Design Report for FSAE India 2009

Mohamed Irfan. N Author

G. Karthick Co-author

Copyright © 2009 SAE International

Technocratz racing

College of engineering, Guindy

ABSTRACT The aim of the competition is to design a formula style car catering to the needs of the weekend racer. The motive of our participation is to design a car without getting into the fabrication process. The principle aim was to understand the finer aspects of vehicular design with the ulterior motive of fabricating a competitive car for the 2011 edition of FSAE India, while strictly adhering to the FSAE rules. The vehicle modeling was done in CATIA, the FEA analyses of which were carried out ANSYS. Flow analysis for the air intake and chassis were carried out using FLUENT. INTRODUCTION The design gives prime importance to satisfy the end user’s needs. The vehicle was assumed to be fabricated in a fictitious firm and the conditions for the feasibility of manufacturing such a vehicle were studied. The analysis threw light on factors such as optimal performance, easy maintenance, ease of handling and aesthetics. For making a standout design, we’ve tried to incorporate some new ideas, viz, outboard mounting of suspension struts, an additional lever for the front brake to provide more braking force during emergencies, a variable rack pinion for reducing steering effort for better handling, an aerodynamically sound chassis by CFD analysis using FLUENT, which was also used to validate the flow efficiencies of our air intake system. DESIGN VISION STATEMENT

- To make a reliable and simple design, primarily depending upon extensive analysis using virtual data, due to our inability to procure various components necessary to make detail practical studies and to come up with a prototype due to time and financial constraints.

- Being the pioneer FSAE team from our college, it is of prime importance to provide a firm footing for future teams. So, we took a conscious effort to avoid too many frivolities and keep things as simple as we possibly could.

POWERTRAIN Engine It was decided to use a 2000 Yamaha Yzf-R6 engine (carbureted) amongst other available options such as the Honda CBR600F4i and the indigenous Bullet Machismo 500, principally for the following reasons - Highest power to weight ratio (With a measured peak horsepower of 96.5 bhp at 12800 rpm without a restrictor, R6 has a power to weight ratio of 1.81 hp/kg, whilst the Honda CBR600F4i, its nearest rival, has a ratio of 1.75 hp/kg). This ratio was due to the lower mass of rotating components when compared to the Honda - a desirable facet for an engine to be used for FSAE. - Carburetion. Fuel injection, in spite of its various advantages, was avoided, mainly because of non availability of a FI engine. This prevented a closer study of the different sensors (like the fuel sensor), without the proper knowledge of which, a complete and satisfactory power train design is impossible. Intake The inclusion of a restrictor whose diameter is to be no more than 20 mm (according to FSAE rules) would cause potential power losses of up to 35 % due to lower specific air input (about 0.12 kg/s). This would mean that the maximum amount of fuel that can be burnt is theoretically 8.3 g/s. To compensate for this a venturi arrangement is incorporated to minimize pressure losses. The venturi-restrictor system is to be made of brass, since air flow is very smooth over a brass surface. Also the restrictor necessitates the construction of an intake plenum. CFD analysis was done using FLUENT and, by trial and error, a satisfactory plenum and runner arrangement was arrived at. Some of the results of this analysis are shown in Appendix D. Drive train The stock 6-speed gearbox of the Yamaha YZF-R6 was retained. It was decided to use a 14 teeth Al 7075 sprocket as our front sprocket.

No of teeth on smaller/front sprocket = z1= 14

Pitch = P = 15.875

Diameter of the smaller sprocket = d1 = P/(sin(180/z1)

= 15.875/(sin(180/14)) = 71.34 mm

The commercially available choices for the rear sprocket are with 40, 42, 45 and 52 teeth. The larger/rear sprocket was chosen to be a 52 teeth Al 7075 sprocket. In spite of the obvious loss in top speed due to the relatively larger number of teeth, it is a known fact that a larger sprocket would produce more torque, and more torque to the wheels would mean better acceleration. Since acceleration is given priority over top speed at FSAE, our selection is justified. Also a larger sprocket reduces the working load for a given amount of transmitted power. No of teeth on larger/ rear sprocket = z2 = 52

Diameter of the larger sprocket = d2 = P/(sin(180/z2) =

15.878/(sin(180/52)) = 262.925 mm

Pitch for both the sprockets = 15.875 mm (5/8”). The standard motorcycle chains which correspond with the above pitch are the 520, 525, 530 and 532 O ring chains, in increasing order of their sizes. The 520 chain was chosen since a smaller chain would spin faster, resulting in a more efficient drive, better power transmission and better fuel efficiency. Speed Table After calculations, an approximate speed table for a tire of dimensions 185/60/R13 was obtained. Taking into account a speed reduction factor of .965 due to reduced tire diameter caused by factors such as atmospheric pressure, temperature, humidity, car weight, car weight distribution, weight shifting caused by acceleration, and rotational speed (centrifugal force).

1 2 3 4 5 6

0 0.0 0.0 0.0 0.0 0.0 0.0

3000 9.1 13.2

16.6

19.3

21.8

23.8

6000 18.2

26.5

33.2

38.7

43.5

47.6

9000 27.2

39.7

49.8

58.0

65.3

71.4

12000 36.3

53.0

66.4

77.3

87.0

95.2

12800

redline

38.7

56.5

70.8

82.5

92.8

101.6

After a market survey, the Torsen University special differential (Limited Slip Differential, Torque Sensing / Torque Biasing) was chosen among a wide variety of possible differentials. Its 3:1 torque bias ratio will more than satisfy our requirements. CHASSIS

Ergonomics: The entire frame is designed around the driver, giving him or her just the right amount of space. To have the best compromise between driver’s CG and comfort (visibility, seating position, and steering wheel angle) a backrest angle of 50 degrees was selected. Our chassis has been designed in such a way that all drivers will be able to exit to the side of the vehicle in no more than 5 seconds.

Firewall and Floor plan: We planned to use panels constructed from aluminum sheets as firewall. They will be bolted to the chassis. The use of fiberglass for this component was investigated but rejected because of greater manufacturing costs. Floor pan of aluminum sheet was made in two sections: a 2mm section at the front of the compartment extending to the seat where the driver could step, and a 1.2mm section for the remaining rear section. Fuel tank: Aluminum sheet metal (corrosion resistant) was selected for the primary tank material due to its light weight, low price, and ease of manufacturability. A sheet thickness of 0.060 inch was chosen to provide sufficient strength to contain the required weight of fuel in a non-load bearing tank while minimizing weight. Sidepods: Structural sidepods are bolted to the chassis to provide safety and increased torsional stiffness. The frame has a calculated torsional stiffness of about 3759 Nm/deg, which is almost 14 times greater than the roll stiffness of the suspension. Space frame:

We chose AISI 4130(chromium-molybdenum steel) to construct our space frame because of its lightness yet its yield strength is very high compared to mild steel of same weight. The space frame is provided with necessary triangulation to provide stiffness and strength.

Impact attenuator: With the attenuator, the mean deceleration at a crash with a velocity of 7 m/s was calculated as 12.5 g. A honeycomb structure made of aluminium composite was selected for constructing the impact attenuator due to its

nearly linear crush resistance force versus displacement, rate independence, and high energy absorption per volume.

Ansys test: An FEA analysis was done using ANSYS and some of the results are shown in the Appendix C. The test results show that the deflection is within the permitted limit. STEERING The steering system constitutes a vital part in an automobile. There are numbers of steering mechanism the one that we employ in our design is variable ratio Rack and pinion steering. This is same in functioning when compared to normal rack and pinion. All the components are the same, and it all works the same except that the spacing of the teeth on the rack varies depending on how close to the centre of the rack they are. In the middle, the teeth are spaced close together to give slight steering for the first part of the turn - good for not to oversteer at speed. As the teeth get further away from the centre, they increase in spacing slightly so that the wheels turn more for the same turn of the steering wheel towards full lock. The pinion used is helical pinion against a variable rack. The steering takes three turns to move from lock to lock positions. The material used for making gears is 50Cr4. We use feather key for the removal of steering wheel when desired. The angle of inclination of steering shaft is 35 deg with horizontal. We set the wheel to have negative camber angle of about 1 deg to generate more grip on the tires. KPI (King pin inclination) is the angle of the king pin when viewed from the front. We set angle to 10 deg in order to have sufficient jacking effect when the car is steered. Caster is an angle that gives directional stability to the car in straight ahead position, and this contributes to weight jacking when the steering is turned. So, we provide a positive caster angle of about 4

o

SUSPENSION For FSAE, since handling is given a higher priority over ride quality, a very stiff suspension with very good anti-roll properties is required to carry more speeds around the corners. To minimize the car’s weight, a light suspension is preferred. As per the FSAE rule book we’ve designed our suspension for a jounce of 2 inches and a re-bound of 1 inch. As this car is going to be used by weekend Autocross racer the suspension should be easily tunable.

Front Suspension: Double Wishbone, Coil over Strut (outboard), Torsion Anti roll bars Rear Suspension: Double Wishbone, Coil over Strut (outboard), Torsion Anti roll bars

Owing to the lower weight of wishbones, to maintain a lesser ground clearance, we chose Double Wishbone Suspension. The Double Wishbone is made of Mild Steel with Carbon Content of nearly 0.2%. Another advantage of the double wishbone suspension is its kinematic possibilities. The positions of the suspension control arms relative to one another can determine both the height of the body roll centre and the pitch pole. We’ve incorporated a coil over strut for each wishbone. After analysis, the Yamaha R15’s strut, amongst the stiffest in India, was chosen. The strut would be mounted outboard in both the front and rear suspensions. The advantage of using Monocross Suspension Strut is that it is stiffer than a normal strut, with a suitable length of 20 cm. Also the stiffness of a monocross suspension can be easily adjusted. Though inboard mounting of struts is preferred in Formula cars, we’ve gone for outboard mounting as it is cheaper as it eliminates the necessity of bell cranks and push rods. Another advantage is easy accessibility for tuning. Torsion anti roll solid bars of higher torsional stiffness has been selected. After a study of the track layout in the proposed venue for the 2011 competition, the toe-in setting, which ensures better grip around the corners and better straight line stability, was chosen. A positive caster of 4

0

has been used to ensure self straightening of the steering. Tie rod from the rack of steering column is attached to wheel hub, which serves to easily adjust the toe setting of the front wheels. An additional tie rod is deliberately given in the rear suspension between space frame and wheel hub to adjust the toe setting of the rear wheels. In both front and rear suspensions, the frame pivots are bolted to the frame using long bolt nuts. So for the adjustment of camber, shim plates are given in between pivot clamps and frame in the bolts. Suspension Analysis: Elastokinematic properties such as wheelbase, track, roll centre, camber, and toe-in were analyzed with the aid of suspension analyzer. The inputs provided were

• Height and location of the ball joints which hold the wheels to the wishbone

• Spring rate

• The point of spring mounted on the frame, which was determined from the CATIA model. With these inputs, properties such as length of A-arms, static and dynamic camber, roll centre and the estimated track width were calculated. A sample of the tests are shown in Appendix B

BRAKES The brake system of a FSAE vehicle should have the following requirements

• Provide effective and safe braking at various speeds and situations.

• Have minimal weight.

• Ignition should get cut automatically when brakes fail.

• Providing a switch using which the racer may cut off ignition manually, as per FSAE rules.

• When brakes are applied, a clearly visible light should glow at the rear of the car, as per FSAE rules.

Of the two basic types of hydraulic brakes available (drum and disc), we will use disc brakes for all 4 wheels as they provide better braking and superior heat dissipation.

As per FSAE rules, one pedal (foot pedal) can

activate all 4 brakes. The foot pedal will activate a tandem master cylinder which can apply hydraulic pressure to all 4 brakes with a bias of 70% to the rear wheels and 30% to the front wheels. This mechanism, achieved by flow control valves, is designed to provide optimum turning and cornering capabilities to the rear loaded car.

If higher braking torque is needed at any time, as in

the case of emergency stops, there will be a separate lever/pedal close to the racers hand, which will activate the front brakes to their full potential. The detailed circuit is shown below in Appendix A.

Automatic cut-off of engine when brake line fails is

achieved by means of 4 pressure transducers. These continuously generate a small current as long as there is no drop in pressure below a specified value in the brake lines close to the cylinders (a sign of brake failure) and a series of AND logic gates leading to ignition system. Manual ignition cut-off is also provided by means of a simple normally closed switch.

With such a system we calculated a satisfactory

stopping distance of 104 ft from a speed of 60 kmph. CONCLUSION Since alleviation of safety concerns is one of our main

priorities, utmost importance was given to the design of the roll cage and extensive ANSYS analysis was done to ensure that our frame was devoid of any glaring safety issues. Special attention was paid to the optimization of intake air flow and a highly satisfactory intake plenum design was procured by employing CFD using the FLUENT software. Recognizing the importance of vehicle handling in an event like the FSAE, a variable steering rack a suspension system whose Elastokinematic properties were thoroughly analyzed using the SUSPENSION ANALYZER software. Again, keeping safety in mind, an additional brake lever has been incorporated to provide immediate additional braking force to the front wheels in case of emergencies.

Most importantly, the design of the vehicle was strictly in accordance with the FSAE rules.

ACKNOWLEDGEMENTS

Team CEG would like to thank and acknowledge the following persons for their support and guidance.

1. Faculty Advisor – Dr. G. Nagarajan.

REFERENCES

1. 2009 FSAE rulebook and 2011 Formula

SAEINDIA rulebook.

2. Fundamentals of Vehicle Dynamics – Thomas

D. Gillespie

3. Race Car Vehicle Dynamics – Millikan

FRONT VIEW OF THE VEHICLE (CATIA DESIGN)

SIDE VIEW

TOP VIEW

APPENDIX A

APPENDIX B

Rear Suspension (Front View) Front Suspension (Front View)

Front Suspension (Top View) Front Suspension (Side View)

APPENDIX C

Deflection due to Rear Impact

Deflection due to Front Impact

Deflection due to Side Impact

APPENDIX D

Velocity Analysis

Pressure Analysis

Analysis of flow magnitude