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2013 Baja Car Rear Suspension Redesign

A Baccalaureate thesis submitted to the

Department of Mechanical and Materials Engineering College of Engineering and Applied Science

University of Cincinnati

in partial fulfillment of the requirements for the degree of

Bachelor of Science

in Mechanical Engineering Technology

by

Sahil Patel

September 2014

Thesis Advisor:

Dean Allen Arthur

ii

ACKNOWLEDGMENTS

I would like to thank Dean Allen Arthur for his ongoing support, patience, and guidance

throughout this project. I would also like to thank Mr. Ronald Hudepohl, Mr. Douglas Rife,

Mr. David Conrad, and Mr. Bill Hansel for their assistance with the production and

assembly. This project would not be able to have been completed without them.

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TABLE OF CONTENTS

ACKNOWLEDGMENTS ........................................................................................................ II

TABLE OF CONTENTS ........................................................................................................ III

LIST OF FIGURES ............................................................................................................. IVV

ABSTRACT ............................................................................................................................. V

INTRODUCTION .................................................................................................................... 1

PROBLEM STATEMENT........................................................................................................................................ 1 INTERVIEWS ....................................................................................................................................................... 1

CUSTOMER REQUIREMENTS ............................................................................................. 2

PROOF OF DESIGN ................................................................................................................ 3

PREVIOUS DESIGN ............................................................................................................... 4

RESEARCH .............................................................................................................................. 5

DESIGN PROCESS.................................................................................................................. 8

REFERENCES ....................................................................................................................... 11

APPENDIX A - MANUFACTURING AND ASSEMBLY .................................................. 12

APPENDIX B - BUDGET...................................................................................................... 17

APPENDIX C - QUALITY CONTROL ................................................................................ 18

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LIST OF FIGURES

Figure 1 – List of Customer Requirements……………………………………………………2

Figure 2 – Figure 2 – Proof of Design ….…………………………………………………….3

Figure 3 – Previous Linkage Mounting Points ……………………………………………….4

Figure 4 – Double A-Arm Suspension………………………………………………………...5

Figure 5 – MacPherson Strut Suspension……………………………………………………..6

Figure 6 – Multi-Link Suspension…………………………………………………………….7

Figure 7 – Motion Sketch of Rear Side Wheel ……………………………………………….8

Figure 8 – Motion Sketch of Camber Change…………………………………………….......9

Figure 9 – Table with New Mounting Points………………………………………………….9

Figure 10 – 3D Final Model………………………………………………………………….10

Figure 11 – Dynamic Mass in Rear Axle…………………………………………………….10

Figure 12 – von Mises Stress Analysis………………………………………………………11

Figure 13 – Bottom Mounting Bracket………………………………………………………12

Figure 14 – Top Mounting Bracket………………………………………………………….13

Figure 15 – Bottom Link……………………………………………………………………..14

Figure 16 – Top Link………………………………………………………………………...15

Figure 17 – Bung Fitting……………………………………………………………………..16

Figure 18 – PVC Pipes……………………………………………………………………….18

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ABSTRACT

The Society of Automotive Engineers (SAE) hosts a yearly intercollegiate Baja race at

various locations throughout the nation. Every college that chooses to compete in this event

assembles a team of undergraduate and / or graduate students to compete. This competition

will include competitions such as acceleration, hill climb, land maneuverability, suspension

and endurance. I will work on the redesign of the rear suspension that will be safe, reliable,

cost effective, and easy to maintain. The suspension redesign will be my senior design

project.

2013 Baja Car Rear Suspension Redesign Sahil Patel

1

INTRODUCTION

PROBLEM STATEMENT

Design an improved rear suspension in the 2013 UC Baja Car. The previous rear

suspension design of the 2013 Baja Car has an understeer causing poor handling. The new

design of the rear suspension must have a roll center of the rear suspension that is closer to

the current center of gravity which is 24”.

The relationship between the roll center and the center of gravity creates a moment on

the vehicle in turns. The greater the distance between the roll center and the center of gravity

the greater the moment the vehicle must overcome (2).

INTERVIEWS

Ricardo Hinojosa was a former Bearcats Baja member, class of 2013. He is

knowledgeable on the 2013 car due to his experience from building the car. He has expressed

his desire to reduce the understeer on the car. According to him, the geometry of the linkages

doesn’t match the suspension. This bit of information was important in developing my

problem statement revolving around the roll center of the vehicle.

Joe Kobs was the former president of the Bearcats Baja team in the year 2014. He

helped with the manufacturing of the 2013 as a 4th year Mechanical Engineering Technology

student. He stated that the 2013 car also has a problem with understeering. According to him

reliability of the suspension is imperative from his previous experience with the team. It was

critical to him that I have a design that will be able to withstand the maximum forces during

competition.


2

CUSTOMER REQUIREMENTS

There are two primary customers for this project. The first is the Society of Automotive

Engineers (SAE). Every year SAE host an intercollegiate Baja Competition at 3 various

locations throughout America. These completions consist of 4 dynamic events; acceleration

hill climb, maneuverability, and rock crawl/suspension. On a yearly basis they issue rules and

regulations at which a vehicle must adhere to.

The second customer will be the University of Cincinnati Bearcats Baja team. They will

be using this car for future competitions and also for practice purposes.

The following are the requirements that have been determined.

1. Safety

The vehicle must be safe to operate.

Vehicle must adhere to SAE Baja rules.

2. Reliable

Must support person 200 lbs. weight in seat.

The design must be able to last in a SAE Baja race.

The camber angle must change less than 5° in the travel motion.

The travel motion of the rear wheels must at least be within the shock limit of

+4.00 inches and -0.50 inches vertically.

3. Easily maintained

Quick disconnects for the assembly parts.

Must be independent.

Can be maintained by using standard tools.

Minimal fabrication will done

The left and right side must mirror each other

4. Cost

Must stay within the allotted budget

Figure 1 – List of Customer Requirements


3

PROOF OF DESIGN

Proof of design will be conducted by performing a test of the travel motion of the newly

designed linkages. The travel motion must be willing to travel up from resting position a

minimum distance of 4.25 inches. The distance of 4.25 inches is the distance determined by

the range of movement in the shock shown in figure 7.

Figure 2 – Proof of Design


4

PREVIOUS DESIGN The previous design of the rear suspension linkages did not form a roll center. The outer

linkage mounting points (side mounted wheel side) were closer in the Y axis than the inner

mounting points (side mounted to the chassis), as shown in the figure below. The outer side

distance is 5.5 inches and the inner side is 5.75 inches. This results in a roll center that does not exist. When this happens the forces in a turn go

in two different directions. The forces going in the two different directions assist in the issue

of the understeer on the car (4).

Figure 3 – Previous Linkage Mounting Points


5

RESEARCH

Figure 4 – Double A-Arm Suspension

Double A-Arm Suspension

This type of suspension is also referred to as the double wishbone. It

utilizes a dual “A” shaped arm. The two arms are on the same

horizontal plane and have a shock absorber running through the

arms. The top arm will be shorter than the bottom arm (4). Each arm

is has two mounting points connected to the frame and one joint at

the knuckle. This type of system is good for increasing negative

camber angle. In turn there are great traction qualities for turns. The

camber of the tire is also easily adjustable in this type of suspension.

(4)

This system is heavier

and takes up more

space than the

counterparts. This

suspension is

complex and has

multiple parts

associated with it.

The costs associated

with this particular

suspension are higher

than the counterparts.

Any maintenance for

the double A will be

time consuming.


6

MacPherson Strut Suspension

The MacPherson Strut system is very common for modern day cars.

The shock absorber will be mounted vertically to the wheel hub. A

control arm will mounted from the frame to the wheel hub for some

additional support (1). This is a simple design with low maintenance

required. There is less room taken up by the suspension than the

double A arm and in turn this is a lighter option. This will help with

the car’s acceleration due to the fact that there will be less weight to

carry. This is the most economical of the three mentioned

suspensions.

(4)

It is difficult to

maintain traction

while cornering. This

is not a good design

for lowered vehicles.

The camber is set and

not easily adjustable.

This type of

suspension will have

trouble moving

horizontally.

Figure 5 – MacPherson Strut Suspension


7

Figure 6 – Multi-Link Suspension

Multi-Link Suspension

This type suspension is a cousin of the double A suspension. The

multi-link suspension uses three or more lateral arms and one or

more longitudinal arms. There is a spherical joint at each end of the

connecting arms. The shock absorbers can mount to the longitudinal

arms if desired or mounted elsewhere if that is a more desirable

option. This allows for more flex for the vehicle which makes it

great for off road applications(3). This suspension is a happy

medium in terms of space in between the MacPherson and double A.

(4)

This can get to be

expensive to build.

The design of this

suspension is the

most complex of the

three types

mentioned. This type

of suspension

absolutely must be

finely tuned and if

there are any

deviations from the

plan, it can cause

damage to the vehicle

and possibly driver.

The car currently has

a multi-link

suspension, re-

designing the

linkages would be the

most cost-effective

plan moving forward.


8

DESIGN PROCESS

The very first thing to be determined in the designing of linkages is the appropriate

travel motion of the rear wheels. Figure 7 is a motion sketch of the side view of the rear side

of the vehicle.

Adjusting the axis shown above shows the compression of the shock. When the car is

motionless on the ground the shock is compressed at 0.25 inches. When the shock is in full

compression it is 4.25 shorter meaning the travel needs to be +4.00 inches and -0.25 inches.

The second thing to consider in the design process was the change in camber while the

Figure 7 – Motion Sketch of Rear Side Wheel

Axis

Shock

4.00 inches

0.25 inches


9

wheels move up and down. The optimum camber change rate I was looking for was ± 5°.

This was done by creating another motion sketch (Figure 8).

By moving the wheels up and down the camber of the wheel can be measured as well as

determining the optimal mounting points for the new linkages. The new changes in the

mounting points and camber change were recorded in a table (Figure 9). This table was

paramount in finding the best combination of roll center, new mounting points, and camber

change.

Figure 8 – Motion Sketch of Camber Change

Figure 9 – Table with New Mounting Points

CG

Roll Center


10

The only issue with this method of design was the length of the linkages was only

considered in the X and Y direction. The total length of the linkage was determined via 3D

modeling which is shown in figure 10.

The final step in the design process was the Finite Element Analysis (FEA). The FEA

was conducted using SolidWorks software. The benefit of FEA is that I will allow me to

know where the possible failure would lie. The equation below in figure 11 shows the

dynamic mass in the rear axle.

Figure 10 – 3D Final Model

Figure 11 – Dynamic Mass in Rear Axle


11

Figure 12 below is the von Mises stress analysis. The analysis shows how the assembly

will deform in the worst case scenario. A force of 131.1 lbf is applied in the study. There are

not any critical points that need to be worried about; they would be indicated by the bright

red.

REFERENCES

1. "Mechanical Engineering." Comparison between MacPherson & Double Wishbone

Suspension System ~. Web. 9 Feb. 2015.

2. Milliken, William F., and Douglas L. Milliken. Race Car Vehicle Dynamics. Warrendale,

PA, U.S.A.: SAE International, 1995. Print.

3. Raiciu, Tudor. "How Multi-Link Suspension Works." Autoevolution. 17 June 2009. Web.

25 Oct. 2014. <http://www.autoevolution.com/news/how-multi-link-suspension-works-

7804.html>.

4. "The Suspension Bible." Http://www.carbibles.com. Web. 19 Nov. 2014.

<http://www.carbibles.com/suspension_bible.html>.

Figure 12 – von Mises Stress Analysis


12

APPENDIX A- MANUFACTURING AND ASSEMBLY

BRACKETS

Figure 13 – Bottom Mounting Bracket


13

Both brackets were made via profile milling.

The brackets were the MIG welded to the frame tab.

Figure 14 – Top Mounting Bracket


14

LINKAGE ASSEMBLY

Figure 15 – Bottom Link


15

Bung fittings MIG welded to aluminum tubing cut to desired length for both top and

bottom links.

Each link has a left and right hand thread at opposite ends to allow for camber

control.

3/8” bolts and nuts were used to mount the linkages to the car.

Figure 16 – Bottom Link


16

The bung fittings were cut to 1” pieces.

Lathe turned to a diameter of 0.675” at 0.50” length.

Figure 17 – Bung Fitting


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APPENDIX B- BUDGET

Build of Materials

Item Size Quantity Price

Heims

Joints

4 Left Hand

Thread

4 Right Hand

Thread

8 $75.84

AISI 4130

Steel Plate

12” X 12”

.08” Thick 2 $45.12

Aluminum

6061

Piping

72”

1" DIA x

0.125" THK

x 0.75"

1 $38.67

Aluminum

6061

Billet

12”

1” Diameter 1 $9.29

Total with Shipping ($32.09) $201.01


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APPENDIX C- QUALITY CONTROL

PVC replicas of the tubing lengths were created and fitted to ensure the tubing was

modeled correctly in SolidWorks

Figure 18 – PVC Pipes

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