram reduzidos a um único a ser estudado em detalhe.
Numa segunda fase é feito o dimensionamento estrutural preliminar do veículo, nomeadamente a
análise estática do chassis em diferentes situações de carga, o estudo dos modos de vibração e análise
das ligações aparafusadas. Finalmente, é estudado em maior detalhe o circuito elétrico a implementar.
Foi possível concluir que a implementação de motores elétricos neste tipo de veículos é um tema atual
que começa a ser explorado pela principal marca do sector, e que apesar de esta integração apresentar
alguns desafios permite também um conjunto de vantagens das quais se destaca a possibilidade de
utilização do veículo em condições atmosféricas sob as quais sem este sistema não seria possível.
Palavras-Chave:
Kart à vela
Sistema de Propulsão Elétrico
Geração e Seleção de Conceitos
Projeto Mecânico
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Table of Contents
Acknowledgements ....................................................................................................................... iii
Abstract ............................................................................................................................................. iv
Resumo .............................................................................................................................................. v
Figure Index ..................................................................................................................................... ix
Table Index ....................................................................................................................................... xi
Acronym List .................................................................................................................................. xii
Part A: Descriptive Analysis ............................................................................................................ 1
1. Introduction ................................................................................................................................. 1
1.1 The main challenge and motivation ................................................................................ 1
1.2 Brief history of the electric vehicle ................................................................................. 2
1.3 The challenge ........................................................................................................................ 3
1.4 Land yacht general structure............................................................................................ 5
2. Market and Patent Research ................................................................................................... 6
2.1 Electric drive solutions ...................................................................................................... 6
2.1.1 Electric bicycle motors ............................................................................................... 7
2.1.2 Electric bicycle batteries .......................................................................................... 12
2.1.2.1 Battery Regeneration ......................................................................................... 14
2.1.3 Electric bicycle controllers ...................................................................................... 15
2.1.4 Other possible solutions .......................................................................................... 16
2.1.5 Variables to consider when choosing the electrical system .......................... 17
2.2 Structural solutions ........................................................................................................... 18
2.3 Brief Patent Research ....................................................................................................... 20
2.4 Analysis of options ........................................................................................................... 21
2.4.1 Type of control ............................................................................................................ 22
2.4.2. Motor and batteries ................................................................................................... 22
2.5 Available commercial options ........................................................................................ 24
2.5.1 Complete sets available in the market .................................................................. 25
2.5.2 Conversion kits ........................................................................................................... 26
2.5.2.1 Motors/controllers ............................................................................................... 26
2.5.2.2 Batteries................................................................................................................. 27
3. Hybrid land yacht concepts .................................................................................................. 28
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3.1 Concept generation ........................................................................................................... 28
3.1.1 Concept A1 – Battery Below .................................................................................... 29
3.1.2 Concept A2 – Battery on Top .................................................................................. 30
3.1.3 Concept A3 – Two Batteries Below ....................................................................... 30
3.1.4 Concept A4 – Geared Motor with SLA Battery Below ....................................... 31
3.1.5 Concept A5 – Geared Motor with SLA Battery on Top ..................................... 31
3.1.6 Concept A6 – Geared Motor with Li Ion Battery Below .................................... 32
3.1.7 Concept A7 – Geared Motor with Li Ion Battery on Top .................................. 32
3.1.8 Concept A8 – One motor on each rear wheel with two SLA batteries below
................................................................................................................................................... 32
3.1.9 Concept A9 – One motor on each rear wheel with two Li ion batteries
below ........................................................................................................................................ 32
3.1.10 Concept A10 – Mid Drive Motor with SLA Battery ........................................... 33
3.1.11 Concept A11 – Mid Drive Motor with Li Ion Battery ........................................ 33
3.1.12 Concepts B1 to B11 ................................................................................................. 33
3.1.13 Concept B12 – Mid Drive Motor to Rear Wheels with SLA Battery ............. 34
3.1.14 Concept B13 – Mid Drive Motor to Rear Wheels with Li Ion Battery .......... 34
3.2 Concept selection .............................................................................................................. 34
3.2.1 Screening: first stage ................................................................................................ 34
3.2.2 Selection criteria ......................................................................................................... 36
3.2.3 Reference product for the selection ...................................................................... 37
3.2.4. Screening: second stage ......................................................................................... 38
3.2.5 Final selection ............................................................................................................. 40
3.3 Winning concept detailed description ......................................................................... 41
3.4 Assembly and disassembly ............................................................................................ 42
Part B: Project’s Calculations ....................................................................................................... 43
4. Structural Dimensioning ........................................................................................................ 43
4.1 Introductory considerations ........................................................................................... 43
4.2 Modifications to the existing structure ........................................................................ 44
4.2.1 Aerodynamic impact of the modifications ........................................................... 44
4.3 Forces transmitted by the wind ..................................................................................... 44
4.3.1 Type of sail ................................................................................................................... 44
4.3.2 Apparent wind and resulting forces on the sail ................................................. 45
4.3.3 Resulting forces on the structure .......................................................................... 48
4.3.4 Rollover ......................................................................................................................... 49
4.4 Materials used in the chassis ......................................................................................... 51
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4.5 Factor of safety................................................................................................................... 52
4.6 Static Analyses of the Chassis ...................................................................................... 53
4.6.1 All wheels on the ground ......................................................................................... 55
4.6.2 One rear wheel on the air ......................................................................................... 57
4.6.3 Conclusions ................................................................................................................. 60
4.7 Vibration Analysis ............................................................................................................. 61
4.7.1 Normal Modes ............................................................................................................. 61
4.8 Weld Beads .......................................................................................................................... 62
4.9 Bolted Joints ....................................................................................................................... 63
4.10 Brakes ................................................................................................................................. 69
5. Electric Components............................................................................................................... 71
5.1 Circuit .................................................................................................................................... 71
5.2 Location and placement of the components .............................................................. 73
5.3 Expected consumptions and battery duration........................................................... 74
5.4 Cost analysis....................................................................................................................... 75
5.4.1 Prototype ...................................................................................................................... 75
5.4.2 Production .................................................................................................................... 75
6. Future Developments .............................................................................................................. 76
References ......................................................................................................................................... 77
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Figure Index
Figure 1 – Percentage of EU urban population exposed to different pollutants [1]…………………………….1
Figure 2 – Expected variation of sectorial contributions to the totality of CO2 emissions [3]……………..2
Figure 3 – First production electric vehicle [4]…………………………………………………………………………………….3
Figure 4 – Land yachts in action [8]…………………………………………………………………………………………………….4
Figure 5 – Blokart standard model………………………………………………………………………………………………………5
Figure 6 – Brushless (left) and brushed motors [12]……………………………………………………………………………7
Figure 7 - Pedelecs for rental [14] ………………………………………………………………………………………………………8
Figure 8 – Direct drive hub motor [16] ……………………………………………………………………………………………….9
Figure 9 – Geared hub motor [18] …………………………………………………………………………………………………...10
Figure 10 – Mid drive motor [19] …………………………………………………………………………………………………..…11
Figure 11 – Shaft drive e-bike [20] ……………………………………………………………………………………………………11
Figure 12 – Friction drive motor [21] …………………………………………………………………………………………….…12
Figure 13 – Typical e-bike battery (lithium-based) - adapted from [22]…………………………………………….13
Figure 14 – Image of a torque sensor used on the Easy Motion Neo Jumper e-bike [26]………………….15
Figure 15 – Image of a cadence sensor in a Pedago City Commuter e-bike [26]……………………..………..16
Figure 16 – DC motor found in a treadmill [27] …………………………………………………………………………….…17
Figure 17 – Base chassis structure A for the hybrid kart project [28]………………………………………………..18
Figure 18 – Base chassis structure B for the hybrid kart project [30]………………………………………………..19
Figure 19 – Motor assisted e-bike patent [31]…………………………..…………………………………………………..…20
Figure 20 – The A7AM20 model [37] …………………………………………………………………………………………….…25
Figure 21 – GNGELECTRIC mid drive motor kit [39] …………………………………………………………………………26
Figure 22 - LeafBike 16’’ 36V 250W conversion kit [40] …………………………………………………………………..27
Figure 23 – Sketch of concept A1 (battery inside the protective case)……………………………………………..30
Figure 24 – Sketch of concept A2……………………………………………………………………………………………………..30
Figure 25 – Sketch of concept A3……………………………………………………………………………………………………..31
Figure 26 – Blokart electric motor [44] ……………………………………………………………………………………………37
Figure 27 – Structure of the winning concept A1 in perspective………………………………………………………42
Figure 28 – Real wind and apparent wind speeds on a land yacht) (adapted from [46])…………………..45
Figure 29 – Forces acting on the sail of the vehicle [46]……………………………………………………………………47
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Figure 30 – Location of the C.G in relation to the origin of the referential ………………………..………………..51
Figure 31 – Failure probability due to variations in the stress and resistance values………………………..53
Figure 32 – Placement of the loads resulting from the passenger’s weight………………………………………54
Figure 33 – Von Mises stress distribution on the chassis for β=0 59………………………………………………….55
Figure 34 – Nodal displacement for β=0……………………………………………………………………………………………56
Figure 35 – VM stress distribution on the chassis for β=0 (one back wheel on the ground)………………58
Figure 36 – Nodal displacement for β=0 (one back wheel on the ground)……………………………………..…58
Figure 37 – Deformation for β=40º (seen from the front) ………………………………………………………………..60
Figure 38 – Refined mesh in the convergence analysis (detail of the bended part of the tripod)………60
Figure 39 – First (left) and second normal modes (right): qualitative deformations………………………….61
Figure 40 – Location of the critical weld bead…………………………………………………………………………………..62
Figure 41 – Location of the bolted joints…………………………………………………………………………………………..63
Figure 42 – Bending moments cause different axial loads according to the position of the bolt…….…65
Figure 43 – Rotation of half of the flange around the hinge line……………………………………………………….65
Figure 44 – Shear forces in joint 1…………………………………………………………………………………………………….67
Figure 45 – Shear forces in joint 2…………………………………………………………………………………………………….68
Figure 46 – Main Circuit Breaker [34] ………………………………………………………………………………………………72
Figure 47 – Layout of the electric circuit to implement……………………………………………………………….……72
Figure 48 – Location of the different components of the electric system …………………………..…………….73
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Table Index
Table 1 – Concept Comparison…………………………………………………………………………………………………………38
Table 2 – Quantifiable Criteria Values………………………………………………………………………………………………39
Table 3 – Final Selection…………………………………………………………………………………………………………………..41
Table 4 – List of constituents……………………………………………………………………………………………………….……41
Table 5 – Values of the lift and drag coefficients for different α angles………………………………………………47
Table 6 – Forces and moments on the connections between the sail and the chassis for α=20º.........48
Table 7 – Forces and moments in the kart’s referential for different values of β, for α=20º…………....49
Table 8 – Necessary information to calculate the center of mass of the vehicle with an occupant……50
Table 9 – Most relevant properties of the materials used in the chassis…………………………………52
Table 10 – Important maximum values occurring in the chassis elements………………………………56
Table 11 – Von Mises stress at the location of the weld for the different load situations……………….57
Table 12 – Forces and torques at the locations of the bolted joints………………………………………57
Table 13 – Important maximum values occurring in the chassis elements………………………………58
Table 14 – Von Mises stress at the location of the weld for the different load situations……………….59
Table 15 – Forces and torques at the locations of the bolted joints………………………………………59
Table 16 – First five resonance frequencies of the chassis for different passenger’s weights………...61
Table 17 – Data of the used connection elements………………………………………………………….64
Table 18 – Constants A and B for different materials [55] …………………………………………………………………64
Table 19 – Maximum loads acting on joints 1 and 2………………………………………………………..65
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Acronym List
AC – Alternate Current
CAD – Computer Aided Design
CG – Center of Gravity
CO – Carbon Oxide
DC – Direct Current
EU – European Union
EV – Electric Vehicle
FEM – Finite Element Method
FISLY - Federation Internationale de Sand et Land Yatching
NACA - National Advisory Committee for Aeronautics
NOx – Nitrous Oxides
SLA – Sealed Lead Acid
1
Part A: Descriptive Analysis
1. Introduction
1.1 The main challenge and motivation
The main challenge of this work is to study solutions to transform a regular land yacht into a hybrid
vehicle. In the last few decades there has been an increasing effort in the research and development of
vehicles that use non fossil fuel based sources of power. There are several reasons for this: the
environmental impacts of conventional internal combustion engines (both global effects, like climate
change, and local hazards, i.e. the release of pollutants dangerous to human health like NOx, CO,
unburned hydrocarbons and particles), the concern in increasing energy independence by countries that
do not produce oil, economical savings for the user in fuel spending during the vehicles life, less noise,
among others. In Figure 1, it can be seen that a large percentage of the European urban population is
regularly exposed to several pollutants [1], being transports the main responsible for their release.
Figure 1 – Percentage of EU urban population exposed to different pollutants [1]
The European Union (EU) has taken comprehensive measures to mitigate the impacts of conventional
fuels in the environment, including setting for its member states the goal of together reducing the
emissions of greenhouse gases by 80% (having the levels of 1990 as reference) by 2050 [2]. To reach
this goal, it was defined a set of routes, known as the roadmap 2050, to transition between energy
paradigms: these include investments in new technologies, renewable energy, energy efficiency
programs and infrastructures; all expected to also bring along increases of competiveness, economic
growth and security of supply. In order to avoid postponing action, there were set milestones to ensure
a gradual change: 40% below 1990 levels by 2030 and 60% by 2040, as well as sectorial goals, where
2
as it can be verified in Figure 2, transports have a significant share. This sector could have a 60%
reduction by 2050, mainly due to increasing fuel-efficiency in the short term and a transition to hybrid
and fully electric vehicles in the mid to long-term [3].
Figure 2 – Expected variation of sectorial contributions to the totality of 𝐶𝑂2 emissions [3]
Pollution and global warming are not the only factors driving the need for a change from fossil fuels to
alternative sources: there is also a growing economic need in several countries to reduce their exposition
to the fluctuation of prices of oil and concerns related with the finite nature of said resources.
For all these reasons, not only governments but also individuals are showing an increasing concern with
this subject, thus it has been possible to notice a growth in the demand of so called green products and
technologies.
1.2 Brief history of the electric vehicle It is not clear which was the first electric vehicle and who invented it. In fact, this is one of those cases
where a particular achievement is responsibility of several different people working separately and
making small contributions during a long period of time. In 1828, the Hungarian Ányos Jedlik invented
a type of electric motor and built a small model car powered by it. In 1834, Thomas Davenport made a
car that circulated in an electrified track [4]. These vehicles were mere concepts and had no actual
practical use.
The viability of this type of idea first began to take form in 1859, with the invention of lead-acid batteries
by Gaston Planté: there was now a way of storing energy on board to use in the electric motor. However,
as it happens with most inventions, the batteries were far from optimized and had great improvements
over time.
Although many prototypes and improvements were made in the meantime, only in 1884 appeared the
first production electric car (seen on Figure 3), by the English inventor Thomas Parker.
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Figure 3 – First production electric vehicle [4]
This signaled the start of an era where there was a great interest in this type of vehicles and technology.
In the late 1890’s electric cars outsold gasoline cars ten to one, even though there were problems with
the lack of power infrastructure. There was then a decline at the beginning of the 20th century, due to a
number of improvements of the internal combustion engine vehicles, which made them clearly superior
to the end user. The period of lack of importance of EVs lasted until the end of the century, when there
was a revival of the interest due to economic and environmental factors, as well as technical progresses,
and they are now seen as a key technology for the future of human mobility [4]. In fact, in the last few
years, there has been a surge in the purchase of electric vehicles in some important markets [5].
1.3 The challenge The land yacht (usually identified as Blokart, although this name referrers to a specific brand [6]) is an
example of a vehicle that does not use fossil fuels and that is relatively unknown: they are wind powered
(by means of a sail or a wing), usually wheeled land vehicles (ice blades for example are used instead
of the wheels), typically with a single occupant, which are operated in flat areas where wind speed is
moderate to strong, as a leisure activity. The number of users of this vehicles is rising in countries like
New Zealand, USA and Australia. However, it is still far from being widespread in Portugal, despite the
fact the country has favorable conditions for this activity to take place, namely a long coast line, a high
average number of hours of solar exposition and a moderate wind potential [7]. A possible factor
contributing for this reality is the lack of infrastructures in Portugal dedicated to practicing this hobby. To
correct the situation, it would be important to have a permanent track where races could take place. The
most adequate location would combine conditions of higher wind speed with a high number of tourists,
which is why it is thought of the region of Algarve. Land yachting has also become a competition sport,
having its own international federation (FISLY). On figure 4 it can be seen the use of these vehicles on
a beach.
4
Figure 4 – Land yachts in action [8]
In this project, the main goal is to design and incorporate an electric propulsion system in one of this
wind powered vehicles (making it a hybrid), allowing to utilize the vehicle in atmospheric conditions in
which is impossible or difficult to do with the conventional models. The idea is that the power generated
by the electric motor would for significantly compensate the lack of thrust impinged in the sail in low wind
situations but also in other circumstances where additional power may be needed, such as when going
uphill. With this project it is intended to generate a number of competing concepts for this wind
powered/electric vehicle having in consideration the required characteristics of the motor, batteries,
controller and circuit and the structural additions and modifications necessary to accommodate them,
choose the best of them and carefully project and analyze it from the structural point of view.
A more generic objective is to take a first step in the direction of proving this kind of vehicle concept: it
should be obtained the project of a viable hybrid land yacht prototype. Even though this is a project
dealing heavily with matters such as electric motors, circuits and controllers, the primary concern is to
obtain a structurally sound solution to accommodate this equipment and transport the occupant safely.
Once the competitive advantages of this type of vehicle are demonstrated there can be considered
further improvements on future theses (since it is not the goal of this work to have every aspect fully
optimized, and therefore it always exists a compromise between the functionality, simplicity and price)
and eventually a possible commercialization, taking benefit of the rise of the number of people who
practice outdoors sports and activities such as this as a form of pastime.
It is imperative to mention that some of the ideas put forward and some of the options chosen throughout
this work have a certain lack of support from the literature. Due to this fact and to the very nature of
every venture consisting in the development of a new concept, it is recognized it would be important to
actually build a working prototype of the vehicle and experimentally assess the viability of the electric
hybrid land yacht projected. However, that exceeds the ambit of the present document mainly due to
time restrictions and for that reason would be an interesting topic for future researches.
With this project it is also wished to make a modest contribution to the dissemination of land sailing in
Portugal and to add to the effort of finding electric solutions for vehicles, something that, as it was
possible to verify previously, is becoming more and more important each year. Finally, this type of works
may also be a first step towards the use of the land yacht as more than a vehicle for recreation: it is
possible to imagine it being used without the sail and just using the electric power for certain short
5
distance trips in some particular environments, although for this it would likely be necessary to make
some alterations and adjustments to the vehicle.
1.4 Land yacht general structure
Wind powered land yachts usually have a number of components common to all of them. In order to
illustrate and understand this general layout it is presented a diagram on figure 5 with the indication of
each of the components in the model of the most sold brand (Blokart) [9]. This model is going to be used
as a reference from this point onwards.
Figure 5 – Blokart standard model [9]
6
The model designed in this project will have the same basic configuration (i.e. two rear wheels and a
front wheel, a simple chassis made of metal tubes, a large sail or wing, etc.) but will also feature the
necessary components to make the vehicle acquire electric propulsion as well as several structural
components fundamental to place the electric ones, and others that will become necessary, e.g. brakes.
2. Market and Patent Research
In order to proceed with the generation of concepts, in an initial stage is necessary to conduct a research
about the options available for the different elements of the system. This is going to be divided in two
stages: an exploration of the market (i.e. already established products), and an investigation of related
patents.
2.1 Electric drive solutions
The main goal of this thesis is to propose an integration of an electric motor in wind powered “karts”.
With this, it will be possible to use the vehicle even in lower wind velocity atmospheric conditions
because there will be an auxiliary source of power to the wheels.
Being a vehicle, the motor will necessarily have to be fed by some type of battery or group of batteries,
which provide DC (direct current). Although it is theoretically possible for the motor to use AC (alternate
current), as in some electric cars, it would be necessary to use complex conversion and control systems
that may be very expensive and take additional space. Therefore, the simplest and most economically
viable solution for the vehicle is to use a DC motor.
DC motors can be classified in brushed or brushless (see figure 6). The latter are more efficient, less
noisy, are less affected by wear (so they are more durable), have higher developed torque to own weight
ratio and require no commuter and no brushes (hence the name). All this advantages made them
replace brushed motors in many applications. They are however more complex and therefore more
expensive and demanding to control (it is done electronically, which allows greater flexibility) [10], [11].
For this project brushless motors are preferred.
7
Figure 6 – Brushless (left) and brushed motors [12]
The focus of this research is on small motors for the following reasons: small power needs considering
the nature of the application (complementing the main driving force, the wind), limited space in the kart
to keep the motor and other components of the electric system (a more powerful motor generally means
a bigger motor and more batteries, which may take a lot of space) and obviously also cost concerns.
The goal of this section of the study is to explore the market and identify various types of possible
components and solutions to conceive a number of different concepts for the electric propulsion system
of the kart based on products that currently exist, to then analyze them carefully and choose the one
that better integrates the characteristics needed to achieve a desirable, reliable and (as much as
possible) cheap hybrid kart.
2.1.1 Electric bicycle motors
Bearing in mind the characteristics of the application, namely the approximate power interval it is
anticipated to be required to assist the wind propulsion, it is expected that the motors found in electric
bicycles (as well as their control systems) might be an appropriate solution to the problem: they are
small DC motors used to complement the main power source in light vehicles that carry a single
occupant and move at moderate speeds, all of which are aspects shared with land yachts. Also, the
front wheel of a land yacht is usually from a bike or similar to those used in bikes. An electric bicycle,
also referred to as an e-bike or booster bike, is a single-track vehicle, similar in all aspects to a
conventional bicycle, except it combines the traditional human-powered, pedal-driven system with an
electric motor, batteries and controller (which regulates the power flow from the batteries to the motor,
and thus its output), together constituting an auxiliary power source. In this regard they are similar to the
hybrid vehicle to project in this thesis, combining a renewable, free, non-polluting main energy source
with an electric power system.
8
This being the case, it is convenient to do a research of the various types of solutions found on electric
bicycles currently existing on the market and to study the main differences between them, to identify
their pros and cons. There are a great variety of electric bicycles commercially available today, ranging
from the ones that only have a small and low power motor used to assist the rider , which are also known
as “pedelecs”; to those that are closer to a moped, given the higher power of their motor (although
always maintaining the possibility of being pedaled). As mentioned before, the power needs required
for this project are not very high: this means the focus of this research is going to be directed to the
former type of e-bikes.
A pedelec (pedal electric cycle) can be defined as the type of electric bicycle where a low power motor
provides assistance to the rider’s by delivering a source of energy that may be used when would be
needed a greater physical effort (uphill roads, strong headwinds, etc.) or when the rider is tired. Given
this characteristics, for legal purposes, they are grouped with conventional bicycles in most countries
rather than being grouped with full electric mopeds or given a classification of their own. In the last few
years a great number of spaces dedicated to the rental of this type of vehicle (figure 7) appeared in
several cities around the world, having e-bikes become more and more popular (which also translates
into an increase of personal ownership) [13]. Advantages of its use include lower energy costs, savings
in other costs (parking, registration, etc.), environmental friendliness and allowing exercise. China is
currently the largest manufacturer and one of the countries with fastest growing demand.
Figure 7 - Pedelecs for rental [14]
The features and specifications of e-bikes vary widely (as is going to be clear when the different types
are analyzed) but typical values of some of the most important characteristics are [15]: around 30km/h
top speed, 15 to 80 km of electric travel range, 2 to 6 hours of socket full recharging time, from 200 up
to 1000 available cycles of charge and discharge of the battery, 100 to 800 Wh full charge battery energy
storing, power supply of 12 to 48V, cost of 500 up to 3000 US dollars and a cost of 200 to 1000 US
dollars for a conversion kit (equipment necessary to turn a conventional bike in a e-bike). These values
allow to have a rough notion of the range of values to expect going into the next sections of this
document but it should be clear some modern models surpass a few of this values.
In general terms, it is possible to classify the different types of electric bike motor systems in the following
categories [16], [17]:
9
Hub Motors: these are the standard, most common type. They are housed inside a hub in one
of the wheels (front or rear wheel hub motor, which is just a change in location of the motor but
has a very noticeable impact on the behavior of the e-bike). The difference between them that
matters the most to this project is that, due to the fact the front fork does not provide as much
of a solid structural platform as the frame on the rear, front wheel hub motors are smaller and
mostly on the 250 to 350 watt power range; while rear wheel motors often go up to 750 watt,
sometimes even exceeding it.
Hub motors can also be classified in direct drive and geared. Direct drive motors are the
simplest: the hub is not just to enclose the motor but is actually part of it, having the magnets
attached to this shell in a circular pattern. It is the outer shell who actually spins (while it still acts
like a traditional connecting element, holding the tire, rim and spokes to the axle) and the
center stays stationary. They allow higher speeds, are quiet, robust and durable. Its name
comes from the fact it drives the wheel directly, since the hub is attached to the rim of the wheel.
Judging from the way they function and from their dimensions, direct drive motors would not be
easy to use externally to the wheel in the hybrid kart to be projected in this document (even
though a chain transmission or a similar method is conceivable). If they are used, it will likely be
by directly using a wheel from an e-bike to propel the kart.
As it can be seen in figure 8, they tend to be big (and are also usually heavy), which is another
reason in disfavor of them in this market research. A large number of direct drive rear hub
motors allow to partially recharge the batteries when the brakes are used, which is known as
regenerative braking. They don’t all do it because there are some flaws related to this type of
system: there is the tendency to not recoup much energy this way, there is an added cost to
implement the technology and also because the extra complexity of the system can affect the
previously mentioned advantage of being a simple and robust option. This kind of system could
be an interesting bonus to the kart, so it is something to discuss in following stages (along with
alternative ways of achieving the same goal).
Figure 8 – Direct drive hub motor [16]
10
Geared motors are significantly lighter and smaller than hub motors. As shown in figure 9,
these motors have their cases connected to the stator through a planetary gear train working
as a reduction system, allowing for the motor to put out a higher torque (at the expense of speed,
since for each turn of the wheel the rotor will turn several times). Some of the downsides are
the increasing wear on the gears with time, reducing the durability of the equipment, and the
fact they tend to be on the lower side of the power spectrum for electric bike motors (on a front
wheel hub bicycle the motor will be of the geared type on the vast majority of times).
In the traditional hub motors the batteries and other components are placed somewhere else in
the bike, usually along or inside the frame or in a basket on top of the rear wheel. Some bicycles
however use an all-in-one hub which includes the motor, batteries, controllers and sensors.
These bicycles are often a high end product since the motors are smaller and more compact,
and sometimes can even be controlled using a smartphone (all of this means a higher price).
Another potential disadvantage is the proximity of the batteries to the motor, which can overheat
them and thus cause the reduction of its useful life. Unlike other types, the battery pack is not
easily changeable and so if it does not fit the necessities of our application it immediately implies
other motor will have to be chosen. It also means that once the limit number of charge cycles of
the battery is reached, unlike in other models they can’t be substituted easily. Theoretically the
system could be removed and directly used in the hybrid kart, having the advantage of being
more compact and lighter than the total components to be used when applying other kinds of
motors. However they might be too complicated to use in the vehicle, for instance, the sensors
and electronics are specific to the e-bike they are design for, and would not make sense to exist
in the kart.
Figure 9 – Geared hub motor [18]
Mid drive motors (figure 10) are not located in the wheel of the bike like in the previous cases
but on the frame. Instead of transmitting the power directly to one of the wheels, these motors
transfer the generated energy to the drivetrain of the bike, using a chain. They are able to deliver
more torque which helps when going up a road with a steep inclination. They usually include a
set of sensors and electronics that try to make the ride easier and more intuitive, like happens
in the all-in-one hubs (the problem with this was already mentioned in the previous paragraph).
11
Figure 10 – Mid drive motor [19]
Shaft drive electric bicycles: in these there is no chain, since the power is transmitted (both
from pedaling and from the motor) through a shaft (figure 11). The motor is usually hidden inside
the frame of the bike or even incorporated in the shaft itself. These bikes have a higher price
and this technology was considered to not be easy to adapt to the kart, so they will be
disregarded.
Figure 11 – Shaft drive e-bike [20]
Friction drive motors have a completely different way of transmitting the power: they include
a roll that sits on top of one of the e-bike’s wheels (usually the rear wheel, but not always) and
when it rotates it drives it forward by friction. The electric motor is usually located behind the
seat (figure 12). They are probably the simplest systems around and because of that are also
cheaper. However they are not considered to be very reliable or efficient, and cause
considerable wear in the tire (which by its turn reduces the efficiency even more) and usually
provide low power.
12
Figure 12 – Friction drive motor [21]
Besides the previous categorization, the motor could also be classified in brushed or brushless. The
differences between these were already explained. The vast majority of e-bikes uses brushless DC
motors due to their higher efficiency and reduced size.
2.1.2 Electric bicycle batteries
In this section it is going to be made a brief review of the types of batteries [22], [23]. Firstly it should be
noted that theoretically there is the possibility to select a motor from a certain model and a battery pack
from a different one or even from something other than an e-bike (although this could in some situations
complicate the control of the motor, something that will be discussed later).
Lead Acid: These have been the standard, most common batteries found on an e-bike in the
past (as well as in many other applications) and are both simple and low cost while having a
low self-discharge rate (which means they can be stored for a long time and remain with the
charge mostly unaltered). Their major drawbacks are the fact it should never be fully discharged
(since it would damage it), the real amp-hours values are always significantly below the nominal
ones, their heavier weight and higher environmental impact and they usually have a lower
number of charge/discharge cycles when compared to other types [24] . The most common
setups in e-bikes are 12V, 7/12Ah (nominal, not real) connected in series to get a total of
24/36/48V. Energy density usually around 20-30 Wh/kg.
Nickel Cadmium (NiCD): They are robust, have a large number of cycles available in their
useful life, and can be deeply discharged, but have been largely replaced by the other kinds
due to other less advantageous characteristics. Energy density of around 35-40 Wh/kg.
Nickel Metal Hydride (NiMH): Similar to nickel cadmium but have higher energy density and
are more environmentally friendly, despite being a bit more expensive and taking longer to
charge. It should also be noted that in order to keep the battery in working conditions for a longer
period it should not be completely discharged each time (shallow rather than deep discharges
should be done). High self-discharge may also be problematic. 50-60 Wh/kg.
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Lithium: The most expensive and also those that can store the largest amount of energy and
have less weight. In figure 13 is shown one of these batteries. They have no memory effect, are
environmentally friendly and have no maintenance needs. Lithium Polymer (160 Wh/kg) are the
lightest but do not have a good number of life cycles and are structurally weak (often requiring
a rigid enclosure). Lithium Iron Phosphate (around 110 Wh/kg) have a very high number of life
cycles and can handle very high currents. Lithium Manganese (100-150 Wh/kg) are by far the
lithium batteries most used on e-bikes (even though they are a little heavier) since they are safer
[25].
Figure 13 – Typical e-bike battery (lithium-based) - adapted from [22]
The set of batteries of any electric or hybrid vehicle is a crucial aspect of its performance and viability,
since it is a key factor to the range, weight and cost (it is always one of the most expensive and heavy
components). In what concerns this aspect, there are a few points that are important to bear in mind
while doing the selection of the best option. Firstly, the motor should preferably have a low voltage (for
the same power). This will generically mean fewer batteries will be necessary which is important since
the lighter the kart is, the better: less weight means greater portability and this is one of the main features
of any wind powered kart (the main target market are people who use it for recreation on holidays and
weekends, who rely mainly in their own means for the transport of the kart to the areas of usage).
Additionally, since the weight of the batteries when compared to the mass of the other components and
of the vehicle itself may be substantial it is necessary to be careful when planning their placement (key
to the dynamics, maneuverability and safety of the kart, e.g. to avoid instability and the possibility of
rollover). It is also important to stress that there is limited space on the kart to accommodate the electric
propulsion system, this potentially being one of the big challenges faced in this project (for that reason,
a smaller battery is preferred). Finally, it is imperative to remember the relevance of the safe and stable
connection of the battery to the chassis as well as guaranteeing its protection to different possible types
of damage (physical impacts, water, sand are all examples very likely to be of concern given the main
expected form of utilization of the vehicle, i.e. having a large share of its time of use on leisure rides or
races on environments such as the beach, dirt tracks, etc.).
A few important final remarks related with the topic of batteries: 1) often, e-bike manufacturers do not
provide to the client the weight of the battery itself (usually just the total weight of the product), however,
14
knowing the values of the expected energy density each type of battery has and how much total energy
the battery or set of batteries of the hybrid kart carries, allows to easily estimate the added weight to the
vehicle and thus have all the necessary data to make a compromise between autonomy, cost, weight
and stability; 2) the number of recharge cycles (very important to the overall durability of the product)
always depends on the type of battery, the storage conditions and how deep are the discharges; 3)
there is the possibility of connecting a group of small batteries to form a pack with the intended
specifications (connected in series to add the voltages and maintain the value of the current and in
parallel to do the opposite).
2.1.2.1 Battery Regeneration In some electric bicycles (as well as in other electric vehicles) there is a feature consisting in the
capability of recharging the batteries while the equipment is being utilized, usually by a process known
as regenerative braking that consists in partially making use of the energy dissipated when the brakes
are actuated. This allows to save money in energy but the main advantage is to extend the range of the
vehicle on a single trip.
The hybrid kart to be designed in this document is expected to be used sporadically, on weekends or
holidays, since it is a leisure equipment. Having this in mind it must be examined whether the increased
cost of including a regenerative braking system is worth it. The only significant advantage of having the
system would be to increase the time of use of the electric propulsion (since the economic value of the
savings of energy are negligible). However, given the nature of its use, it is not farfetched to assume the
vast majority of people will utilize the vehicle no longer than 3 to 5 hours at a time: considering the
electric motor is mostly expected to be used when the wind velocity is low, not throughout the whole
duration of the journey, it is safe to infer it will be possible to project a vehicle, within the reasonable
weight and cost restrictions, whose batteries will not run out in a day of use. Potential problems with this
type of system would be that in land yachts braking is not very frequent and most fully wind powered
models don’t even have a brake (when there is the need to stop, the rider uses the sail to do so), as well
as a necessity for a higher level of complexity of the motor and of the control system to accommodate
this feature. Taking all of this into consideration, it is not reasonable to include regenerative braking in
the kart.
There is still however another possibility: to include a system that allows to recharge the batteries when
the motor is turned off and the rider is only resorting to the wind and so the kart is functioning as a
traditional land yacht. The first thing to consider when studying the feasibility of such a feature is that
when it is being used there is an added resistance in the vehicle: part of the energy that would contribute
to move the vehicle forward is now being diverted to the battery, which implies a decrease of the
performance (speed) in such situations. Therefore, it could be necessary to determine whether or not
this addition would have a major impact on the performance and if it would be compensated by the
convenience of recharging the batteries. Other aspects to always keep in mind are the necessity for
more complex controls when using any type of battery recharging system and to use batteries capable
15
of handling a very high number of small and repetitive charge/discharge cycles. Another factor could be
to study which solution (higher battery capacity or regeneration when the motor is not being used) is
more economically viable and results in an overall better product. However, considering all the
mentioned disadvantages and that in this project is intended to develop a vehicle with a simple electric
propulsion system, it was seen as preferable to not include this type of battery regeneration either.
2.1.3 Electric bicycle controllers
Similarly to the motor and battery, controllers from electric bikes may also be used in this project, which
means it must be made a research about the different types of control philosophies used on the
mentioned products. There are usually two distinct forms: throttle (manual) or pedal assist, while some
models allow to choose between both (which is known as hybrid power control).
In the first way of activating the electric propulsion the rider operates a switch or paddle that allows to
turn it on and off (so it can turn into a fully electric vehicle) and to select the intensity of the motor delivery.
The output of the motor remains independent of whether the rider is pedaling.
On a pedal assist type bike (also known as half-assist), the motor automatically provides power when
the rider pedals and only in this situation, based on the information gathered by multiple sensors (the
control system could have to be redesigned to be used in the kart) and the user can generally choose
between levels of assistance intensity. It is typically used one of two types of sensor: torque and
cadence. Torque sensors (figure 14) in a way measure the effort being made by the rider and regulate
the power delivered by the motor according to it, thus allowing a more intuitive ride. Cadence sensors
(figure 15) are simpler and cheaper: they measure the number of revolutions of the pedals per minute.
Bikes that use them have their motors deliver power in function of this information and the level of
assistance selected (low, medium, high) [26].
Figure 14 – Image of a torque sensor used on the Easy Motion Neo Jumper e-bike [26]
16
Figure 15 – Image of a cadence sensor in a Pedago City Commuter e-bike [26]
This is one of the features of the kart that must be decided: it may have a throttle to control the electric
motor or it may come in action once some predefined parameter (e.g. speed of the kart) goes below a
certain value. The latter option is more complex and probably more expensive since it would require
sensors and a carefully designed control system, nonetheless it could be a very advantageous feature
to include in the hybrid vehicle since the driver would only have to worry about handling the sail and the
steering. If this option was chosen it still had to be possible to turn off the electric propulsion system to
allow an exclusively wind powered vehicle if the driver so wishes (even when there are low wind
conditions). It can already be defined in this work that if the automatic control is to be used the torque
sensor solution is not suited to the kart, while the basis behind the cadence sensor system could be
easily applied in the land yacht (measuring the rotations of one of the wheels of the kart).
Even if the kart does not use the controller of a bicycle directly (if it is adapted or a different controller is
used) the same principles and logic discussed in this section can be implemented in the used control
system: direct or automatic control.
2.1.4 Other possible solutions Although motors found in electric bicycles might be adequate to reach the goal of this project, there are
other types of motor that could also satisfy the conditions necessary to conceive a good solution for the
hybrid kart. A problem with the electric bike solution, their motors and the transformation kits is their
cost. When searching for other motors there must be kept in mind some key points:
An adequate power range: the power of the motor should be no less than 250W and bellow
500W;
Moderate dimensions;
DC motor, preferably brushless;
Low voltage (for safety reasons and to limit the weight of the batteries associated to them);
Low cost;
Availability in the market.
17
The motors of electric golf karts could be an option given their availability and lower cost, however their
dimensions are too large to the limited space existing in the hybrid kart and the power they develop is
too much for the necessities.
Several other types of motors that are frequently used in projects of electric karts have the same
problem: they are too large and powerful since they are the only power source of the vehicle, which are
supposed to move at a considerable speed. Some of them are starter engines from trucks and forklift
motors.
Given the inadequate nature of other alternatives evidentiated in the previous discussion, at this point it
is going to be defined that in this project is going to be used a motor prevenient from an e-bike or an e-
bike conversion kit, also due to the fact it is easier at this early stage of prove of concept to apply the
controller the e-bike brings along with the motor in the hybrid kart rather than to build a controller or
adapt one from a third party system. This is nonetheless one of the areas where in future works, following
the previously stated guidelines, there could be found ways of improving the vehicle, namely in terms of
cost. An example of a possibility is to use the DC motor from a treadmill (figure 16) in upcoming
refinements, since it satisfies the defined conditions (only the least powerful ones, since some of these
motors go up to 1500W of power and therefore are not fit for this application). It would, however, have
to be placed in an exterior configuration, which could bring several potential inconvenient consequences
that will be discussed later.
Figure 16 – DC motor found in a treadmill [27]
2.1.5 Variables to consider when choosing the
electrical system
Based on this research, it is possible to resume that some of the variables to consider when choosing
the electric power system are:
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Number of motors (a single motor or one for each rear wheel);
Power of the motor or motors;
Number of batteries;
Total energy storage capacity of the fully charged set of batteries;
Whether to have the ability to recharge the batteries when the vehicle is being used resorting
only to the wind thrust;
Type of control: having the user define when to use the motor(s), using a throttle or something
similar; or have it come in operation automatically, when a certain condition is satisfied (e.g.
when the speed of the vehicle goes below a certain predefined value);
How easy it is to combine the different selected elements of the system and the complexity of
the necessary adaptations (e.g. it is easier to control some types of motor than others);
Total cost of the “electric pack”;
Possible needed modifications/additions to the chassis, and their cost;
Potential need of having a shock/vibrations damping structure around the electric components
in order to protect them.
2.2 Structural solutions
The structure of the chassis has already been studied in a previous thesis [28], on which the present
document partially intends to build upon. It is going to be considered the main structure from that
reference. The chassis that emerges from it (figure 17) is the result of a process of generation of
concepts followed by the application of certain selection conditions, similarly to what will be done in this
work in subsequent sections, although mainly physic and mechanic criteria were used while in the
present document several other types of criteria will be taken into account to select the best model. In
fact, the very large number of different variables to considerer in the generation of concepts is one of
the reasons for this previous definition of the basic structure of the vehicle.
Figure 17 – Base chassis structure A for the hybrid kart project [28]
As it can be seen, it is a simple configuration that shares one key feature with almost all existent land
sailing vehicles: having a structure with two rear wheels and only one front wheel. The reason for this is
19
to allow to have a lighter frame, better aerodynamics, simplified steering mechanism but mainly to have
a higher maneuverability (although it also depends on other factors), something very important in this
vehicles. The sail is anchored to the tube pointing up and the rider’s sit is located behind the sail. These
are the only two relevant sources of loads applied to the structure of a conventional land yacht. The
brand Blokart refers in their official website a weight restriction of 120kg in their products [29] and it was
decided to use the mentioned value in the simulations and calculations that will follow.
In order to establish a comparison with the previous chassis it was chosen the standard Blokart model
(figure 18), which nowadays is the most sold land yacht.
Figure 18 – Base chassis structure B for the hybrid kart project [30]
There will not be necessarily considered all the features of the land yacht model sold to the public, just
its general layout. Every wheel and their connection rods to the chassis are separated from the main
body, which folds itself into a plane (the rider’s back support, the tube where the sail is linked and its
supports all have hinges used to put them down to facilitate transport, and back up in place ready to
use).
It is important to stress that a structural analysis of the body of the vehicle is required given its importance
to the safety of the person using the kart. Furthermore, it will be necessary to implement alterations to
the structure to accommodate the electric motor and associated components, both to hold them to the
frame and to protect them. In some concepts it may also be indispensable to completely redesign some
elements.
In conclusion, the mentioned chassis (figures 17 and 18) are the foundation of the structural part of
every concept studied in this project, but several modifications are introduced: they are the starting point
but the final arrangement may involve major changes whenever the conducted analyses reveals such
necessity.
20
2.3 Brief Patent Research
Similarly to products existing on the market, issued patents dealing with this type of products may
provide crucial information to complement the concept generation process or to better understand
certain technical details of the components under consideration and their legal restrictions. In this section
are only mentioned 3 of the most relevant found, since there are a great number of them.
Bicycle with electric motor assist
In figure 19 a) and b) is shown a pedal-operated bicycle with an electric motor destined to deliver a
supplementary power source to more easily overcome hilly terrain (Patent number: US3921741 A;
Publication date: 25/11/1975; Inventors: Irwin P Garfinkle, Robert J Mcnair). It includes a wheel mounted
hub DC motor that can also operate as a generator, a battery and a handle bar control that allows to
switch between motor/generator (the latter can be activated when going downhill or when it is wanted
to bring the vehicle to a slower speed). It allows then to recharge the battery while on movement, which
is a feature that could be included in the kart to be projected. In the images can be seen a general
overview of the bicycle, the scheme of the electric circuit existing in the product and a detailed cut view
of the area of connection of the motor to the wheel [31].
Figure 19 – Motor assisted e-bike patent [31]
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Wind driven vehicle with electric motor assist
This is the patent found (Patent number: CA2670036 A1; Publication date: 03/12/2010; Inventors:
Richard Letkeman) that most closely resembled the vehicle to project in this document. The applicant
claims the vehicle is aimed to address some inconveniences of land yachts such as the lack of efficient
directional maneuverability and propelling the vehicle during light wind periods. Some of its key
characteristics are: being propelled by one or more adjustable airfoils or sails; two wheel-hub motors; a
very low center of gravity achieved with a corresponding seating position and the placement of the
batteries as near to the ground as possible; steering achieved by a combination of differential braking
and sail adjustment [32].
Rechargeable sailing recumbent tricycle
This invention (Patent number: WO 2007081411 A1; Publication date: 19/07/2007; Inventors: Philip A.
Mactaggart) is a cycle for sailing on land consisting of a frame with 3 wheels supporting a sailing system,
which may be adjusted during use through the swing of a jib. The application of wind force is the main
source of movement, however the vehicle also includes an electric system comprising a generator in
cooperation with the rear wheels to convert mechanical energy in electric energy and store it in a set of
batteries and an electric motor which may be controlled to allow application of an extra force in addition
to the wind [33].
2.4 Analysis of options
In the previous sections it was made a research about the different types of options available in the
market on each of the main components needed to transform the regular land yacht in a hybrid vehicle.
For this reason every type was presented and was made little to no analysis of the feasibility of each of
them. In this section it is going to be conducted a discussion of the lack of viability of some of the options
and is going to be made as well the explanation on how some of the values of important specifications
of the components to be used in this project were obtained.
At this stage the configuration of the components and their arrangement in the vehicle are still not
decided (will only happen in the phase of comparing concepts).
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2.4.1 Type of control
The motor controller is an essential element of the electric propulsion system to implement: it regulates
the operation of the motor in all its function. Its roles include [34] starting (a gradual start is required in
the kart to allow comfort and protection of the equipment, which could not be obtained if the motor was
directly connected to the batteries), stopping, reversing and regulating the speed (the controller takes in
the fixed voltage provided by the batteries and its output is a variable voltage supply fitting the needs of
the motor at each moment to produce the desired rotation speed). The voltage output of the controller
changes according to control signals, either applied by the user or provided automatically by the system
as a function of the information collected by sensors. Controllers cannot be overvolt: the nominal voltage
rating should not be surpassed, at the risk of damaging the equipment (although it is possible to add a
protection device).
As it can be understood by the discussion so far, the number of variables that could be considered in
the generation of concepts is very large, which could easily lead to several dozens of competing designs.
This would be counter-producing since the comparative analysis of all of them would consume a lot of
the time needed to study and project the concept that emerges as the winning. If it is intended to keep
a higher focus on the structural aspects of the integration of the electric motor it is necessary not to opt
by the most complex solutions in terms of the electric circuit layout and type of control. To be more
specific, it is going to be implemented direct control of the motor since it is simpler than the automatic
control based on sensors (that would measure the speed of the vehicle), while still satisfing the
necessary conditions to have a product capable of showing its advantages when compared with the
traditional exclusively wind powered kart. The automatic control of the motor of this vehicle is, however,
yet another interesting topic to develop in future works, exploring possible improvements on this initial
version.
Having this in mind, in the phase of generation of concepts the variables that are going to be considered
are: type of electric bike/conversion kit/components and their cost, location/disposition of the
components, structure of the vehicle and the transmission to the wheels. Other important choices such
as whether to have regeneration of the battery are going to be made separately in a subsequent stage.
2.4.2. Motor and batteries
As for the types of motors to be considered, friction and shaft drive motors are going to be excluded
right away since their characteristics in terms of the manner they transmit power were considered non-
viable for this work.
23
In the generation of concepts there might be conceived models with two motors (one for each rear
wheel) or one (to propel both rear wheels or to just propel the front wheel). In any case, to determine
the characteristics of the batteries in a first approach, is necessary to know the overall power of the
motors and how much time of use is wanted. As it was explained before, the average use of the land
yacht is done in one morning or afternoon (let’s consider 4 hours), of which only in a part of the time is
used the motor, and not always at full power. In a first reflection it is going to be assumed the batteries
must last for 1 hour of use at full power before running out of stored energy.
In this first effort of projecting the vehicle the maximum amount of power in the electric motor(s) chosen
in going to be 500W. This is an average/moderately high value for electric bicycles, so it is going to be
selected given the already mentioned similarities between them and the hybrid kart to project in terms
of expected needs of power (vehicles to carry one occupant, where their weight is more significant than
the vehicles’ own weight and the electric motor is only a complement to the main power source). We
also have the example of fully electric go-karts, which often have a motor with a power of around 1000W
(in this vehicles the motor is the only power source). The batteries should then have of total capacity:
1h x 500W = 500Wh
Obviously, it is not essential this value is exactly met by the batteries, being just a reference point.
Sometimes the total energy capacity of a battery is not given explicitly by the manufacturer, rather are
presented the values of the voltage (V) and of the electric charge (Ah). To get the energy content of a
battery we multiply these values.
The selected motor(s) will always have a voltage below of 48V or below (primarily 36V) for the following
reasons: no need for higher voltage on this type of vehicle, lower safety risks associated, less batteries
needed and lower cost.
It is important to understand that the selection of the specifications for the components are not precise
but are a reasonable first approach in a project that does not have the objective of having the parameters
optimized, rather intends to reach a stage where the implementation is viable and the competitive
advantages of the hybrid vehicle are apparent.
The developed vehicle is expected to have a large share of its use time in locations where the terrain is
very irregular. This type of conditions apply a significant amount of vibrations and shocks into the vehicle
which is made worse by the fact there is no suspension (there is only a limited absorption of this loads
made by the wheels/tires). Batteries can be damaged when exposed to these, namely from short-
circuiting or loss of connectivity caused by the internal components shifting place, resulting sometimes
in premature failure [35]. Since they are very expensive it is necessary to take measures to diminish
these impacts. On top of that, the failure can also result in fire or explosion, being a threat to the
occupant. The details of the vibration isolation solution to implement will be discussed later.
24
2.5 Available commercial options
Now that some important characteristics of the components are defined it is going to be made a specific
exploration of the commercially available options for motors, batteries and controllers to the hybrid land
yacht. Unlike the initial generic market research, which had the goal of investigating the different existing
technologic variants of the components (in order to make an assessment of what could be used in the
vehicle), this stage of the project has the objective of finding suppliers or sources for the constituents
needed (to have a way of actually ordering them if necessary in the future), to determine whether to
select a complete bicycle, a conversion kit or separate components and to know the prices, something
that is essential given the perceivable final cost will be one of the criteria to choose between the
concepts.
There will only be considered products from available commercial suppliers, excluding all sources where
the transaction is not made with commercial entities. In order to be more concise, instead of presenting
a comprehensive list of the items considered along the research there are only going to be shown those
found to be the most adequate according to the set criteria. Furthermore, this exploration was not very
extensive since this is not among the main goals of the present work. A more detailed approach could
be required in the preparation for actually building the hybrid land yacht (there are several dozens of
products in the market that would satisfy the requirements, so just an appropriate example of each type
is presented here).
When considering wheels with integrated hub motors is important to choose those who have smaller
diameters. Regular 24 or 26 inches wheels, when put in the chassis, would result in a vehicle where the
wheels would be much larger than those usually applied in these vehicles and this would translate in
both an awkward and unappealing design and a deviation from the intended characteristics, namely in
terms of handling and maneuverability. As a result, there are going to be given preference to e-bikes
with smaller wheels, for example those destined to children.
In the cases a bicycle wheel is going to be used, it will come from a subcategory known as fat bikes
[36]. This kind of models are specifically designed to be used at the beach, snow and in other off-road
locations, and for that reason the wheels are wider than normal to allow a better traction and to prevent
them to sink too deeply in the sand. On top of that, it is expectable they have a greater protection on the
electric components to avoid damage by the environmental conditions.
25
2.5.1 Complete sets available in the market
E-bike Shuangye A7AM20 beach model
This model produced by the Chinese brand Shuangye fits the defined parameters: it has a 36V, 500W
geared hub motor; lithium batteries with 48V and 10Ah (480Wh, very close to the defined needed
capacity); manual control (either a twist or a thumb throttle can be chosen) and a tire 20 inches in
diameter, 4 inches wide. Unfortunately, the weight of the battery is not discriminated in the available
information (as usual, only the value of the total weight of the bicycle is given). A good estimative can
be obtained knowing the battery’s capacity and the average value of the energy density for the type of
battery in consideration, as previously discussed. With an average value of 120Wh/kg, we get a
reasonable approximation for the battery’s mass of 480/120=4kg.
Another reason to choose this model is its cost: although it is not explicit on the website of the company,
it was found in another website (Alibaba [37]) to be of 710$ (630€), which is on the lower range. These
are the highlighted characteristics but more detailed technical information can the consulted in reference
[37]. In figure 20 can be seen the thicker, wider tires and the relative size of the battery.
Figure 20 – The A7AM20 model [37]
E-bike Shuangye A7-AQ20
Specially designed to be used on the snow, this model [38] also has 20’’ diameter “fat wheels”. It carries
a 250W geared hub motor and a 36V lithium battery. The value of its capacity is not available, however
it is listed that the expected range on a full charge is of 40 to 60km (let’s assume 50). As a rule of thumb,
it is known that in e-bikes 240Wh allow on average a 30km trip: therefore it is reasonable to assume a
capacity of around 400Wh, which is adequate. Going through the same process as for the previous
bicycle, we have an approximate value for the mass off the battery of 3,3kg. The retail price of the bicycle
is 700$ (612€).
26
All the other 20’’ diameter, fat tire e-bikes found were similar to the one above in type of battery (lithium)
and motor (geared hub): it was not possible to find bicycles having small diameter fat tires with different
types of motor (e.g. direct drive) or battery (lead acid) meeting all the criteria (power, battery capacity,
etc.).
To have other types of equipment to conceive models, in the next subsection are considered alternatives
to complete e-bikes.
2.5.2 Conversion kits
It is frequent to find kits that consist of every necessary part and kits that exclude only the battery, since
this is the most expensive component, and for that reason it is expectable the client may want to choose
it separately. This being the case, the following kits (which belong to the latter category) are going to be
utilized together with the battery packs below. In this section there were some difficulties in finding
elements that met all the prerequisites defined, and on top of that some of the suppliers would only sell
in bulk (e.g. more than 100 units, those were obviously excluded).
2.5.2.1 Motors/controllers
1-GNGELECTRIC chain reduction mid drive electric motorized e-bike kit
Mid-drive motors are frequently found in kits. This is an example of a product (figure 21) that includes
the motor, controller, throttle and other elements (some not used in the kart) but not the battery. This
set can be combined with different batteries in the kart as it would happen if it was being used in its
intended purpose. It has a 48V, 500W mid drive brushless motor and a price of 369$ (322€) [39].
Figure 21 – GNGELECTRIC mid drive motor kit [39]
27
2-LeafBike 16’’ 36V 250W conversion kit
This kit [40] consists of a geared, brushless motor which has 36V, 250W and a price of 293$ (259€).
Besides the motor, it includes a controller and a throttle, as well as the wheel, which is essential due to
the type of motor existent in the kit (it contrasts with the kit of the previous section, allowing a greater
variety in the concepts that will be generated in this work).
As was already explained, direct drive motors are usually heavier than the other types. In this research
was also found they have almost always at least 750W of power, which exceeds the upper limit defined.
For these reasons, they are considered no further.
Figure 22 - LeafBike 16’’ 36V 250W conversion kit [40]
2.5.2.2 Batteries In this section are presented the batteries chosen to complete the conversion kits of section 2.5.2.1. In
order to have as much variety as possible in the options for originating concepts, are chosen lead acid
and lithium batteries to put together with the kits. For each type is presented a 48V and a 36V pack. For
future reference, the packs are numbered from 1 to 4.
The E-BikeKit® Sealed Lead Acid Electric Bicycle Battery Pack [41] is the selected equipment for the
lead acid option, having versions of both 48 and 36V. The first of the battery packs has the following
characteristics: (1) 48V, 9Ah (432Wh), 10kg, dimensions: 203x152x95mm, 150-250 life cycles, 300$
(265€). The 36V option of the same equipment has the following specifications: (2) 12Ah (432Wh),
12,2kg, dimensions: 267x152x95mm, 150-250 life cycles, and a price of 280$ (248€).
For the lithium based batteries, were selected E-BikeKit® LiFePO4 Lithium Electric Bike Battery [42]
(48V and 36V) packs. The first of these has: (3) 48V, 10,5Ah (504Wh), 6,5kg, dimensions:
385x150x82mm, 700 life cycles with 100% discharge/1500 with 80%, 740$ (654€). Conversely, the 36V
version (4) has a capacity of 10,5Ah (378Wh), it weighs 4,8kg, has 382x149x69mm dimensions, is
prepared to have 700 cycles with 100% discharge or 1500 with 80%, and is priced at 628$ (555€).
28
This research has also showed that nickel batteries are virtually not applied anymore in e-bikes, which
is mainly due to the difficulty of recycling or safely dispose them: they are widely regarded as not a good
choice and a thing of the past [43]. For this reason, from this point onwards are only going to be
considered lead-acid and lithium based batteries.
3. Hybrid land yacht concepts
3.1 Concept generation
Before coming up with different concepts it is necessary to define the criteria to meet and according to
which is possible to consider the vehicle could satisfy costumers’ needs as defined to this work. They
are going to be the generic guidelines of the generation but mainly of the selection process. These
include:
Increased range of weather conditions where the land yacht is usable;
Safety for the rider (both in terms of a crash/rollover of the vehicle and electric system stability);
Overall expected cost;
Battery capacity;
Stability and maneuverability (related with the center of gravity);
Weight and how easy it is to transport;
Assembly/disassembly;
Resistance to rough weather conditions (corrosion);
Durability and maintenance;
Ergonomics and comfort.
As was previously mentioned, one of the key features of many traditional land yachts is their
convenience of transport and easy and quick assemble. The goal of the project is to offer an improved
solution, so by including the electric system this characteristic of the product should not be endangered.
To achieve it, is essential to design all the vehicle in such a way that allows to disconnect the
components and put them back together easily. Being this the case, it is possible to imagine models
where the electric system can be taken away and therefore allow to transform the vehicle in a regular
exclusively wind powered land yacht. The modular nature of such concepts is a noteworthy
29
characteristic since it adds possibilities of choice for the user (e.g. on very windy days the electric power
isn’t necessary so it would be a disadvantage to carry the extra weight around in these situations).
It is important to note a partial solution could be implemented: instead of removing every element of the
electric system, some less heavy ones could be left permanently in the vehicle in order to simplify the
design and reduce the assembly and disassembly time. By removing the batteries and motor alone most
of the added weight would be taken away, plus it would be avoided to unnecessarily subject them to
vibrations and shocks, thus contributing to extend their life.
There were defined 3 possibilities for the location of the motor (which are related to the type of motor
selected): to use a complete bicycle wheel (including a direct drive or geared hub motor), to put it inside
the kart’s wheel hub (using a geared motor directly connected to the wheel) and to apply a mid-drive
motor externally.
The concept generation process chosen is very simple: the previously selected e-bikes/conversion kits
are going to be put together in order to form different models. At this stage it is not done a very deep
analysis of the quality and merits of each of the concepts (which will be done in detail in following
sections), only completely non-viable models will be disregarded at this point.
Although it is possible to use a motor and a battery with different nominal voltages, models with those
characteristics are not going to be considered for following reasons: 1) Motors with lower voltage than
the batteries would be constantly under overvoltage and could easily overheat; 2) motors with higher
voltage would not put out all their power; 3) there are already a large number of concepts and those
new ones would not differ substantially and so would just make the analysis more complicated without
adding something relevant.
Only the variation of the location of the motor(s) and battery pack are being considered since they are
the most relevant to the characteristics and behavior of the vehicle. The placement of components like
the controller are going to be discussed once the winner concept emerges from the selection process
since it’s not considered to be a key aspect of this work (it does not influence the center of gravity or the
maneuverability). The concepts are going to be divided in two groups: those that use structure A
(appearing in figure 17) and those that use structure B (in figure 18).
3.1.1 Concept A1 – Battery Below
In this first model (in figure 23) it is directly applied a wheel of an e-bike with a geared motor in the front
fork of structure A. All the components used come from the e-bike A7AM20 [37]. The battery is located
beneath the rider’s seat in a platform or compartment connected to the chassis (to the central element
of the structure, to which the sail is connected to) with screws. As happens with every model that uses
the entire wheel of an e-bike with an integrated electric motor, to be possible to use it without the motor
30
it is necessary to change this wheel with a regular one. For this reason, the spare wheel should be
included in the sold product to allow to do the conversion. The battery is protected by a case designed
for that purpose (which also happens with all other concepts).
Figure 23 – Sketch of concept A1 (battery inside the protective case)
3.1.2 Concept A2 – Battery on Top
This one is equal to concept 1 except that the battery is located along the front wheel connection tube
(figure 24), between the rider’s legs. The battery is put in a removable compartment/box that is linked
to the chassis with screws.
Figure 24 – Sketch of concept A2
3.1.3 Concept A3 – Two Batteries Below
The motored wheels of two e-bikes A7-AQ20 [38] are used (one in each rear wheel) on a structure’s A
layout. The battery sets of each bicycle are applied, so in fact they are parallel systems that were
integrated to be controlled simultaneously. It would be technically easier (and also cheaper) to just use
the elements of one bicycle with motors on both the front and rear wheels, but unfortunately the existing
models greatly exceed the established limit power (usually going up to 2000W, which would completely
modify the set goal of assisting the wind power, going towards the direction of an actual electric kart)
and often the front and rear motors have different sizes and nominal power outputs. This is clearly a
31
solution that is more costly and with a more intricate control system than most of the other competing
concepts (there is also the extra expense of having to include two extra land yacht wheels to make it
modular, i.e. being able to convert the vehicle in a traditional wind powered land yacht). In this concept
each battery supplies only one motor but they are both located on a platform similar to the one used in
concept 1. With two batteries (figure 25) it is not feasible to have them along the front part of the frame
as in concept 2 due to constraints of lack of space to place them. It also would not be possible to use
only one of the batteries since it would run out too quickly.
Since each motor has a 250W nominal power, the total for the vehicle is still within the boundaries (it is
the same as in the previous models).
Figure 25 – Sketch of concept A3
3.1.4 Concept A4 – Geared Motor with SLA Battery
Below
The geared motor from conversion kit 2 is used in the hub of a regular land yacht wheel, in this case the
one in the front. As it was mentioned before, the other components (controller, throttle) come from the
same kit and they are in this concept combined with battery pack 2 (sealed lead acid), introduced in
section 2.5.2.2. The battery is placed in a platform in the same way as in concept A1.
3.1.5 Concept A5 – Geared Motor with SLA Battery
on Top
The same as the previous one, except the battery is located using the method of concept A2. This
change is more relevant between concepts A4 and A5 since the battery is lead-acid, and therefore
larger, heavier and with a greater impact in the change of the center of mass and consequently in the
alteration of the dynamic behavior of the vehicle.
32
3.1.6 Concept A6 – Geared Motor with Li Ion Battery
Below
This concept has the same characteristics as concept A4, with the exception it is used battery pack
number 4 (introduced in 2.5.2.2). This alteration in the type of technology of the battery will allow to more
carefully examine the relative importance of that factor.
3.1.7 Concept A7 – Geared Motor with Li Ion Battery
on Top
The same as A5 except it is used battery 4. Once again, the main goal is to be able to compare the
differences that become noticeable when changing the type of battery in the vehicle.
3.1.8 Concept A8 – One motor on each rear wheel
with two SLA batteries below
Two motors #2 (from the leaf bike conversion kit) are used in this model, one in each rear wheel (same
general idea as in concept A3, and most of the same considerations still apply here). The motors used
have 250W each so whether one or two are applied the set power limits are met; however there is a
very significant power difference between these two models which is positive since it allows to analyze
that parameter. Two batteries #2 (from e-bike sealed lead acid 36V) are used, one for each of the
motors, and both placed beneath the rider’s seat.
3.1.9 Concept A9 – One motor on each rear wheel
with two Li ion batteries below
This one is equal to A8 with the exception that are used two batteries #4 (lithium ion).
33
3.1.10 Concept A10 – Mid Drive Motor with SLA
Battery
In this concept, motor #1 (mid drive) is used to propel the front wheel of the vehicle. Unlike in the previous
possibilities, in this vehicle the motor is external to the wheels and so it is necessary to have a way of
transmitting the power to them. In e-bikes this is done through a chain and this is going to be maintained
in this concept. The motor is placed in the zone connecting the main body and the front wheel. To avoid
too much weight and volume in the front part of the vehicle it is only going to be considered a battery
placement beneath the rider’s seat (in this case battery pack #1 – which as mentioned previously is a
48V SLA battery).
3.1.11 Concept A11 – Mid Drive Motor with Li Ion
Battery
Equal to A10 but it is used battery #3.
3.1.12 Concepts B1 to B11
This section refers to a group of 11 concepts: they are in all aspects similar to the 11 concepts defined
so far, with the exception they use structure B. B1 corresponds to A1, B2 to A2 and so on.
Although structures A and B have different underlying philosophies and some contrasting pros and cons
(namely in terms of transportability and cost), they also have characteristics in common that have as
one of the consequences the lack of space to place the electric components. Moreover, they allow the
same limited locations for these constituents, and as a result the concepts that emerge using both these
chassis are analogous. The only major difference having a significant impact on the generation of
concepts is that, contrarily to A, structure B has a connection of the rear wheels using an axle. This
means it is would be theoretically possible to have models using a mid drive motor where the
transmission of the power is done to both rear wheels. These are described below (B12 and B13).
34
3.1.13 Concept B12 – Mid Drive Motor to Rear
Wheels with SLA Battery
In this concept mid drive motor #1 is used to propel the rear wheels of the vehicle. The transmission is
done using a chain connecting the output of the motor and the axle where a sprocket is installed. The
battery is put in the same location as in concept A2. It is used a lead acid pack (battery #1).
3.1.14 Concept B13 – Mid Drive Motor to Rear
Wheels with Li Ion Battery
The same as B12 except battery #3 (48V lithium ion pack) is used.
3.2 Concept selection
3.2.1 Screening: first stage
Even though before the generation of concepts it was made a discussion of some aspects that led to
the definition of a number of features of the vehicle to be designed (which was done in order to reduce
the quantity of models to handle) there are still a total of 24 concepts. This is not necessarily undesirable
when using this methodology because having a reasonably large number of concepts with as much
variety as possible may be helpful, as well as to include those ideas that at first may seem unreasonable
or strange: some of them may be at some point corrected, improved or combined with others and result
in valuable products.
In a subsequent stage, however, it is important to bring the number down to a manageable amount. To
achieve that it is going to be made a pre selection based on the elimination of concepts that are clearly
inferior, or very similar to others and with no noticeable advantages over them. It is then possible to
apply certain formal selection methods in order to identify the concept with the best overall features.
Concepts A10, A11, B10 and B11 are going to be eliminated because the transmission of the power
using a chain could interfere with the steering of the vehicle and the placement of the motor would be a
problem to the placement of the rider’s feet. Besides these models have no clear competitive pluses
over the others and also have the problems associated with the use of a chain in environments with
sand or dirt (which would require a protective cover).
35
Concepts A3 and B3 would require a much higher investment to have all the components necessary for
the electric system than other concepts, due to the fact they are retrieved from e-bikes. Obviously, in
actual production the parts would be specifically bought for the vehicles and the cost would be smaller
but still they would always be much more expensive than the constituents used (for example) on A1/B1.
Once again, the use of components retrieved from e-bikes instead of getting them separately is done
because this is a preliminary study of the integration of an electric propulsion system on a land yacht,
and there could be compatibility problems when buying the components separately that could endanger
the main goal of the project (this is much less likely to happen when just adding to a kit a battery sold
separately). In subsequent works it would be interesting to study the economic advantages of building
the hybrid vehicle with components sold separately and to evaluate and solve potential incompatibilities
that may exist.
The electric components also take twice the volume (space to accommodate them is very limited) and
make the vehicle heavier, mainly because two battery packs are used (for the reasons already
mentioned). Additionally, since the batteries are put close to each other they will with greater probability
change in a significant way the center of gravity of the kart, and therefore its dynamic behavior in
comparison with what is expected from the traditional wind powered land yachts: this is an important
drawback since the objective of this project is to include the assistant electric energy power source with
minimized interference in the rider’s experience in terms of maneuverability, stability and safety. For
these reasons A3 and B3 are also excluded.
Concepts B12 and B13 use a transmission system instead of directly propelling the wheels: for this
reason they are intrinsically more complex than the other models and have an extra associated cost
because of that (even if it is small and surpassed by other costs in competing models). Two major
problems are common to all of the mentioned concepts: (1) they would require a protective covering to
keep sand grains from entering the mechanism, which could still not guarantee to certainty this would
not happen and cause damage; and (2), the use of a chain would (in a best case scenario) dramatically
increase the assemble and disassemble time of the vehicle and could even compromise the goal of
having a vehicle where it is possible to remove the electric system and use the kart only having the wind
to power it. For these reasons the mentioned concepts are rejected.
Lead acid batteries are substantially larger and heavier than the lithium based ones. In some concepts
they are put in the tube connecting the front wheel to the rest of the chassis which may alter the difficulty
of the rider’s steering. In addition, the placement of the battery and its large volume complicate the
positioning of the rider’s legs, making the experience more awkward and uncomfortable. This brings
concepts A5 and B5 to be discarded.
As mentioned before, there is a problem in accommodating lead acid batteries due to their volume and
weight. In concepts A8 and B8 two of these are used and put together and for that reason this concepts
are going to be excluded.
36
3.2.2 Selection criteria
At this point there are 12 concepts that remain (A1, A2, A4, A6, A7, A9, B1, B2, B4, B6, B7 and B9).
This is an acceptable number to use the screening methods. Before that, however, it is necessary to
define what are the criteria being considered and why they are important to the selection of the winning
concept. For it, consider the following:
-Portability. As it was mentioned before, it would be important not only to be possible to remove the
electric components and use the vehicle as a regular land yacht (something that happens with all the
models still under scrutiny after the first stage of the selection process) but also to have the ability to
completely disassemble the chassis in order for it to be easier to transport.
-Cost. Two things are going to influence the total cost: the part related to the structure chosen and the
added budget due to the implementation of the electric system (both the components and the structural
changes and additions). Although no exact values are known at this point it is possible to have
reasonable estimations, and besides the most relevant thing at this point is being able to make
comparisons between the costs of each model. Structure A is considered to be cheaper due to the lower
number of components and lower complexity. In what concerns the cost of the electric components
necessary, this is given by the price of the bicycle or kit they come from.
-Weight. The total mass of the vehicle is always going to be bigger than the value of the traditional land
yacht due to the electric components, being the battery the most relevant. A lower weight means in
general terms better maneuverability and easier transport. Structure A is lighter.
-Position of the center of gravity (C.G.). This is a crucial factor in having high maneuverability, safety
and stability in the vehicle. It should be as low and as centered as possible in order to achieve these
goals.
-Range. The motors and batteries used vary among the different concepts, which means each vehicle
will be able to obtain different maximum durations of use of the electric system. It is necessary to take
in consideration the values of the nominal power of the motor (W) and the energy storage capacity of
the batteries (Wh). Dividing the latter by the former we get an approximation of the expected time of use
of the vehicle at full power.
-Time of assembly. Related with the previous item is the estimation of how long it would take to
assemble completely the vehicle and have it ready to use and to dissemble and store it. However, these
points cover different aspects since “portability” refers to how easy it would be to do the transport once
the disassembly is completed.
-Power. When the electric power available increases there is usually also an increase of negative factors
such as the weight and the cost, and for this reason (among others) it was set a limit of 500W to the
totality of the motors. However, within this limit and considering all other parameters equal, it is
37
somewhat preferable to have the value of the available power closer to the upper limit since it will help
to overcome steeper terrain and will allow to effectively use the vehicle in weaker wind conditions.
-Maintenance. The “cost” variable will determine the price which represents the initial investment for
the costumer. However, it cannot be neglected the fact that lead acid batteries have considerably less
available charge cycles in their life time compared to the lithium ion based batteries (e.g. 200 versus
700 cycles in the considered 36V batteries). Having in mind that the life cycle of the structure and electric
components other than the battery is a lot larger than the 200 days of use of the lead acid battery
(assuming a discharge per day of use), it will be necessary for the user to periodically buy a new battery
to keep using the vehicle. There is, therefore, an underlying cost in vehicles using this technology that
must be taken into account since it constitutes a significant disadvantage for the concepts that use them.
3.2.3 Reference product for the selection During the process of generation and selection of concepts for this work it was found for sale in a website
[44] a product (figure 26) from the company Blokart that consists in an addition to their land sailing
vehicles: a wheel with an incorporated hub electric motor and corresponding battery pack, controller and
throttle. It is therefore clearly a product that aims to solve the same identified shortcomings of land
yachts and that seems in a first analysis to follow the same product generation steps (namely having as
a base the technology found on e-bikes and adapting it to the land yacht).
Figure 26 – Blokart electric motor [44]
This product will be useful to make a comparison with the developed concepts, namely in the selection
process, where it will be the reference used. The motor in the Blokart product is sold separately, which
means the vehicle can be used in its traditional way or having the electric power assistance: this
characteristic was also sought on the prototypes imagined in the present thesis (to be more clear, the
reference is the traditional Blokart with this addition installed). It has a power of 500W which is the upper
limit set in this work (250 to 500W). The battery used is lithium-based and has 36 volts, is inside a
protective alloy case and is positioned inside a bag behind the Blokart seat with the system’s controller
placed in the side of the structure of the kart. No data on the capacity of the battery was found, however
given its chemistry and the fact the equipment is so expensive, and since the battery is always the most
expensive component of this type of electric systems, it is going to be assumed a value of 750Wh, which
38
is on the higher end of the spectrum of what is seen on e-bikes (the comparison point) but still fairly
common and adequate to this type of application.
Its price (approximately 2540€) is considered very high, which could indicate this type of product has in
its production higher costs than it was initially thought, due to variables that were inadvertently
overlooked in the present process. However, this value could probably be justified having in
consideration this product was recently introduced, being common in these situations for companies to
try to recoup the development investment more quickly by charging a higher price to the first consumers.
The lack of similar competing products on the market and the fact the brand in question is dominant in
the land yacht commercialization area may also play significant roles. Unfortunately, it was not possible
to find the patent presumed to have been issued for this product. This may be due to the recent nature
of the item.
3.2.4. Screening: second stage
In table 1 the concepts under examination are compared to the reference product and rated according
to their performance in each of the selection criteria in relation to it. Plus (+) means the concept has a
better performance in that category, minus (-) inferior and zero (0) means there isn’t a relevant difference
between them. Then it is done the counting of the number of occurrences each concept has in each
grade and based on that information it’s made the ranking of the concepts.
Concepts
Selection
criteria
A1 A2 A4 A6 A7 A9 B1 B2 B4 B6 B7 B9 REF
Portability - - - - - - 0 0 0 0 0 0 0
Cost + + + + + + + + + + + + 0
Weight + + 0 + + 0 0 0 - 0 0 - 0
C.G. position 0 - 0 0 - 0 0 - 0 0 - 0 0
Range - - + 0 0 0 - - + 0 0 0 0
Time of
assembly
+ + + + + - 0 0 0 0 0 - 0
Power 0 0 - - - 0 0 0 - - - 0 0
Maintenance 0 0 - 0 0 0 0 0 - 0 0 0 0
Pluses 3 3 3 3 3 1 1 1 2 1 1 1
Same 3 2 2 3 2 5 6 5 3 6 5 5
Minuses 2 3 3 2 3 2 1 2 3 1 2 2
Net 1 0 0 1 0 -1 0 -1 -1 0 -1 -1
Rank 1º 3º 3º 1º 3º 8º 3º 8º 8º 3º 8º 8º
Continue? Yes No No Yes No No No No No No No No
Table 1 – Concept Comparison
39
Some other parameters, like the safety or comfort of the passenger, could also be considered. However,
it is not expected substantial variation of these characteristics among the concepts. It would also be
important to assess the maneuverability of each of the karts but this would be something that could only
be done satisfactorily by testing each of the models (as said before, it is suggested to test the resulting
vehicle of this project in subsequent works). Nonetheless, albeit is not easy to establish differences
between the concepts, it is generally expected that the introduction of the electric motor will improve this
feature of the land yacht, both by providing to the vehicle a much more stable and controllable power
source and by allowing the rider to address a higher focus to the steering of the vehicle.
Furthermore, it is important to notice that, as explained before, this work is intended to be a preliminary
effort in developing this kind of vehicle and for that reason it is acceptable to introduce some
approximations in the comparison of concepts done in the previous table, as well as overlook some less
fundamental characteristics that could otherwise be included in this sort of analysis.
Some of the considered selection criteria used in the previous selection table are quantifiable while
others are more subjective. For the first type it is presented below table 2 with the respective values,
while for the second it is put forward the justification for the attributed comparative rating.
Concepts
Selection
criteria
A1 A2 A4 A6 A7 A9 B1 B2 B4 B6 B7 B9 REF
Cost
(electric
component
s only) (€)
630
[37]
630
[37]
507
[40],
[41]
814
[40],
[42]
814
[40],
[42]
1628
[40],
[42]
630
[37]
630
[37]
507
[40],
[41]
814
[40],
[42]
814
[40],
[42]
1628
[40],
[42]
2517
[44]
Range
(hours at full
power)
0.96
0.96
1.73
1.5
1.5
1.5
0.96
0.96
1.73
1.5
1.5
1.5
1.5
Power (W) 500 500 250 250 250 500 500 500 250 250 250 500 500
Table 2 – Quantifiable Criteria Values
In what concerns “portability”, it is considered as a first approximation that all the concepts that use
structure A are less portable than the reference model, since the reference is a traditional Blokart where
it is possible to add the wheel with the motor, and it was considered in this work that structure A cannot
be as easily put inside the carrier bag and put in the trunk of the car as it happens with the Blokart and
its electric motor addition. This is mainly because the individual parts of this structure are bigger (A has
fewer parts) and take more space. As an example, the central part of structure A (as seen in figure 27),
where the mast is connected to the vehicle, has a shape that makes it somewhat harder to transport.
40
A standard Blokart model weights around 29kg. Unfortunately, it was not possible to find a source for
the weight of the electric motor system used in the reference model. However, in order to make the
comparison with the reference it is enough to note that the Blokart electric addition uses a lithium battery
and just one motor (and these have approximately the same mass as the corresponding elements used
in the generated concepts, in which this value is known), and that structure A is lighter than B (which is
also the reference). Note: a “+” sign means a positive characteristic, so in this case it refers to a lighter
model.
The cost of the reference product, since they are sold separately, must include the price of the Blokart
classic model (2465€) as well as the price of the electric system (approximately 2517€) for a total of
almost 5000€. The cost of the concepts comprise the electric components (for which the prices are
known from the market research) and the cost of manufacturing the structure, which is not known at this
point and would require a detailed analysis for both types of chassis (for structure B only the retail price
of the model it is based on is known at this point). However, for this study it is enough to understand
that based on the known values, and even including others like those related with the changes
necessary to accommodate the electric propulsion system, it is reasonable to assume both the total cost
and the consequent price to the costumer would be lower than that of the reference.
The time of assembly of each vehicle is difficult to determine exactly, so there are going to be considered
estimates based on the type of structure and on the number of components of the electric system.
Concepts that use structure A have lower assembly times, but the opposite is assumed for those with a
higher number of electric components (A9 and B9).
Since both types of structure provide a similar position to the rider, the most significant characteristic in
influencing the alteration of the position of the C.G. from one model to the other is the type of battery
(consequently, its weight) and its location.
3.2.5 Final selection
In order to have a greater degree of certainty that the winning concept is the best possible solution
among the existing ones considering the set criteria, the number 1 concept in the first table is not
immediately picked as the final. Instead, the 2 best ranked are going to be selected to go through to a
next phase.
At this point is necessary to attribute a value (a scale of 1 being the worst and 5 the best) to each
selection criteria of each concept that reflects its place in the ranking among the totality of the concepts.
Simultaneously, to each selection category is going to be assigned a value that translates the relative
importance of said criteria in the selection of the vehicle. Although the two final concepts are similar in
terms of general structure and disposition of the elements of the electric system, there are differences
41
between them in terms of components (namely motor, battery and front wheel). The result is presented
in table 3.
A1 A6
Criteria Weight Rating Weighted Score
Rating Weighted Score
Portability 15% 3 0.45 3 0.45
Cost 15% 4 0.6 3 0.45
Weight 5% 4 0.2 4 0.2
C.G. position 15% 4 0.6 4 0.6
Range 10% 3 0.3 4 0.4
Time of assembly
15% 5 0.75 5 0.75
Power 10% 4 0.4 3 0.3
Maintenance 15% 4 0.6 4 0.6
Total Score 3.9 3.75
Rank 1º 2º
Develop? Yes No Table 3 – Final Selection
3.3 Winning concept detailed description
The selected concept was A1. It follows at table 4 an enumeration of all the components that make the
vehicle:
1- Sail 2- Mast 3- Tripod
4- Left back connection tube 5- Right back connection tube 6- Front wheel connection tube
7- Fork 8- Steering bars 9- Main Circuit Braker
10- Back wheels 11- Front wheel (bicycle wheel) 12- Motor
13- Battery 14- Controller 15- Throttle
16- Brake 17- Brake lever 18- Bearings
19- Bolts 20- Nuts 21- Seat
22- Pin 23- Battery case (and protective
foam)
24- Fuse
Table 4 – List of constituents
The reason for the existence of some of the components on the table will be explained in the following
sections.
42
From this list, the ones manufactured specifically for this project from metal tubing are: 3, 4, 5, 6, 7 and
8. All other components will be bought. In the image below is displayed the configuration of the structure
of the vehicle.
Figure 27 – Structure of the winning concept A1 in perspective
3.4 Assembly and disassembly The structure of the vehicle allows a balance between assembly time (favored when we have fewer
parts) and simplicity of transport (which decreases when having large components). The chassis is
almost 2 meters long but is constituted by only 5 components and when disassembled they are easily
transported (the longest has about 650mm).
The procedure to assemble the land yacht should start by joining the tripod with both back wheel
connection tubes and the front wheel connection tube, using for the first case four M10 bolts in each
flange and 2 in the second (the reason for the use of these elements is explained in following chapters).
In all situations in this vehicles a nut and two washers are applied as well.
The second stage is to join the fork to the chassis using a pin. At this point the whole structure is
complete and all the wheels can be put in place. Now, the electric system can be assembled: the battery
case (with the battery inside) is connected to the chassis using four M8 bolts and nuts. The controller,
main circuit breaker, the brake lever and the throttle are placed in the positions mentioned in section
5.2. Once they are in place, all the wires are connected according to the electric circuit in section 5.1.
Finally, the mast and sail can be put in position. They are the last elements because of their size and
because the action of the wind on them would difficult the placement of the other components.
To do the disassembly of the vehicle, it is suggested all the operations are done in the inverse order.
From the previous description it is evident that the inclusion of the electric power system is associated
with an increase of the assembly and disassembly complexity (and consequently, the time necessary
to complete it). However, in the opinion of the author, this disadvantage is overshadowed by the opening
of new possibilities for the users.
43
Part B: Project’s Calculations
4. Structural Dimensioning
4.1 Introductory considerations Several types of analysis are necessary to guarantee the good practices of mechanical engineering in
regards to the structure of the vehicle and the expected life of the product. There are going to be
conducted the following: static analysis of the chassis, dynamic analysis (vibration behavior and
resonance frequencies), verification of the connection elements (screws, etc.) and verification of other
bought components (e.g. the brakes).
In this project, in order to simplify the process, it was first made a 3D CAD model of the land yacht
having as a base the characteristics of the concept chosen in the selection phase and using dimensions
common in this type of vehicles, with the goal of being able to conduct the previously mentioned
analyses. If one or more of them reveals the model does not assure the integrity and safety of the
structure to a satisfying degree, alterations are then implemented (either on the basic layout of the frame,
on details of it, on the dimensions of the sections of the tubes used or even on the materials, always
according to the results of the analyses). Alterations will also occur if the studies reveal over
dimensioning, in this case not for safety reasons but for cost, size and weight concerns.
As it was previously stated, in this project will only be considered two types of external loads applied on
the structure of the vehicle, since they are the most relevant: those applied by the wind on the sail
(causing a reaction in the structure) and the rider’s own weight (considered to be 120kg, since this is
the limit for which the vehicle is designed, in accordance with the value used by the company leader of
the market). In the static analysis of the vehicle’s chassis are going to be used only the highest expected
loads, since by doing this it can be checked the structure’s safety in static conditions.
The forces applied by the mast on the tripod are the ones that have a more complex variating behavior,
influenced by the type of sail, its position and the wind conditions, among other factors. This being the
case, they require a previous analysis in order to determine an estimation of the values to be used in
the calculations that will follow.
As in the vast majority of the land yachts on the market, the sail on this project is attached to the structure
by the mast and by a rope.
44
4.2 Modifications to the existing structure
Although the basic structure (figure 17) was defined in previous sections, it is considered convenient to
implement right away some changes to it. The most obvious alteration was done in the fork: in the
original structure it was used only one tube to connect the axis of the wheel to the body of the fork, now
it is present in both sides. The reason for this is the use of bicycle wheels in the vehicle (contrarily to
what happens in [28], where the structure’s layout is originally from), which are much wider than the
wheel typically used on these vehicles. With this alteration it is intended to have a more stable and rigid
connection, also necessary due to the presence of the electric motor.
Other key differences are the necessary elements to accommodate the battery beneath the passenger’s
seat. Finally, the connection between the different tubular components of the structure is done
integrating flanges at the end of each of them, and using bolts and nuts. The integrity of these
connections is going to be checked in subsequent sections.
4.2.1 Aerodynamic impact of the modifications
Some of the changes in the concept, namely the addition of the battery and its compartment, have an
implication on the aerodynamic behavior of the vehicle, since these constitute a perturbation to the
existing flow of air. Assuming that the perturbation is not very significant, since this is a vehicle for
recreation, not destined to competition, it is not considered to be necessary to conduct a time consuming
detailed analysis of the degree of influence of this parameter on the performance of the vehicle.
4.3 Forces transmitted by the wind
4.3.1 Type of sail
Several types of equipment are used to “capture” the energy of the wind and transform it in a way of
propelling land yachts, namely sails with different sizes, shapes and profiles. These apply on the vehicle
loads that differ dramatically and thus impact significantly the behavior of the vehicle and its
performance.
It is not, however, the focus of this project to study in detail the influence of this choice in the performance
of the land yacht. For that reason, it is going to be assumed the vehicle utilizes the type used in reference
45
[28], which is a NACA 0015 wing profile [45]. It is therefore made an approximation of the sail as a rigid
wing, done in order to simplify the estimation of the loads.
It would still exist, however, a choice to be made: since the vehicle now has a secondary source of
propulsion in the electric motor, it is conceivable to utilize a smaller sail and still maintain an adequate
total thrust in the vehicle. Nonetheless, if this option was taken, it could mean there would be an
important loss of the modular nature of the product (i.e., the option of removing the electric system and
use the vehicle in a traditional way). It would still be present, but there could be a noticeable decrease
of the performance in these conditions. For this reason, the same dimensions are considered as in
reference [28].
4.3.2 Apparent wind and resulting forces on the sail
The first thing necessary to mention, and that may be counterintuitive at first glance, is that it is possible
to achieve higher speeds in wind powered vehicles at a given moment than the speed of the wind at
that time. This is done by setting the sail at an angle relatively to the wind direction and by using the
lateral resistance of the surface on which the vehicle is travelling to maintain a direction other than the
one of the wind. An important concept to understand the mechanism of motion of sail vehicles is the
apparent wind, which is the wind experienced by any observer in motion on said vehicle. Figure 28
illustrates the real speed of the wind (𝑉𝑇), the speed of the vehicle (𝑉𝐿) and the resulting apparent wind
speed (𝑉𝐴) [46].
Figure 28 – Real wind and apparent wind speeds on a land yacht) (adapted from [46])
46
β is the angle between the direction of the apparent wind velocity and the direction of movement of the
vehicle (apparent wind angle, a very important parameter in the following analysis) and Φ is the one
between the real wind direction and the movement of the vehicle (real wind angle).
As it can be seen, the apparent speed of the wind is the vector addition of the two other speeds.
Therefore [46]:
(1)
𝛽 = {
𝑎𝑟𝑐𝑡𝑔𝑉𝑇𝑠𝑖𝑛𝜙
𝑉𝑇𝑐𝑜𝑠𝜙+𝑉𝐿 𝑖𝑓 𝛽 ≤ 𝜋/2
𝑎𝑟𝑐𝑡𝑔𝑉𝑇 sin(𝜙−
𝜋
2)−𝑉𝐿
𝑉𝑇cos (𝜙−𝜋/2)+
𝜋
2 𝑖𝑓 𝛽 > 𝜋/2
(2)
It is the actual flow of air acting on the sail, and that is the reason why it is possible to propel a land
yacht (or other sailing vehicle) at a higher speed than the real wind speed.
The force of the wind acts on the sail, which then transmits the loads to the tripod and impels the vehicle
forward. 𝑉𝐴 is the most important speed to consider both for the determination of the
behavior/performance of the vehicle and of the loads acting on the structure, since it is its intensity and
direction that determine the forces on the sail.
The overall force resulting from 𝑉𝐴, which is a consequence of the pressure differences between each
side of the sail, is designated aerodynamic force 𝐹𝐴 and is generally vectorially decomposed into the
aerodynamic drag force 𝐹𝐷 (which is the component along the direction of 𝑉𝐴) and aerodynamic lift force
𝐹𝐿 (normal to it). Both components have influence on the propulsion (being the relative importance
dependent on the angle between the sail and the apparent wind, the attack angle α) since both have
components along the direction of movement of the vehicle. The forces are considered to be exerted on
the center of pressure, which is the point on the sail where the total sum of the pressure acts (the real
distribution of forces on the sail is equivalent to having the forces applied to this point).
In addition to the attack angle, the force on the sail (and the mentioned components) are dependent of
the apparent wind and the type and dimension of the sail. The resultant along the direction of movement
of the land yacht is the aerodynamic drive FT, and perpendicularly to it acts a lateral force in the vehicle,
FS (which in some circumstances may cause instability on the vehicle) [47], [48]. Figure 29 illustrates
this (R, RD and RS are the reactions opposing FA, FT and FS).
47
Figure 29 – Forces acting on the sail of the vehicle [46]
In order to get the necessary results, it is important to have available the loads in all three directions
(i.e.,𝐹𝑥, 𝐹𝑦 and 𝐹𝑧) as well as the torque components (𝑀𝑥, 𝑀𝑦, 𝑀𝑧) that are transmitted to the structure,
calculated from the previously discussed forces on the sail.
The lift and drag forces acting on the wing can be calculated from the following equations [46], [49]:
𝐹𝐿 = 1
2𝜌𝑎𝑖𝑟𝑉𝐴
2𝐶𝐿𝐴𝑊 (3)
𝐹𝐷 = 1
2𝜌𝑎𝑖𝑟𝑉𝐴
2𝐶𝐷𝐴𝑊 (4)
ρ𝑎𝑖𝑟 is the density of the air (1.22 kg/𝑚3 at 20ºC and 1atm), 𝐴𝑤 is the effective area of the sail (i.e. the
projected area, which changes with the attack angle imposed by the user, being the maximum the actual
area of the sail, 4,1 𝑚2), 𝐶𝐿 is the lift coefficient, 𝐶𝐷 the drag coefficient. The lift coefficient can be
assumed to be approximately a function of only the apparent wind angle. Both coefficients can be
determined with certainty experimentally, when the values of the forces are not known (which is the
case, since the goal is to calculate them).
In the present situation, the values of 𝐶𝐷 and 𝐶𝐿 for different attack angles can be obtained by considering
results in the literature for the NACA 0015 profile [45]. For α bigger than 15º, it is considered that
𝐶𝐷 reaches a limit value of 0.2. The values for both coefficients are shown in table 5:
α 𝑪𝑳 𝑪𝑫
0º -0.0044 0.0101
5º 0.5438 0.0141
10º 0.9067 0.0270
15º 0.4365 0.2
Table 5 – Values of the lift and drag coefficients for different α angles
However, it can be done for 𝐶𝐿 a theoretical calculation, which was used to obtain the values actually
applied [50], [51]:
48
𝐶𝐿 = 2𝜋𝑠𝑒𝑛(𝛼+2
ℎ
𝑐)
1+2
𝐴𝑅
(5)
AR = 𝑏2
𝐴𝑝 =
𝑏
𝑐̅ (6)
Here c is the root chord, c̅ is the mean chord, b is the wingspan, h is thickness, AR the aspect ratio
and Ap is the wing area. Having determined the values of the lift and drag coefficients, it is
straightforward to calculate the respective forces by using equations (3) and (4) and then calculate the
average pressure on the wing. The results show 𝐹𝐿 has a far bigger magnitude, and for that reason 𝐹𝐷
was neglected.
In a first approximation, it was only considered the situation in which the sail is in such a position relative
to the land yacht that only forces on the direction of the movement appear, i.e. angle θ of figure 29 is
equal to 90º.
It was conducted a FEM analysis [28] of the selected wing (for which the geometry is fully known),
applying the value of the pressure that was obtained as discussed in the previous sections and the
boundary conditions that are a consequence of the connection though the mast and the rope. The attack
angle was of 20º, value for which a stationary regime begins. It was used an apparent wind speed of
14,9m/s and an average lift pressure of 294Pa. The forces and moments that result from this analysis
are those in the table 6 [28].
𝐅𝐱 (N) 𝐅𝐲 (N) 𝐅𝐳 (N) 𝐌𝐱 (Nm) 𝐌𝐲 (Nm) 𝐌𝐳 (Nm)
Base of
the mast
-485.3 -808.1 -242.7 1909.5 -86.7 450.9
Point of
connection
to the rope
485.3
-404.4
242.7
0
0
0
Table 6 – Forces and moments on the connections between the sail and the chassis for α=20º
4.3.3 Resulting forces on the structure Having in mind the use of the same wing and identical wind conditions as in the reference [28], the same
forces are transmitted to the chassis. These forces (which are what really matters in terms of
dimensioning the vehicle) are dependent on the previous values of the forces on the mast and on the
rope, but are also a function of the angle β (the angle between the direction the vehicle is moving and
the direction of the apparent wind). When this angle is null, the sail is in a position that results in a
maximum forward force and no lateral force.
49
As the angle increases, the lateral component of the force increases and the frontal component
decreases. For this reason it is important to keep the angle below values that would destabilize the
vehicle, reduce its speed and even could make it rollover. There were made analyses for 3 values of β:
0º, 20º and 40º. In each case it is necessary to transpose the forces to the kart’s referential, using a
coordinate transformation. For each situation the results can be seen in table 7 [28].
β 𝑭𝒙 𝑭𝒚 𝑭𝒛 𝑴𝒙 𝑴𝒚 𝑴𝒛
Mast 0 -808.1 485.3 -242.7 -86.7 -1904.5 450.9
Rope 0 -404.4 -485.3 -757.4 0 0 0
Mast 20 -925.3 179.7 -242.7 569.9 -1819.2 450.9
Rope 20 -214.0 -594.3 -757.4 0 0 0
Mast 40 -930.9 -147.7 -242.7 1157.8 -1514.7 450.9
Rope 40 0 -631.7 -757.4 0 0 0
Table 7 – Forces and moments in the kart’s referential for different values of β, for α=20º
Obviously, in static conditions, the rope only transmits tensile forces. Its negative values are due to the
orientation of the referential.
Although the values of the reactions caused by the wind were directly taken from a previous work [28],
the discussion done in the last few sections is essential to understand the phenomena that originate
them. With this data, it is now possible to conduct the analyses on the modified structure that is
considered in the present thesis.
4.3.4 Rollover
It is crucial for practical and safety reasons to know for which conditions the land yacht may rollover on
its side. As it was denoted earlier (figure 29), the aerodynamic forces on these vehicles include a lateral
component FS, perpendicular to the direction of movement of the vehicle, which may cause the instability
of the vehicle. Bearing in mind the structure of the land yacht (two rear wheels and a front wheel), let’s
consider an axis A passing through the center of the front one and the center of a back wheel. In order
to avoid the rollover, it is necessary that the lateral force does not overcome the weight of the system,
causing the vehicle to turn on its side with a movement in turn of the mentioned A axis. FT was not taken
into account because it is in the direction of movement of the vehicle, and for that reason it does not
contribute to the moment along axis A (
In this case, and contrarily to what is done in all other situations in this work, the weight of the rider is
not assumed to be the maximum value of 120kg because it would not be associated with the worst case
scenario. A higher mass makes it harder for a rollover of the vehicle to happen: the situation that favors
this is the minimum. Since the designed land sailing kart is not suitable for children (given its
dimensions), it is going to be established a value of 50kg as the lowest expected weight of an occupant.
50
First, it is necessary to calculate the center of gravity of the entire system, i.e., the kart with the sail and
the electric components in place, plus the rider. Table 8 has the information of the location of the center
of gravity of each of the main constituents of the system (elements like the controller of the cables were
neglected since their mass is not relevant compared to the rest).
Chassis Occupant Wing Battery Motor
x (m) 1.24 1.37 1.1 1.17 0
y (m) 0.11 0.32 1.6 0.07 0
z (m) 0 0 0 0 0
Weight (kg) 12.2 50 17 4 1
Table 8 – Necessary information to calculate the center of mass of the vehicle with an occupant
The data concerning the chassis was directly obtained from the computer program Siemens NX’s
assembly model, after introducing the materials in the final model. The considered coordinate system
xyz has its origin located between the two supports of the front wheel. The total weight of the system
considering a 50kg user is 84.2kg. To determine the coordinates of the center of gravity of the global
system, it is used the following equation [52]:
𝑟𝐶𝐺 = ∑ 𝑟𝑖𝑚𝑖𝑖
∑ 𝑚𝑖𝑖, (7)
Here 𝑚𝑖 is the mass of the ‘i’ element and 𝑟𝑖 its x, y or z coordinate. Therefore, we have:
{𝑥𝐶𝑀 =
1.24×12.2 + 1.37×50 + 1.1×17 + 1.17×4 + 0×1
12.2+50+17+4+1=
107
84.2= 1.27 𝑚
𝑦𝐶𝑀 = 0.53 m𝑧𝐶𝑀 = 0 m
(8)
Assuming just the lateral wind force and the weight of the system as relevant in generating moment
along axis A (figure 30):
∑ 𝑀𝐴 = FS x 𝐷𝑊 – P x 𝐷𝑃 (9)
𝐷𝑝 is the shortest distance between the C.G. and axis A and 𝐷𝑊 the one between the virtual point of
application of FS and the same axis. These dimensions are influenced by the length/width ratio (L/W) of
the vehicle. Equation (9) is an approximation because it does not take into consideration the influence
of inertia, since it would vary with the mass of the passenger and the speed of the vehicle. In figure 30
is showed the location of the C.G in relation to the origin of the referential.
51
Figure 30 – Location of the C.G in relation to the origin of the referential
The rollover is imminent when ∑ 𝑀𝐴 = 0. To calculate the critical value of FS we have:
FS x 𝐷𝑊 = P x 𝐷𝑃 FS = P x 𝐷𝑃
𝐷𝑊 (10)
This value of force that makes rollover imminent will occur under diverse wind conditions for different
sail angles, not to mention the influence of the rider’s weight. Using the 3D CAD model of the vehicle is
straightforward to determine (drawing a line between the C.G. and the axis and checking the smallest
distance) that 𝐷𝑃= 0,334m and 𝐷𝑤= 1,5m. Substituting, we obtained the maximum value of FS under
these conditions:
FS = = P x 𝐷𝑃
𝐷𝑊 =
84.2x 9.8 x 0.334
1.5 = 184 N (11)
4.4 Materials used in the chassis
In order to maintain the general tendency of this project to have a focus on the structural aspects of the
integration of an electric system in a land yacht, instead of conducting a detailed comparative analysis
from scratch concerning the types of materials, with the goal of selecting the ideal kind to use in the
chassis of the vehicle, are going to be used (at least in this first approach) the materials defined in [28].
However, in order to assure those materials meet the necessary requirements of this project, it is going
to be conducted a critical review of the choices made. These are: 304 stainless steel for the fork and
6061-T6 aluminum for all the others structural elements listed in section 3.3. Table 9 shows the most
relevant characteristics of these materials for the project [53], [54].
52
Properties 6061-T6 aluminum 304 stainless steel
Modulus of Elasticity 70 GPa 193 GPa
Tensile Yield Strength 276 MPa 215 MPa
Ultimate tensile Strength 310 MPa 505 MPa
Fatigue Strength (at 5x108 cycles)
96,5 MPa Assumed infinite
Endurance Limit [40] - 252 MPa
Density 2700 kg/𝑚3 7900 kg/𝑚3
Table 9 – Most relevant properties of the materials used in the chassis
A characteristic that must be met by any material used in this product is a high resistance to corrosion,
since it must be possible to use the vehicle in rough environments from this viewpoint (like the beach,
where is expected the vehicle is going to be subjected to salt water splatters, for example). Besides this,
the materials of the structure obviously must be able to support the loads and provide enough rigidity. It
is also a key factor to maintain the weight low. Finally, the materials should have good machinability and
weldability, factors that are very important in the production of the vehicle.
Examining the properties of the materials, it is possible to say it is expected they meet the needed
characteristics (although the rigidity and strength will only be verified in the analyses to come). Their
resistance to corrosion is acceptable for the environments anticipated to be faced, and even though
aluminum is not as easy to weld as steel, it is still viable to do it (the previously discussed advantages
outweigh this drawback).
4.5 Factor of safety
Before performing the different analyses, it is necessary to establish the factor of safety for the project,
n=𝜎𝑦𝑖𝑒𝑙𝑑
𝜎𝑉𝑀, according to good engineering practices to deal with uncertainty in the forces, state of the
material and other unknown conditions. Knowing the material properties, fixing n allows to define the
maximum admissible value of tension along the structure: this limit cannot be exceeded in any of the
FEM simulations to be conducted (if this happens either the model or the materials must be altered).
There are several ways for determining it, one of the most commons being Pugsley’s method [55], which
is the one being used in this project. The value of n is given by:
n = 𝑛𝑠𝑥𝑛𝑠𝑦 (12)
Being 𝑛𝑠𝑥 a factor that translates the level of confidence on the accuracy of the values of the material
characteristics and of the stresses in the structure and 𝑛𝑠𝑦 is related with the level of danger failure could
represent to people or property. Following [55], we have n = 1,5 x 1,4 = 2,1 (which is also a common
value in mechanical construction under dynamic loads).
53
On figure 31 [56] it can be better seen the meaning of the factor of safety. In a given situation, it is
expected the maximum stress not to surpass a determined value, however, there is a probability it will
go beyond it. Similarly, the resistance of the material used can, due to several factors, be inferior to the
value assigned to it (in this case there is also a distribution of probability for this to occur, which gets
lower to bigger deviations from the expected figure). When these two deviations over cross, and
therefore the load reaches the value of the resistance of the material, failure happens.
Figure 31 – Failure probability due to variations in the stress and resistance values (adapted from [56])
By choosing a higher value for the factor of safety, the curves of figure 31 are brought further apart and
the chances of failure occurring get smaller.
4.6 Static Analyses of the Chassis
Having modeled the chassis and defined the loads it has to withstand, as well as the materials, the next
step is to conduct an analysis that allows to determine the deformations and tensions on the structure.
For this goal, as well as in all other studies in this document that require a computational approach, it
was used the commercial finite elements method (FEM) software Siemens NX [57].
In this section only the chassis is under examination, so the simulation was run without the wheels or
the elements that connect each part (e.g. bolts). The analysis utilizes beam elements to simulate the
structure (with the actual tubular cross section), in which each element of the chassis is assumed to the
rigidly connected to the ones in its vicinity with every movement constrained in the connection. The main
advantage of this approach is the great reduction of the number of elements necessary to be able to
achieve acceptable results, when comparing with the use for example of a 3D-mesh on the modeled
assembly, and consequently a reduction of the duration of the analysis. When using beam elements, is
necessary to allocate a large number of them to the parts with significant curvature, in order to replicate
54
its geometry better (since each element is necessarily straight). To each portion of the mesh was
allocated the respective section geometry and the materials as discussed in section 4.4
Two situations have to be investigated: when the vehicle has all three wheels on the ground
(corresponding to normal operation) and the situation where one of the back wheels is hanging in the
air with the other two on the floor (a situation that is common in this type of vehicles and for that reason
must also be checked). On both cases the forces applied were the weight of the passenger and the
previously mentioned reactions of the sail in the structure. Only the maximum values of each were
considered (and that was done simultaneously), since this is the most critical situation possible.
This study is also necessary in order to obtain the loads to use in the verification of the bolts used in the
connections and of the weld bead.
It’s important to notice that in analysis no loads on the x direction are considered as they result from the
sail and are transformed into movement and therefore do not influence the deformation of the structure.
This would not be the case when braking, however this is a separate situation that was not addressed
in this work. On the other hand, in the boundary conditions, the movements along x were constrained in
the back wheels in order for the problem to be more realistic (leaving the front wheel unrestrained in x
allows the vehicle to deform in that direction).
The 120kg of the passenger is assumed to correspond to a force of 1200N. In this simulation it is divided
into 3 equal forces of 400N applied as showed in figure 32. The rope is connected to the outside of the
tripod, 20 cm below its top section. Although its magnitude is much smaller, the weight of the battery
was also applied in the tripod (the location of the battery in the winning concept). It weights 4kg so it
was assumed it would be adequate to apply 10N on each of the 4 supports of the battery.
Figure 32 – Placement of the loads resulting from the passenger’s weight
55
4.6.1 All wheels on the ground
The boundary conditions chosen for this first case were: fixing every degree of freedom on one of the
rear wheel hubs while allowing movement just on the y direction (lateral movement) on the other and,
on the front, restricting every degree of freedom except the linear movement in the x direction. As
mentioned before, there were employed beam elements: the mesh selected was of 6 elements in each
of the straight segments, while for the bended region of the tripod were used 20 elements (since these
type of regions require a higher number of elements in order to be obtained satisfying results).
There were defined tubular cross sections for the model, and from the library of materials of the program
were chosen “AISI_SS_304” for the fork and “Aluminum_6061” for the elements in other locations. It
was done a research of the aluminum tubes available in the market. The tubes will be bought from the
company Alu-Stock, which has available the profiles in reference [58], having been chosen tubes with
60x40 dimensions for the tripod and the front and rear wheel connection tubes.
After repairing the mesh (i.e. correcting some errors in it) and setting the geometric and material
properties as well as the boundary conditions, for each situation where applied the discussed loads in
the appropriate locations (e.g. the forces of table 7 in the mast) and a static analysis was run. The
software gives back to the user graphic information on several parameters (e.g. stress, displacement)
which allow to identify the areas where they are more severe, and also allows to retrieve information on
specific points. The simulation in the used software displays the centerline of the elements, not the
actual section. The results for this geometry and β=0, which is one of the situations defined previously
(see table 7), are shown in figures 33 and 34.
Figure 33 – Von Mises stress distribution on the chassis for β=0
56
Figure 34 – Nodal displacement for β=0
From figures 33 and 34 is possible to verify that the maximum stress is around 112 MPa and the
maximum displacement just above 4mm (the showed deformation of the centerlines does not
correspond to the real deformation, it is magnified in order to have a clearer notion of how it occurs
qualitatively). These are satisfying results, since the magnitude of the displacement does not interfere
with a safe utilization of the vehicle and the stress is below the admissible limit.
In tables 10 to 13 is presented the key information extracted from the analyses done for each load
situation. Besides the overall maximum values of displacement and stress for each of the β angles
considered, it was also retrieved the values of tension at the weld and of the forces and moments at
each of the bolted joints, which will be necessary in upcoming sections. To obtain this data, the
procedure was repeated, only changing the loads according to the case in analysis (constrains,
boundary conditions, mesh, materials, etc. were all maintained unaltered).
β=0º β=20º β=40º
Maximum
Displacement (mm)
4.2 4.7 6
Maximum Rotation (º) 1.2 1.2 1.5
Maximum Von Mises
Stress (MPa)
112.1
107.1
97.4
Table 10 – Important maximum values occurring in the chassis elements
57
β=0º β=20º β=40º
Stress at the weld
(tripod)
112.1 107.1 89.2
Table 11 – Von Mises stress at the location of the weld for the different load situations
Bolted Joint 1
β=0º β=20º β=40º
𝐅𝐱 (N) 527 -474 -565
𝐐𝐱𝐲 (N) 1505 1546 2018
𝐐𝐱𝐳 (N) 222 604 689
𝐌𝐱 (Nm) -26 -72 -102
𝐌𝐲 (Nm) -271 -337 -438
𝐌𝐳 (Nm) -15 -127 -161
Bolted Joint 2
β=0º β=20º β=40º
𝐅𝐱 (N) 75 73 55
𝐐𝐱𝐲 (N) 736 710 538
𝐐𝐱𝐳 (N) 558 -90 33
𝐌𝐱 (Nm) 24 40 65
𝐌𝐲 (Nm) -416 -428 -302
𝐌𝐳 (Nm) 122 52 -12
Table 12 – Forces and torques at the locations of the bolted joints
4.6.2 One rear wheel on the air
The same analysis and gathering of data was done for the situation where one of the back wheels lifts
from the ground. In this case, one of these wheels is left unconstrained to replicate the scenario while
the other is constrained in all degrees of freedom and the front wheel is kept as is was in the previous
analysis. Every other input (materials, mesh, etc.) was kept the same, and an equal procedure was
taken, i.e. it was once again conducted a static analysis of the chassis.
The representation of the distribution of the stresses and of the displacements can be seen on figures
35 and 36, while in the tables 13 to 15 is once again presented the most important information.
58
Figure 35 – Von Mises stress distribution on the chassis for β=0 (one back wheel on the ground)
Figure 36 – Nodal displacement for β=0 (one back wheel on the ground)
β=0º β=20º β=40º
Maximum
Displacement (mm)
16.9 22 28.1
Maximum Rotation (º) 1.8 2.3 3.1
Maximum Von Mises
Stress (MPa)
112.1
107.1
110
Table 13 – Important maximum values occurring in the chassis elements
59
β=0º β=20º β=40º
Stress at the weld
(tripod) (MPa)
112.1 107.1 89
Table 14 – Von Mises stress at the location of the weld for the different load situations
Bolted Joint 1
β=0º β=20º β=40º
𝐅𝐱 (N) -876 -1046 -127
𝐐𝐱𝐲 (N) 2794 2740 529
𝐐𝐱𝐳 (N) -619 -431 1224
𝐌𝐱 (Nm) -552 -745 -141
𝐌𝐲 (Nm) 81 303 507
𝐌𝐳 (Nm) -342 -505 -278
Bolted Joint 2
β=0º β=20º β=40º
𝐅𝐱 (N) 76 70 54
𝐐𝐱𝐲 (N) 741 620 2595
𝐐𝐱𝐳 (N) -212 -50 31
𝐌𝐱 (Nm) 81.2 116 71
𝐌𝐲 (Nm) -427 -397 -297
𝐌𝐳 (Nm) 115 29 -11
Table 15 – Forces and torques at the locations of the bolted joints
In this analysis, the chassis is subjected to a somewhat similar stress distribution when compared to the
all wheels on the ground situation, with the highest value in the same location (the major difference is
the higher stress in the rear connecting bar). The displacement in the chassis, however, has increased
significantly, which is a consequence of a less restricted structure. As it would be expected, the
maximum displacement occurs next to the unrestrained wheel.
60
4.6.3 Conclusions
From the information above is possible to realize that the highest forces on the chassis occur in the
region of the weld of the tripod, except when β=40º. In this case, they occur in one of the rear connection
bars. Also, in this situation the main deformation of the tripod does not occur in the x direction as in the
previous cases, instead happening a bending to the side (y direction), which can be seen in figure 37.
This is mainly due to the higher bending moment applied around the x axis.
Figure 37 – Deformation for β=40º (seen from the front)
The analyses had the goal of determining whether the chassis of the vehicle would be submitted,
through the course of its expected normal use, to loads or deformations that could endanger the integrity
of the vehicle and the safety of the rider. It was possible to verify that this is not the case, since the
stresses in all cases studied were below the defined limit (the factor of safety of the project was met in
all situations) and the deformations were not very accentuate. The stress around the location of the weld
(figure 40) decreases with the angle β, and for that reason this is only the location of the highest stress
for β=0 and β=20.
It was also done an analysis of convergence of the solution, increasing the number of elements in just
the curved portion of the tripod (since the straight tubes are correctly represented by few elements, even
just one). The results obtained were just slightly different from the previous ones, both in terms of
stresses and displacements. This showed that the original analyses had been done with sufficient
elements. In figure 38 is showed the mesh used in this refinement.
Figure 38 – Refined mesh used in the convergence analysis (detail of the bended part of the tripod)
61
4.7 Vibration Analysis
4.7.1 Normal Modes
The goal of the present study is to determine the natural frequencies of the structure. Contrarily to the
previous static analyses, in this one are considered inertia forces due to the movement of mass.
A plausible way for this type of structure to be subjected to problematic oscillating forces that could
cause it to be in a resonating state is when the vehicle moves in a terrain with regular/irregular
discontinuities (e.g. cobblestone roads).
To determine the wanted values, it was first added the mass of the passenger, of the sail and electric
equipment to the structure (all in form of a concentrated mass placed in the respective location), since
this may dramatically influence the results. The weight of the passenger may vary, so analyses were
made for different values. The mass of the battery (4kg) and of the sail (17kg) [28] are the remaining
relevant loads applied. It was also possible to verify that the difference in having the electric system or
not (maintaining all other masses and conditions unaltered) is very small, mainly in the first few natural
frequencies. There were used the same boundary conditions as in the first set of static analyses (all
wheel on the ground), i.e., fixing every degree of freedom on one of the rear wheel and allowing
movement just on the y direction on the other and, on the front, restricting every degree of freedom
except the linear movement in the x direction. The results obtained (the natural frequencies, in Hz) are
presented in table 16 for the first five normal modes, while figure 39 show the deformation in the two
first modes:
120kg 85kg 50kg
1st 1.41 1.66 1.8
2nd 1.98 2.27 2.62
3rd 4.09 4.07 4.35
4th 5.1 5.03 5.84
5th 8.15 8.21 8.22
Table 16 – First five resonance frequencies of the chassis for different passenger’s weights
Figure 39 – First (left) and second normal modes (right): qualitative deformations
62
The natural frequencies found, namely the first two, are in a range in which is conceivable the vehicle
could be exposed to vibrations of that magnitude. However, it is still very unlikely this behavior becomes
a concern for the structure’s integrity since the exterior loads come from the terrain and a slight deviation
from the problematic conditions would rapidly end the issue. Furthermore, it was not considered the
damping the wheels provide to the system: these elements dissipate energy and thus significantly
mitigate potential resonance of the chassis.
Looking at the results is possible to state the variation of the weight of the person in the vehicle has an
impact on the resonance frequencies (they decrease as the weight increases). Keeping in mind the
weight can vary continuously between 50 and 120kg, there is an infinity of values possible for the natural
frequencies of the system. However, under this analysis, the range of possible frequencies for each of
the modes has its inferior and superior limits in the values for a passenger of 120kg and 50kg that are
present in table 16. It was also possible to verify the change in mass does not affect the normal modes.
It should be noted, however, that the values obtained for the first natural frequencies were considerately
lower than those expected for this situation (which was for them to be around 25Hz). This is therefore a
topic that could be address in future experimental works, preferably including the wheels in the system
to analyze.
4.8 Weld Beads Although weld beads are used in some other areas in the structure, section 4.6 showed that by far the
most critical one in terms of the tension it is subjected to is the one located in the highlighted area in
figure 40.
Figure 40 – Location of the critical weld bead
Both tubes to join have 60x40 diameters and are made of 6061 aluminum. The horizontal tube is the
one cut to make it fit and the weld is done all around.
63
The highest stress value in the region occurs for β=0º and is 112.1 MPa, as seen on the static analyses
of the chassis presented above (Von Mises equivalent stress, which is directly used in the calculations).
Considering this information, we have (assuming the same resistance for the welds):
n = 𝜎𝑦
𝜎𝑉𝑀 = 276/112.1 = 2.46
It is good practice to avoid having welds in the regions where the stresses are higher, contrarily to what
we have in this particular case. However, the factor of safety of the bead is high enough to guarantee
its integrity. Nonetheless, it is recommended particular attention to this feature in the manufacturing
stage because of its vulnerable location.
The main reason for the use of this simplified approach is that the weld is subjected to intricate combined
loads (including torsion, bending and axial loads) which make it more difficult to analyze. It is also
important to notice that the chassis is in fact subjected to dynamic loads that induce fatigue damage
after a high number of cycles, which can affect the weld beam.
For the reasons mentioned above, it is clear that the approach taken in this section of the preliminary
design done in the present work may be too simplistic, therefore it is advisable to do the analysis in
further detail in future studies that should precede the construction of a prototype.
4.9 Bolted Joints
There are two types of bolted joints to be studied: the ones connecting the tripod to the tubes that support
the rear wheels (there are two of these that are equal) and the one linking the central element to the
front wheel connection tube. They can be seen in figure 41.
Figure 41 – Location of the bolted joints
The former is constituted by 4 steel bolts and the latter by 2: their characteristics are presented in table
17, along with the used nuts and washers [60]:
64
Standard Class Total Length
(mm)
Number of
elements used
Bolts ISO 4017 M10 35 10
Nuts ISO 4032 M10 10 10
Washers ISO 7089 M10 2 20
Table 17 – Data of the used connection elements
Both joint types 1 and 2 use the elements of the previous table. Considering the thickness of the flanges
to connect (each being 16mm in total in every situation) and the fact that 2 washers of 2 mm each are
used in every joint, the connection lengths are given by:
𝑙1 = 𝑙2 = 16+2+2 = 20 mm
Considering the data obtained in the static analyses of the chassis, is possible to verify that all bolted
joints in the structure are subjected to normal loads and sheer loads (as mentioned in the respective
section these loads were obtained using the post analysis capacities of the Siemens NX software).
Following once again the methods of reference [55], we have that the tensile stress 𝜎𝑏 in a bolt is given
by:
𝜎𝑏= 𝐹𝑏
𝐴𝑡 =
𝐹𝑖+𝐶𝐹
𝐴𝑡, (13)
Here 𝐴𝑡 is the tensile stress area of the bolt, 𝐹𝑏 is the resultant load on the bolt, C is the stiffness constant
of the joint, F is the external separation load and 𝐹𝑖 the applied pre-load. C is given by:
C = 𝑘𝑏
𝑘𝑏+𝑘𝑚, (14)
In equation (14), 𝑘𝑏 is the effective stiffness of the bolt in the clamped zone and 𝑘𝑚 the stiffness of the
members of the connection in that same zone. These constants are calculated using the following
equations [55]:
𝑘𝑏 = 𝐴𝑑 𝐴𝑡𝐸𝑏
𝐴𝑑𝑙𝑡+𝐴𝑡𝑙𝑑 (15)
𝑘𝑚 = A𝐸𝑚d𝑒𝐵𝑑
𝑙 (16)
In the equations above 𝐸𝑏 , 𝐸𝑚 are the respective Young’s modulus, 𝐴𝑑 is the bolt major diameter area,
𝑙𝑑 is the length of the unthreaded portion in grip, 𝑙𝑡 the length of the threated portion in grip, l the total
length, A and B are constants.
Table 18 – Constants A and B for different materials [55]
65
To perform the analyses is necessary to determine the forces occurring in the bolts and for that are
going to be used the values presented in section 4.6. In each case are being considered only the
maximum moments and forces in every direction. Therefore, the considered combined loads are
conservative. It is also considered that the loads go through the centroid of the connection.
Bolted Joint 1 (back) Bolted Joint 2 (front)
𝐅𝐱 (N) 527 76
𝐐𝐱𝐲 (N) 2794 2595
𝐐𝐱𝐳 (N) 1224 558
𝐌𝐱 (Nm) 745 116
𝐌𝐲 (Nm) 507 428
𝐌𝐳 (Nm) 505 122
Table 19 – Maximum loads acting on joints 1 and 2
The axial force on the bolts depends on their location due to the existence of the moments 𝑀𝑦 and 𝑀𝑧.
These moments have as a consequence for half of the flange to tend to rotate away from the other half
about an axis (hinge line). This is illustrated in figures 42 and 43 for joint 1 but the it is also applicable
to joint 2.
Figure 42 – Bending moments cause different axial loads according to the position of the bolt
Figure 43 – Rotation of half of the flange around the hinge line
66
In the situation of figure 42, a moment along y would exert the same axial stress in bolts 1/2 and another
in 3/4, while a moment along z would do it in 1/3 and 2/4. To make sure the joint is not vulnerable, it
would only be needed to verify the bolt under the highest load, which is designated the critical bolt. The
direction of the moment determines which of the bolts will be more affected: as an example, in figure 43
the bolts in the upper part of the joint have to support greater stresses because there is a moment
applied in the plane of the sheet of paper that points inwards. Therefore, to determine where the hinge
line is located in a certain situation it is required to observe the direction of My/Mz.
To determine which one of the bolts in the joint is the critical one, it may be necessary to calculate the
loads caused in each bolt by each moment, and sum them. However, in the situation of joint 1 is sufficient
to notice that attending to the directions of the moments applied, bolts 1 and 2 are the ones the furthest
away from the hinge line when considering My (thus the ones under higher loads) and 1 and 3 when
considering Mz. Therefore, bolt 1 must the critical one.
For both types of joints the axial and shear loads are going to be analyzed separately.
Joint 1
The load on each bolt caused by the moments is given by the following equations (𝑟𝑖 is the distance
between the bolt i and the hinge line in the respective situation):
𝐹𝑀𝑦 =
𝑀𝑦𝑟𝑖
∑𝑟𝑖2 ; 𝐹𝑀𝑧
= 𝑀𝑧𝑟𝑖
∑𝑟𝑖2 (17)
This equation is also valid to calculate the shear force caused by the torque, in which case 𝑟𝑖 is the
distance between the centroid and the bolt.
The shear force originated by Qxy and Qxz on each bolt is:
𝐹𝑧’ = 𝑄𝑥𝑦
𝑛 =
2794
4 = 699 N (18)
𝐹𝑦’ = 𝑄𝑥𝑧
𝑛 =
1224
4 = 306 N (19)
On the other hand, Mx causes a shear force on each bolt that is equal in magnitude but changes in
direction, as illustrated in figure 44. This means that the sum of the vectors of the shear forces varies
from bolt to bolt. The force caused by Mx is:
𝐹𝑀𝑥 =
𝑀𝑥𝑟𝑖
∑𝑟𝑖2 =
𝑀𝑥
4𝑟 =
745
4.0,0407 = 4576 N (20)
In order to obtain the critical one regarding shear, was used the parallelogram rule. It was found the
critical bolt is number 1, and the total force is 𝐹𝑠1= 5337 N.
67
Figure 44 – Shear forces in joint 1
The shear stress on the critical bolt is given by:
𝜏𝑠1= 𝐹𝑠1
𝐴𝑠 =
5337
58 = 92 MPa (21)
The shear area 𝐴𝑠 of the bolt is equal to 𝐴𝑡 (58𝑚𝑚2). The shear strength is obtained considering [55]:
𝑆𝑥𝑦= 0.577 𝑆𝑦 = 0.577 x 640 = 369 MPa (22)
𝑛𝑠1 = 𝑆𝑥𝑦
𝜏𝑠1 =
369
92 = 4 (23)
Bearing in mind the location of the bolts in relation to the hinge lines in each case and the way in which
the respective moment tends to rotate the flange, and substituting in equations (17), we have for bolt 1
of joint 1:
𝐹𝑀𝑦1 =
𝑀𝑦𝑟𝑖
∑𝑟𝑖2 =
507 x 0,085
0,0852+0,0852+0,0132+0,0132 = 43,095
0,0148 = 2914 N (24)
𝐹𝑀𝑧1 =
𝑀𝑧𝑟𝑖
∑𝑟𝑖2 =
505 x 0,049
0,0492+0,0492+0,0112+0,0112 = 24,745
0,005 = 4906 N (25)
Assuming that Fx is distributed equally by each bolt, its contribution to the load in bolt 1 is:
𝐹𝑥1= 527/4 = 132 N (26)
The total axial force in the critical bolt of the joint is therefore:
𝐹𝑡1 = 𝐹𝑀𝑦1+ 𝐹𝑀𝑥1
+ 𝐹𝑥1 = 7952 N (27)
M10 steel bolts are used, so A=0.787, B=0.628, 𝐴𝑡1= 58𝑚𝑚2, 𝑙𝑑 = 4mm, 𝑙𝑡 = 16mm, 𝐴𝑑1= 78,5 𝑚𝑚2 .
We have:
𝑘𝑏1 = 78,5.10−6x 58.10−6 x 207.109
78.5.10−6 x 0,016+58.10−6 x 0,004 = 6.33 x 108 N/m (28)
68
𝑘𝑚1 = 0,787x 207. 109x 0.01 x 𝑒0.628𝑥10
20 = 2.23 x 109 N/m (29)
C = 𝑘𝑏
𝑘𝑏+𝑘𝑚 = 0.221 (30)
The recommended pre-load for a reusable connection is given by the following equation (𝑆𝑝 is the proof
strength):
𝐹𝑖 = 0.75 x 𝐴𝑡 x 𝑆𝑝 (31)
Consulting the appropriate data tables [55], we see that for the used bolts 𝑆𝑝 = 580 MPa
𝐹𝑖 = 0.75 x 58. 10−6 x 580.106 = 25230N (32)
The factor of safety to axial stress for the critical bolt is therefore:
𝑛𝑏 = 𝐴𝑡𝑆𝑝−𝐹𝑖
𝐶𝐹𝑡1
= 8410/1757 = 4.79 (33)
Joint 2
Although this joint is geometrically different and only has 2 bolts, the same approach used for joint 1
applies. In this case, the distance between the centroid and each bolt is 41mm.
Figure 45 – Shear forces in joint 2
Repeating the same procedure used for joint 1, we have:
𝐹𝑧’ = 𝑄𝑥𝑦
𝑛 =
2595
2 = 1298 N (34)
𝐹𝑦’ = 𝑄𝑥𝑧
𝑛 =
558
2 = 279 N (35)
69
𝐹𝑀𝑥 =
𝑀𝑥𝑟𝑖
∑𝑟𝑖2 =
𝑀𝑥
2𝑟 =
116
2.0,041 = 1415 N (36)
Once again, using the parallelogram rule to determine the total load each bolt has to withstand, was
determined that the critical bolt is subjected to 𝐹𝑠2= 2134 N
𝜏𝑠2= 𝐹𝑠2
𝐴𝑠 =
2134
58 = 36.8 MPa (37)
𝑛𝑠2 = 𝑆𝑥𝑦
𝜏𝑠2 =
369
36,8 = 10 (38)
As for the axial loads, the force in the critical bolt may be obtained by:
𝐹𝑀𝑦2 =
𝑀𝑦𝑟𝑖
∑𝑟𝑖2 =
428 x 0,03
0,032+0,032 = 12,8
0,0018 = 7111 N (39)
𝐹𝑀𝑧2 =
𝑀𝑧𝑟𝑖
∑𝑟𝑖2 =
122 x 0,094
0,0942+0,0122 = 11,47
0,009 = 1274 N (40)
The influence of Fx may be ignored since it is of much smaller magnitude.The total axial force in the
critical bolt of the joint is therefore:
𝐹𝑡2 = 𝐹𝑀𝑦2+ 𝐹𝑀𝑥2
= 8385 N (41)
Having in mind that the same type of bolt is used in both joints, we have:
𝑛𝑏 = 𝐴𝑡𝑆𝑝−𝐹𝑖
𝐶𝐹𝑡2
= 8410/1853 = 4.5 (42)
Therefore, it is possible to state that in all joints the factors of safety for axial and shear stresses are
met.
4.10 Brakes
Some models of land yachts have brakes [59], while in others the decrease of speed is done exclusively
using the sail. In this project it is considered that the developed vehicle will necessarily have to include
brakes because of the use of the electric motor, namely due to the possibility of using the vehicle without
the sail. It would not be wise to project a new set of brakes for the vehicle in this work, instead this
components will be bought.
The first and most obvious choice for this constituent is to use the brakes in the bicycle selected to be
used in the winning concept (i.e. e-bike A7AM20). However, the forces propelling the land yacht (wind
and electric motor) are not the same as those that propel the vehicle the brakes originally come from,
and for that reason it could not be suited for this application to use the same brakes.
70
The braking distance, i.e. the distance a vehicle travels between the moment its brakes are activated
and the moment it comes to a complete stop, is given by (d is the distance in meters) [61]:
d = 𝑣2
2𝑓𝑔 + 𝑣𝑡𝑟 (43)
In this equation v is the velocity of the vehicle (m/s), g is the gravitational acceleration (9.81 m/𝑠2), f is
the coefficient of friction between the tires and the surface and 𝑡𝑟 (s) is the reaction time of the driver.
0.75 seconds is a typical value for this last parameter, so it will be used in this work. The most relevant
value for v is the maximum, since this will lead to the biggest braking distance. It is assumed that the
land yacht has a top speed of 40km/h [62]. An expectable value for f in this situation is around 0.25.
Substituting the values in equation (44), we have d = 33m.
It is important to notice that the distance will always be smaller than the 33m found, since when the sail
is being utilized it can also be used as a means of braking and for that reason the brakes won’t have to
do all the work; if not, the vehicle will never go up to the highest speeds. Being this the case, it is
reasonable to postulate that the mass and the speed of both vehicles (e-bike and land yacht) are
somewhat similar and therefore possibly the e-bike’s brakes are adequate to the land yacht.
Besides the generation of enough braking torque, another usual concern with this components is the
heat generation and dissipation. However, in this case the brakes are not expected to be used very
intensely nor very often, and their placement in the front wheel leaves them exposed to flows of air that
have a cooling action. For these reasons, there is not going to be a concern with this area.
The choice made will also allow to save money in the construction of the prototype, since it eliminates
the necessity of buying the component separately.
With this discussion it is clear this is another area in which it would be beneficial to conduct tests in order
to determine with more precision whether the selected components are in fact of viable use. If the
experiments reveal the brakes of the e-bike are not suitable, an obvious alternative would to use the
brake set of the company Blokart, specifically design to be used in this kind of vehicles [59]. It is given
priority to the brakes of the e-bike due to this and the mentioned cost concerns.
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5. Electric Components
5.1 Circuit
One of the most important parts of this preliminary project are the considerations about the electric
components of the vehicle and the circuit. Unlike some of the preceding sections, this one is not based
on earlier theses; in fact, the introduction of the electric propulsion system is the key difference between
this work and previous works.
Another factor that adds to the relevance of this section is its importance to the safety of the user, which
has always to be a major concern when dealing with electric systems. Although the currents and
voltages are not high in this project they are enough to provoke injuries. This is why it is so important to
carefully design and implement the circuit (avoiding malfunctions such as short circuits), to properly
isolate the wiring and electric components (as well as firmly hold them in place) and to include
components in the system that can allow to shut it down in case of necessity. Furthermore, usually it
would be necessary to isolate and protect the constituents of the electric system from being subjected
to sand, dust and water (the exposure to these is very likely due to the kind of utilization expected from
this type of vehicle). However, the components are taken from an e-bike specially intended to be used
on the beach, and therefore the components are already capable of dealing with those problems, since
they were designed specifically for it.
The constituents that are going to be part of the circuit are already mostly defined, since they were
chosen in the concept selection stage:
36V, 500W electric motor (geared hub);
Lithium ion battery, with 48V and 10AH (480Wh);
Controller;
Throttle (manual/twist).
Besides these, are going to be included the following elements in order to contribute to avoid accidents
in the electric part of the vehicle:
Main circuit breaker (figure 46): The function of this device is to allow a quick disconnection of
the batteries from the rest of the components, immediately interrupting the circuit. It is a safety
component which is supposed to be used in case of emergency (e.g. the detection of sparks,
flames, etc.).
72
Figure 46 – Main Circuit Breaker [34]
Safety fuse: It interrupts the circuit when an unexpected spike of current occurs due to a short
circuit or accident. Normally they are placed in-line with the battery. While the circuit breaker
may be manually activated in case the occupant detects an anomaly, the safety fuse does it
automatically (adding a second layer of protection).
Besides these, the only other elements necessary are the wires to do the connections. Their isolation
must be without defects and during use they should be held closely to the chassis (using supports) to
avoid being pulled.
The electric circuit implemented (figure 47) is very simple due to the characteristics of the problem (only
a few components are necessary and in a simple configuration). The control is done using a bought
controller (used specifically with the motor and battery of the vehicle) and for that reason it is presented
in the circuit as a black box (it is unnecessary to present the arrangement of its inner components).
Figure 47 – Layout of the electric circuit to implement
73
In figure 47 is presented the circuit that will be used in the vehicle. It is a simple layout that is typical in
electric vehicles. S1 represents the throttle, S2 the fuse and S3 the circuit breaker.
5.2 Location and placement of the components
Motor (1): It comes integrated in the wheel of the e-bike (placed in the front), and cannot be
removed from it. To make the transition to a fully wind powered vehicle the wheel has to be
replaced.
Battery (2): This component is placed in a compartment (the battery holder), which is itself
situated beneath the horizontal bar of the tripod, and connected making use of 4 bolts and nuts
which go through the lateral supports specifically design for that purpose. Batteries are
susceptible to damage caused by vibrations and impacts, which may cause their irreparable
failure (a very serious problem when considering their high cost) and even pose a danger to the
occupant. For that reason, in the inner walls of the battery holder are used silicon foam pads
specifically design for this purpose. Besides absorbing energy of the shocks, this material is
very resistant to high temperatures and flames, a key property in this type of application [63].
Throttle (3): Positioned beneath the seat (like the brake lever, but on the opposite side). It was
considered the use of a handlebar to steer the vehicle, and in this case it would be placed in
there. Once it was decided that would not be the best option, this location was chosen for the
throttle because it is easy to reach and does not present safety concerns.
Controller (4): Placed behind the seat, a location that offers some protection. This component
does not need to be touched during use, so there is no inconvenient in this location. It is held in
place by being put in a support integrated in the seat.
Main circuit breaker (5): Given its function and importance, it must be in an accessible location.
For that reason, it is also placed beneath the seat, but in a posterior position relatively to the
throttle since, unlike this element, it is only used sporadically.
In figure 48 are identified and illustrated the locations of the mentioned elements.
Figure 48 – Location of the different components of the electric system
74
5.3 Expected consumptions and battery duration
Knowing all the components of the system and their characteristics, is possible to do an estimate of the
consumptions in different wind conditions, and therefore the time of use available. The two extreme
circumstances are when the vehicle is being operated only resorting to the wind power (electric system
removed) and when the wind is negligible and therefore the kart only has the motor as a power source.
The battery has 480Wh of energy storage capacity and it is going to be assumed in every case the use
of the vehicle is started with full charge. It is important to notice this is another example of a situation
where accurate values could only be determined by doing tests with the vehicle (instead, an
approximation is utilized).
In section 2.4.2, in order to choose the necessary capacity of the batteries, was made the following
approach to the problem: it was considered the average utilization of the vehicle was 4 hours and that
in normal wind conditions the energy drained from the battery in this period corresponds to the use of
the motor at full power during one hour. This 4 hour period is therefore from that point onwards set as
the average expectable duration of the battery in regular wind conditions.
On the other hand, and considering the utilization of this type of vehicle requires in most cases a
moderate/high speed, in a no wind situation (or when it is very weak) the expected duration of the battery
will be close to one hour (the motor will be working close to full power to compensate for the lack of
thrust from the sail). This is a very short duration that in most cases would not satisfy the needs of the
user: it is important to remember, however, the electric power system is supposed to be only an
assistance to the wind propulsion since the vehicle, namely the battery, was not design to operate in
these conditions (a greater battery capacity would be necessary, and this would translate in an important
cost increase).
The batteries should be fully charged before being used and fully discharged after, mainly to maximize
the time of use in each utilization (lithium ion batteries are not as susceptible to being damaged from
“memory effect” as other types).
These numbers can be utilized in the future as reference points when testing the vehicle. It is also
expected the values to some extent vary with the type of terrain. Either way, since the motor is controlled
with a twist throttle that allows to regulate the power gradually, it’s possible for the user to roughly predict
the duration of the batteries.
Finally, this discussion is only valid for a new battery since, as mentioned before, even lithium ion
batteries, correctly stored and only subjected to complete charge/discharge cycles tend to loose capacity
over time with each utilization, although it only becomes relevant after several hundred repetitions. The
expected life of the equipment in analysis is around 1000 cycles (from this moment is considered the
battery has lost too much of its capacity and should be replaced).
75
5.4 Cost analysis
In this section it is going to be made the discussion of the cost of the implementation of the electric
power solution in the land yacht, including both the direct and the indirect shares. The cost of parts not
related with the existence of an electric motor (structure, sail, etc.) will not be considered. Once again,
this option was taken to focus attention primarily on the exploration of the specificities of the hybrid
vehicle, and also because in previous works a cost analysis of the standard components was already
done. There are going to be considered two situations: prototype stage and production stage (although
only in the first situation can be found exact values).
5.4.1 Prototype
In this phase, the components of the electric system are all taken directly from the e-bike Shuangye
A7AM20 beach model. Therefore, to build the prototype it would be necessary to purchase it, at a cost
of 630€ [37].
There is still another factor to consider: since the land yacht projected in this work has 3 wheels and a
fourth one is required to replace the motored front wheel when the user opts to use the vehicle without
the electric system (when the wind speed is high enough), two extra non-motored wheels are needed.
This represents an estimated extra cost of 100€. The other elements of the electric system (main circuit
breaker, safety fuse, wiring) have a negligible cost when compared to the former shares.
These portions englobe every cost with components that have to be bought except the sail and
fasteners. All other cost are related with the purchase of the necessary aluminum and steel tubes and
the production operations required to obtain the chassis.
5.4.2 Production
In a situation of production of the vehicle, the unitary costs with the electric components would obviously
be smaller, since they are now bought in bulk and without the need to buy them together with extra
unused parts. It is, however, difficult to quantify this difference given that it would greatly depend on the
quantity of vehicles produced and the specific research done to find suitable suppliers. This research
would also determine whether the different elements would still be bought as a set or whether it would
now be advantageous to acquire them separately.
76
6. Future Developments
One of the main goals of this thesis is to propose a way to transform a regular land yacht in an electric
hybrid vehicle. The basic structure of the land yacht used in this work was the subject of a previous
thesis. Likewise, there are some key points that may be explored in greater detail in future works. Some
of the most evident are:
- Study the structural integrity of the weld beam connecting the tripod (namely the fatigue damage),
since this is the most sensitive zone of the chassis;
- Some aspects of the vehicle and values used in this project, as explained in the respective sections,
were decided or obtained using approximations that were somewhat imprecise. The reason for this is
frequently the necessity of performing tests in order to get accurate data. That type of work could bring
valuable improvements to the vehicle;
- Study in detail the viability and advantages of using an automatic control system, i.e., having the electric
motor come into action once some predefined parameter (e.g. speed of the kart) goes below a certain
value, and applying it to the vehicle if it reveals to be preferable.;
- Explore other types of electric motors as mentioned is section 2.1.4. in order to try to decrease the end
cost of the land yacht;
- Extend the research of the available commercial options in terms of e-bikes and/or components and
redo the concept generation in order to reduce the cost;
- Verify experimentally if the used brakes (taken from the e-bike) are in fact adequate for the projected
vehicle.
77
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