baja sae india design report

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Paper Number- TTTITM2 Baja 09 design report Raman Sarin Captain, Member design team Ajay Goyat Steering and brakes department Copyright © 2006 SAE International ABSTRACT The design report focuses on explaining engineering and design process behind each system in the Baja vehicle that is developed till now. The report also throws some light on the alternatives considered. The design of the vehicle is in accordance with the specifications laid down by the rule book. This design report is a cumulative effort towards explaining the design process to the readers. INTRODUCTION The design process of the vehicle is iterative and is based on various engineering and reverse engineering processes depending upon the availability, cost and other such factors. So the design process focuses on: Safety, Serviceability, Cost, Standardization, Strength and ruggedness, Driving feel and ergonomics, Aesthetics The design criterion followed here is design for the worst and optimize the design while avoiding over designing, which would help in reducing the cost. We proceeded by setting up the budget for the project. Throughout the design process we distributed the budget in such a way that if we assign more money to one system, we reduce that amount from some other system. Our last year vehicle design was based on the criterion of prevention of failure, as that year no one knew the track and the obstructions prevalent over there. So the procedure of over designing was followed as the safety of the driver is of utmost importance. The design targets of our vehicle for Baja 09 are as follows: 1. Maximum speed – 45 km/hr 2. Weight – 270 kg 3. Ground clearance – 20 cm or 8 inch 4. Track width – 160 cm or 64 inch approx 5. Wheel base – 190 cm or 75 inch approx 6. Braking distance – 1400 cm 7. Turning radius – 240 cm or 96 inch Further, as designing is based on prevention of failure so let me define the condition of failure of each system of our vehicle. For roll cage, failure is yielding as this would change the distance between various parts and thus their working is affected. It should be rigid and the mountings should be able to bear its load.

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Page 1: Baja Sae India Design Report

Paper Number- TTTITM2

Baja 09 design report

Raman SarinCaptain, Member design team

Ajay GoyatSteering and brakes department

Copyright © 2006 SAE International

ABSTRACT

The design report focuses on explaining engineering and design process behind each system in the Baja vehicle that is developed till now. The report also throws some light on the alternatives considered. The design of the vehicle is in accordance with the specifications laid down by the rule book. This design report is a cumulative effort towards explaining the design process to the readers.

INTRODUCTION

The design process of the vehicle is iterative and is based on various engineering and reverse engineering processes depending upon the availability, cost and other such factors. So the design process focuses on:

Safety, Serviceability, Cost, Standardization, Strength and ruggedness, Driving feel and ergonomics, Aesthetics

The design criterion followed here is design for the worst and optimize the design while avoiding over designing, which would help in reducing the cost.

We proceeded by setting up the budget for the project. Throughout the design process we distributed the budget in such a way that if we assign more money to one system, we reduce that amount from some other system.

Our last year vehicle design was based on the criterion of prevention of failure, as that year no one knew the track and the obstructions prevalent over there. So the procedure of over designing was followed as the safety of the driver is of utmost importance.

The design targets of our vehicle for Baja 09 are as follows:

1. Maximum speed – 45 km/hr

2. Weight – 270 kg

3. Ground clearance – 20 cm or 8 inch

4. Track width – 160 cm or 64 inch approx

5. Wheel base – 190 cm or 75 inch approx

6. Braking distance – 1400 cm

7. Turning radius – 240 cm or 96 inch

Further, as designing is based on prevention of failure so let me define the condition of failure of each system of our vehicle.

For roll cage, failure is yielding as this would change the distance between various parts and thus their working is affected. It should be rigid and the mountings should be able to bear its load.

For brakes, failure is their inability to lock all the four tires simultaneously.

For tires, it is failure to transmit the required torque maintaining the traction with the track surface.

For suspensions, failure occurs if they are not able to isolate the driver from the shocks or if they are so soft that they compress to their solid length while working.

For transmission there is less scope of failure but failure is if any part is not able to transmit the required torque or also if torque provided in the first gear is unable to drive the vehicle from a halt.

For steering, failure is defined in terms of effort applied by the driver and ability of the various components to facilitate the function of steering.

Failure of various other mechanisms like pedals, levers, electrical components occurs if they are not able to fulfill their desired function.

Page 2: Baja Sae India Design Report

Hence, our designing process targets on the above lying facts to ensure the proper working of our vehicle.

MAIN SECTION

ENGINE AND TRANSMISSION

A quick look at the engine:

Power - 8 kW at 4400 rpm

Max Torque – 19 Nm at 3000 rpm

About gear box, we have 4 forward and 1 reverse gear box with built in differential and universal joint.

As engine and gear box were given to us. Thus we had a little choice while working on transmission. Configuration of our vehicle would be rear engine rear wheel drive. We decided to keep the maximum speed of the vehicle at 45 km/hr as the vehicle is not about larger speed but greater torque and stability. For attaining this speed, the only thing we can vary was the outer diameter of the driving tire. For 45 km/hr O.D. of the tire came out to be 16 inch. This diameter is too small as ground clearance decreases.

Hence in order to counter this problem options available were:

1. Manipulation of power transmission outside the gear box using gears, sprockets and chain.

2. Engaging the reverse gear lever while driving in all the forward gears and using the first gear in forward as reverse gear.

We decided to work on the latter option and so did reverse engineering process trying to find if the gears would be able to transmit the increased torque. Also following this method,

1. We were able to check the weight

2. Reduce the cost of the vehicle as we avoided the use of additional gears, sprockets and chains.

3. We used standard parts, thus increased the reliability of the transmission system.

To find the speed of the vehicle corresponding to different gear ratios, the formulae used is

Velocity on road = 2π×N×R×60÷ (1000×G) Km/hrWhere,G=gear ratioN=revolutions per minuteR=outer radius of the tire in meters.

Some of our calculations for reverse and forward orientation are as follows:

Normal orientation

Final Gear Ratios

Speed (km/hr)

Speed (km/hr)

D=22 D=24 inch inch

First 31.45:1 0.65D 14.5 15.8

Second 18.70:1 1.109D 24.4 26.6

Third 11.40:1 1.82D 40 43.6

Forth 7.35:1 2.82D 62 67.7

Reverse 55.08:1 0.38D 10 9

Reverse orientation

Final Gear Ratios

Speed (km/hr)

Speed (km/hr)

D=22 D=24 D=28 inch inch inch

First 55.08:1 0.38D 8.3 9 10.5

Second 32.75:1 0.63D 13.9 15.2 17.7

Third 19.96:1 1.04D 22.8 24.9 29

Forth 12.87:1 1.61D 35.45 38.7 45

Reverse 31.45 0.65D 14.5 15.8 18.5

Hence for maximum speed of 45 km/hr, we selected tires of 28 inch outer diameter.

Further, for better economy, we assume engine rpm to be ranging from 2750 to 3250 as maximum torque produced by the engine is at 3000 rpm. In between this range the torque produced by the engine is almost constant (from engine characteristics graph; fig e1). Thus, for better economy, the range of speed in each gear, for the driving tires of O.D. 28 inches; operating in reverse orientation is:

First - 6.7 to 9 km/hrSecond - 11 to 14 km/hrThird - 18 to 24 km/hrForth - 29 to 37 km/hrReverse - 12 to 15 km/hr

Apart from this, for mounting the engine we are going to use neoprene rubber mountings.

Page 3: Baja Sae India Design Report

TIRES

Selecting the tires is one of the most important things as the whole vehicle is in contact with the road on these 4 points or rather patches. Also for designing an all terrain vehicle tires form the most important part. They should be such that they are able to provide enough traction on all kind of surfaces so as to transmit the torque available at the wheels without causing slipping.

LAST YEAR:

Front and rear same tiresOuter diameter of tire – 24 inchOuter diameter of rim – 12 inchTread width – 6 inchAspect ratio - 1Number of plies – 6Tread design – mud cuttingSide with – 210 mm

THIS YEAR:

FRONT

Outer diameter of tire – 24 inchOuter diameter of rim – 12 inchTread width – 8 inchAspect ratio - 1Number of plies – 6Side with – 198 mm

REAR

Outer diameter of tire – 28 inchOuter diameter of rim – 12 inchTread width – 10 inchAspect ratio - 1Number of plies – 6Side with – 231 mm

Shown in fig t1

One of the most important parameter for the selection of the outer diameter of the tires in rear was the maximum speed of the vehicle. The relation between outer diameter of the tires and the vehicle speed is as given below:

Velocity on road = Angular velocity × (Outer radius of tire ÷ gear ratio)

For the reverse orientation of the transmission system and maximum speed of the vehicle as 45 km/hr radius comes out to be 28 inches. Apart from outer radius of the tire, other factors for the selection of tires include tread width, tread design, side wall width, load handling capacity, number of plies and treads on side wall etc which define the traction ability, tire resistance to wear and puncture and performance of the tire on various terrains.

ADVANTAGES:

1. Built with a 6 ply rating and a reinforced casing makes these one of the most puncture resistant tires in the market today.

2. Large shoulder knobs wrap down the sidewall to provide excellent side to pull out of the ruts without causing sidewall failure.

3. The deep tread and open wing design provides excellent clean-out with each lug and an improved traction.

4. Special natural compound delivers added traction.

5. Smaller tires in front results in a smaller magnitude of moment on the wishbones due to cornering forces during steering.

6. Use of the larger outer diameter tire at the rear helps to provide good ground clearance and also 10 inch treads provides good traction to the power wheels.

BRAKES

The criterion for designing the brakes stated as per the rule book is that all the four wheels should lock simultaneously as the brake pedal is pressed.

LAST YEAR‘S BRAKING SYSTEM:

Front Disc brake of Maruti800 (91 mm)Rear Drum brake of APE (180×30mm)

THIS YEAR:

In the last years vehicle we found that the braking force was not enough to lock all the four wheels simultaneously

For designing the braking system this year, we calculated the weight of our vehicle in static condition as well as in dynamic condition as per the deceleration (0.6 g) and stopping distance. In static condition it is around 60kg on each front tire and 110kg each on the rear tire. But in dynamic conditions, we consider weight to be 85kg on each tire, the front and the rear. We have calculated the dynamic weight using the formulae as given below:

Front axle dynamic load = w1 + (α ÷ g) ×W× (H ÷ L)

Rear axle dynamic load = w2 – (α ÷ g) ×W× (H ÷ L)

Where, W1=Weight on the front axle in the static condition.W2=Weight on the rear axle in the static condition. g = Acceleration due to gravity.

Page 4: Baja Sae India Design Report

W= Total weight of the vehicle.H=Height of center of the gravity.L= Length of the wheel base.Deceleration of the vehicle is α.

We planned to use disc brake in front and drum brakes in rear. Initially we thought of using disc brakes for all four wheels but disc with parking brakes have higher cost and we found it necessary to use the parking brakes to increase the all terrain capabilities of the vehicle.Some formulas that we used for designing our brakes:

T (disc) + T (drum) = m × f ×R

T (disc) = µ×R1× (P × A) ×2

T (drum) = (P × A) × Brake factor×R2

Where,T(disc) = Frictional torque on the discT(drum) = Frictional torque on the drumf = decelerationm = mass of vehicleR = radius of tiresP = Pressure applied by the TMC.µ= Coefficient of frictionR1=Radius of the discA= Area of the caliper for disc brake and wheel cylinder for the drum brake.

Using these formulae, we have done our calculation and selected our brakes. Some of calculations are shown in the table:

F kg Pr D1 D2 D3 R1 R2 R3 R4

1 20 2.5 16.25 50 30 91 90 12” 14”

2 20 5 22.96 50 30 91 90 12” 14”

3 20 3 17.78 50 30 91 90 12” 14”

4 15 5 19.88 50 30 91 90 12” 14”

5 26.93 3 20.64 50 30 91 90 12” 14”

Where the parameters shown above are as under:F=Pedal force required for braking (kg)Pr = Pedal ratio D1=Diameter of the TMC (mm)D2=Diameter of caliper cylinder for the disc (mm)D3=Diameter of the wheel cylinder for the drum (mm)R1= Radius of the disc (mm)R2=Radius of the drum (mm)R3=Outer radius of the front tires (inch)R4=Outer radius of the rear tires (inch)

The above highlighted specifications have been selected for our vehicle. These are the standard specifications of Maruti Zen’s braking system with vacuum booster. We

selected these as per our design of the braking system for 5.9 m/s^2 deceleration. The pedal force would decrease further by a factor 3 due to the use of booster. So the force on pedal would be 9 kg approx.

COMPARISON WITH PREVIOUS YEAR BRAKES:

1. The friction material is semi-metallic which has got better frictional properties. So we have a higher coefficient of friction

2. Vacuum booster is used for giving the better comfort in applying the paddle force.

3. Use of X-type brake fluid lining which will give us better response and has a higher reliability.

4. Ventilated disc for higher heat dissipation rather than a single disc.

ADVANTAGES

1. Standardization of parts is there. Thus reliability is there.

2. The cost of standard parts is lower.3. Ergonomically, the use of booster would get the

pedal force to a lower value. Thus facilitating the driver.

4. Friction material is in the disc and drum is of semi-metallic which has very good frictional property.

STEERING SYSTEM

LAST YEAR:

Reticulating ball type, pitman arm 4 tie rods used Steering ratio 19.53:1 Turning radius - 9 feet IBJ to IBJ - 50 mm OBJ to OBJ - 1090 mm Column inclination from horizontal - 40 degree One flexible coupling is used in column One universal joint was used. Two pivots were used as shown in fig above

THIS YEAR:

Reticulating ball type (ZF steering) Turning radius – 9 feet Gear ratio - 19:1 Steering ratio – 17:1

While designing the steering system the constraints that we possessed were centre alignment of steering system, track width, human effort at the steering wheel and the desired response of the steering system.

Apart from deciding the steering ratio we have not been able to design the linkages, tie rods etc as presently we do not have the gear box of steering.

Page 5: Baja Sae India Design Report

The formulae used for steering calculations are:

C^2 = X^2 + Y^2

X = c sin (p) + (a+ b sin (q) – a cos (q))

Y = b cos (q) + a sin (q) - R

Where,C – length of tie rodX, Y – lengths as shown in fig s1p, q – angles as shown in fig s1a – length of steering knuckle from center of tireb – perpendicular distance of steering knuckle from pivot point as shown in fig s1

We stick to reticulating ball type steering as we had a good experience using it in last year vehicle in terms of response. The only problem lying with it is its higher weight than rack and pinion type steering.

ADVANTAGES

1. It has an advantage that being worm type, only driver effort would be transmitted to the wheels. But unlike rack and pinion, the wheels reaction generated from the track would not be transferred to the driver.

2. Reticulating ball type steering has lower wear and tear as compared to rack and pinion steering.

3. It can withstand 25000 cycles at the constant load of 250N/m, at the pitman arm speed of 20 to 25 RPM.

Further, apart from keeping the steering ratio to be 17:1. Our main concern in the design of the steering mechanism, using reticulating type steering, is to reduce the weight of the mechanism and to incorporate as minimum joints as possible which would help to reduce the human force required to steer the vehicle.

SUSPENSIONS

Suspensions act to provide cushioning action to the driver by absorbing the shocks from the road and also help the tires to maintain good traction.

Last year

Front - Mac Pherson strutRear – Mac Pherson strut and rubber bumpers

Last year, the problem with out suspensions was that they were too stiff. So the movement of springs was too small ride was not comfortable.

This Year –

Unequal wishbone suspension in both front and rear

Reason:

Wishbone suspension give more movement of the tires and hence the vehicle for the same movement of the spring.

Independent suspension.

In double wishbone suspension, force is distributed at 5 points on the roll cage unlike at only one point in Mac Pherson strut.

It can be slightly adjusted for different parameters of suspension tuning like camber angle, ground clearance at the time of testing and then finalized (proper adjustments are made at the time of fabrication).

Design of suspension system should be such that it is able to sustain the worst of the conditions.

For example, in the case when the vehicle is falling on ground after jumping from a speed breaker, just the two wheels support the vehicle as it lands on the ground. But if we design our springs according to this situation, our spring will be a lot stiffer and hence the ride will not be comfortable. Also if we choose stiffer springs, they would not be able to facilitate tire traction. On the other hand softer spring mean that a larger spring travel should be more otherwise they would reach to their solid length. Hence the suspension system would fail.

This criterion can be fulfilled by the 2 alternatives:

1. By putting a spring of gradually changing pitch and hence stiffness. This is the best method to encounter this problem but we could not find a vendor who could manufacture for us a continuously varying spring.

2. By putting a very long soft spring which has enough uncompressed length left so that it would remain in its working range without reaching its failure limit. This method was used by one of the team last year. But the main problem is that the spring might buckle. Even with a damper, the spring-damper system might buckle. We might be able to solve the problem using guides but this is making the system unnecessarily complex.

3. By putting a system of compound spring (in parallel) in which only one spring is acting in normal conditions and a stiffer spring starts to work only after reaching a certain amount of load. This is the method that we will follow because:

It can be said to be equivalent to the first system.

The range of travel is small as compared to the previous two methods and hence our

Page 6: Baja Sae India Design Report

damper buckling problem is also solved to an extent.

Spring Design started with some arbitrary parameters within the constraints

Constraints: Weight, ground clearance required and space limitations

Weight of the last year’s vehicle

472

Estimated weight of this year’s vehicle

270 kg approx.

Driver with accessories 90 kg approx.

Total weight with driver 360 kg approx.

Unsprung mass 75 kg approx.

Sprung mass 300 kg (at max. with driver)

The spring design is to be for the total weight of around 300 kg now.

Since the major components of the sprung mass(in terms of weight) like engine, transmission, driver etc…. are at the back only, the weight distribution is taken as 50 kg on each suspension in the front and 100 kg on each suspension at the back. Also, this was the approximate ratio of distribution of the vehicle weight of last year vehicles.

FRONT SUSPENSIONS

The spring damper would be placed at the centre of the lower wishbone.

Taking ground clearance to be around 8 inches and load of 50 kg on each tire. Thus static load on each spring would be 100 kg as spring is mounted at the centre of the wishbone

Length of spring = 300 mm

Total length (spring + damper) = 430 mm

Wire diameter (d) = 9mm

Mean coil diameter (D) = 70 mm

Allowed travel of the spring = 160 mm

Maximum travel of the spring = 192 mm

Spring stiffness (K) = 20 N/mm

Pitch = 25 mm

No of active turns = 10

Total no of turns = 12

Springs are squared and grounded

Initial compression (after driver is seated) = 50mm

Thus springs can take up a load of 220 kg apart from the weight of vehicle and driver

The dimensions and assembly of front suspensions is as shown in fig sus1

REAR SUSPENSION

Here also the constraints were ground clearance 8 inches, vehicle weight 110 kg on each tire and movement of transmission shaft as shown in fig sus2; full angle being 15 degree, full jounce 3 degree and full rebound 12 degree

In here, we keep the mounting point of the spring on the upper wishbone and at its end. The rear suspension system is as shown in fig sus3.

For the smaller half drive shaft, the distance between spring mounting point and shaft hinge point is 12 inch approximately. Thus, for 15 degree spring movement is 80 mm as calculated by the formulae:

LENGTH OF ARC = RADIUS * ANGLE SUBTENDED

So for 1 degree movement of shaft deflection of spring is 5.3 mm

Now,

Length of spring = 230 mm

Total length (spring + damper) = 490 mm

Wire diameter (d) =11.1 mm

Mean coil diameter (D) = 80 mm

Allowed travel of the spring = 72 mm

Maximum travel of the spring = 96.8 mm

Spring stiffness (K) = 30 N/mm

Pitch = 19 mm

No of active turns = 10

Total no of turns = 12

Springs are squared and grounded

Initial compression (after driver is seated) = 33.3mm

Page 7: Baja Sae India Design Report

From initial compression we conclude that the movement of shaft required is 6.3 degrees

Further, if one of the rear tire falls in a ditch, their will be load on the spring. Assuming tire and brake assembly weight to be 20 kg, deflection of spring required is 6.7 mm or in terms of shaft movement we can say that 1.5 degree of shaft movement would be sufficient for allowing the movement of tire if it encounters a ditch.

Hence under static but loaded conditions, position of shaft below the horizontal level is 4.5 degree (12-7.5 degree).

Now, the allowed movement of shaft under dynamic conditions is 7.5 degree or we can allow spring movement of 39 mm. Thus, the rear suspension can accommodate an additional load of 117 kg.

ROLL CAGE AND MATERIALS

The kind of body we are required to manufacture is a unitized body. The roll cage is of utmost importance for us as it would be the one which would provide safety to the driver, mounting points for various systems and even ergonomics and looks to the vehicle.

It should be strong enough to bear the laden load and should be designed against impact load that it might encounter. The failure criterion for the roll cage is yielding.

Our design of the roll cage started with ergonomic and driver comfort study. We also studied the rules and safety instructions as per Baja SAE INDIA 2009 rulebook. This was followed by study of compatibility of various other systems with the roll cage, as these systems were developed in the process. Based on these, we designed a layout which was modified again and again to take the present shape as shown in fig r1. Adjacent to fig r1 we also have the roll cage of last year vehicle as fig r2. The software used by us is Pro-E for 3-D modeling and design and Auto-CAD for 2-D drafting.

Initially I assumed the ratio of total height of driver to length of driver below waist as 1.65 (considering myself as standard) and designed a roll cage model for a person of height 6 feet 3 inches. Then slowly as the other systems of the vehicle were developed, the roll cage design got modified.

Dimensions of the roll cage are see (fig r3):

Length – 2300 mm

Width – Max - 870 mmAt front end - 540 mmAt rear end - 720 mm

Height – 1440 mm

The FEM analysis of the roll cage is still pending and would be included in the final design report.

The material that we are going to use is mild steel, IS: 1239 (part 1):2004. The material has chemical composition as:

CARBONMANGANES

ESULPHUR PHOSPHORUS

0.2 1.30 0.040 0.040

The pipe we are using is of electric resistance welded type, heavy duty pipe with the following specifications:

Bore – 20 mm

Wall thickness – 3.2 mm

Outer diameter - 26.5 to 27.3 mm

Weight per meter – 1.87 kg

Yield strength – 480 N/mm^2 (as per UTM test)

As the yield strength is as per UTM test so we assume working yield strength of 400 N/mm^2

The pipe of above specification has a higher bending strength and rigidity than the material specified by the rule book.

For safety of the driver, Ethan foam padding would be used over the pipes in the adjacent of the driver.

For fabrication of the roll cage, we are going to use metal inert gas welding and cold bending techniques.

OTHER MECHANISMS

This section includes all the levers, electrical equipments etc that form an important part of our vehicle. Apart from the accessories provided by Lombardini, we are going to use

Battery: 12 V, 44 Ah

Kill switches: 2

Odometer, speedometer, fuel indicator, oil pressure lamp, brake switch, brake lights, reverse alarm

We are going to use a separate reversing lever

We also worked on a gear shifting mechanism which would be available near the steering wheel

CONCLUSION

Page 8: Baja Sae India Design Report

As discussed earlier, our approach is to design for the worst and still optimize so that we avoid over designing. This would help us to reduce the cost.

The approach that we followed is iterative in nature and processes like reverse engineering are adopted in order to select various systems from the ones, existing in the market. This step would ensure standardization and reliability would follow as a by part.

Our top priority would always be the safety of the driver and working in this direction, we will strive to add aesthetic value and a sense of ergonomics to the

The design process is not a single handed effort and so it is my team, whom I wanted to thank for standing with me under all circumstances. I would also like to express my gratitude towards our Mechanical department and on the whole towards the college for supporting us and believing in us. SAE has provided us with an excellent platform for learning and showcasing real life projects. While working on the project, it was really heartening to see that the people from industry were willing to help us and they provided us with their precious time.

CONTACT

Raman SarinMechanical Engineering student Institute of Technology and Management,

Email I.D. – [email protected]

Address - #1178, Sector 18-C, Chandigarh

Page 9: Baja Sae India Design Report

Fig e1

REAR and FRONT

Fig t1

Page 10: Baja Sae India Design Report

Steering mechanism(Tie rods to steering knuckle)

Fig s1

Fig sus1

Page 11: Baja Sae India Design Report

Fig sus2

Fig sus3

Page 12: Baja Sae India Design Report

THIS YEAR ROLL CAGE LAST YEAR ROLL CAGE

Fig r1 Fig r2

Fig r3