car #8_bangalore institute of technology_team stratos

8/9/2019 Car #8_Bangalore Institute of Technology_TEAM STRATOS

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Paper Number 2

FINAL DESIGN REPORT

TEAM STRATOSTeam #8, Car #8

Copyright 2009 SAE International

ABSTRACT

The aim of this report is to highlight the final design ofTeam Stratos mini-Baja vehicle which will compete inBaja SAEASIA 2010.

The teams primary objective was to design a safe andfunctional vehicle based on a rigid and torsion-free rollcage and chassis, well mounted powertrain, anddynamically tested steering and suspension systems.The secondary objective was to enhance performanceand maneuverability of the vehicle.

The team was divided into core groups responsible forthe design and optimization of major sub-systems whichwere later integrated into the final blueprint. Current CADmodeling and FEA approaches were used.

INTRODUCTION

We approached our design by considering all possiblealternatives for a system & modeling them in CADsoftware like CATIA, AutoCAD etc. to obtain a modelwith maximum geometric details.

The models were then subjected to analysis usingANSYS FEA software. Based on analysis results, themodel was modified and retested and a final design wasfrozen.

Dynamics analysis was done in Lotus suspensionanalysis software and MSC ADAMS. The aim was tooptimize suspension variables to improvemaneuverability. Theoretical calculations of performancecharacteristics were also done.

Extensive weight reduction techniques were followed atevery stage of the design to improve performancewithout sacrificing structural integrity.

DESIGN OF MAJOR SYSTEMS

FRAME DESIGN

Material chosen for the frame is ASTM A106 schedule40 steel with a radius of 1 and a wall thickness of 3mmSome bracing members have 2mm wall thickness. Inaccordance with section 31.5 sub-section A of the rulebook, the material chosen has a carbon content of0.265% which is >0.18%.

Joining method used will be Flux Metal Arc WeldingThis method was compared with Metal Inert GasWelding and found to be giving welds of equal strengthFMAW was chosen since it is more economical.

The earlier frame design is shown below. The forcesused in its analysis were too low in magnitude. Newvalues of impact and torsional forces were calculated.

fig i

When the entire powertrain was modeled, the enginebay area was found to be insufficient. A mockup of thepowertrain was done and the engine bay was resized.

When space between A-pillars was increased to improvefield of vision, the rigidity of the frame was significantly


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reduced. Also, front structural members were toocomplicated to manufacture. Therefore, the front of thecar was redesigned.

The new frame design is shown below. This wassubsequently analyzed in ANSYS for frontal impact,torsion and rollover tests.

fig ii (a & b)

Frontal Impact Test:

For a perfectly inelastic collision, energy transferred isDE = (m1m2/m1+m2)(u2-u1)

2where m1 and m2 are

masses of two vehicles and u1 and u2 arecorresponding velocities. Assuming m1=m2=350kg andu2=0 (vehicle at rest),

DE = 1/4 m1u12& F=DE/t where t=100ms

Then, F= [.25 x 350 x (16.67)^2] / [10x.1] = 24315N

Hence, a frontal impact force of 6000N was applied at 4

points on the frame. The back of the frame wascompletely constrained.

The deformation and stresses are shown below. For astress of 67MPa, the FOS obtained was 5.15.

fig iii (a & b)

Torsion Test:

For torsion test, a force equivalent to the gross weight ofthe vehicle (3500N) was applied at one of the 4 cornersof the frame while constraining the other 3.

Deformation and stresses were as follows. For a stressof 163MPa, the FOS obtained was 2.12


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fig iv (a & b)

Rollover Test:

In the rollover test, a force equivalent to the gross weightof the vehicle (3500N) was applied to one of the top

corners of the frame while constraining the base.

For a stress of 36MPa, the FOS obtained was 9.58.

fig v (a & b)

SUSPENSION DESIGN & WHEELS

A double wishbone suspension setup was chosen for thefront as well as rear as it is lightweight, independent andprevents deflection during hard cornering which ensuresthat the steering and wheel alignment stay constant.

Other types like McPherson strut and trailing arm wererejected because of weight considerations.

Wishbones:

Material used for wishbones is same as the framematerial. As seen below, for a 1KN force on the ball joinand shock absorber mounting, the max stress obtainedis 63Mpa, which gives a FOS of 5.46.

fig vi

For the rear upper arm, a force of 1KN was applied tothe hinges and the shock absorber mounting. Maxstresses were within limits.

fig vii

Hubs:

Front hubs are OEM and are made of cast iron with ahardened steel stub axle. Rear hubs are made of mildsteel (hardened). Rear hubs were designed toincorporate the double wishbone suspension and also toenable mounting of disc brakes.


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fig viii

Front and rear hubs were both analyzed for 3500N forceapplied at the bearings and were found to be withinlimits. Front hub shows a stress of 157MPa while therear hub shows a stress of 65Mpa. The design is wellwithin yield limits for the materials used.

fig ix

Shock Absorbers & Wheels:

Shock absorbers used are completely adjustable gasfilled dampers (OEM from Maruti Omni) coupled withcompression springs.

Wheels used are tubeless bias type having R10 175 infront and R10 250 in the rear. Rims used aremagnesium alloy.

Dynamic Analysis:

During wishbone design it was found that size of theengine bay and track width limitations were resulting inextremely short rear wishbone lengths. This would inturn limit travel of the shock absorbers and result in anextremely harsh ride and possible damage to the enginemounts. The back of the frame was then extended as anarrow portion to make longer wishbone lengthspossible.

Dynamic analysis was done on the front suspensionsetup to check the response of the vehicle for bump, inroll and while steering. Keypoints were obtained from theCAD model. Variables were tuned to reduce bump steercamber angles and wayward movement of roll center.

Bump:

fig x (a & b)

Above are the graphs for bump (mm) (x-axis) versus toecamber and castor angles. For a bump and rebound o100 mm each the camber was restricted within 0.5 degand toe within 2 deg. This minimizes the forces on theknuckle ball joints during bumps.

Roll:


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fig xi (a & b)

Values of toe angle, camber angle and roll center heightversus roll angle (deg) (x-axis) indicate that driver willexperience good control over the vehicle whilecornering.

Steer:

fig xii (a & b)

Steering angle (deg) (x-axis) vs. camber angle, toe angleand roll center indicates minimum deviations of all three.The Ackermann error is only 6%, which indicates anaccurate and responsive steering.

ENGINE & DRIVETRAIN

A Mahindra Alfa transmission (4 forward 1 reverse) wilbe used and will be directly coupled to the wheels. Gearratios will not be modified. Engine will be mounted onrubber bushings to reduce NVH characteristics.

Using a directly coupled final drive also enables theengine to be mounted as low as possible, thus loweringthe C.o.G of the vehicle.

STEERING & BRAKES

Steering is a rack and pinion system having a lock-to-lock of 2.5 turns. Steering ratio is 15:1 with Ackermannangles of 24deg and 36deg. The turning radius of thevehicle is 3.46m. The rack is placed ahead of the fronwheels center axis to improve handling.

fig xiii

Brakes are disc type in front and rear, with 180mm discsin front and 130mm in rear. Brake force is distributed via2 master cylinders so that system is independent.

SAFETY & ERGONOMICS

fig xiv

Shown above is the Impact Energy Diffuser (IED) usedin the front of the vehicle to absorb energy from impactsand prevent damage to the wishbones and tie rods. Iwill incorporate springs and dampers to absorb forcesand keep vehicle functioning after a crash.


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fig xv

The driver cabin is ergonomically designed keepinganthropometry in mind. The seating is adjustable. Shownabove is the tilt steering assembly used to providedifferent steering settings depending on the userspreferences. It utilizes a spring loaded lockingmechanism to hold the steering column in preset

positions. It can also be moved completely out of theway to enhance ease of ingress/egress.

EXPECTED PERFORMANCECHARACTERISTICS

POWER & TORQUE:

Power to Weight ratio is (10.72/275)*1000 = 39 bhp/ton.Torque is calculated as follows.

BRAKING DISTANCE:

Using OEM master cylinders & assuming force appliedby driver on pedal to be 85lbs = 386N, force on mastercylinder = 386 x 0.26 (dist in m from pedal to cylinder) =100.36N

Now, this is equal to F x ram length, i.e. 100.36=Fx.08so F=1254.5N

Then, pressure delivered by the cylinder P=F/A =1254.5/314.15e-4 = 39,933N/m^2

Assuming front:rear brake bias as 68:32 givesP(f)=27154.4N and P(r)=12788.6N.

Hence, force applied by the rear cylinder F(r) = P(r)*A =490.9e-4*12788.6 = 627.70N and similarly, F(f) =1333.1N.

Also Force applied on the discs by the cylinder F(R) =

2*F(r)* = 2*627.70*0.3 = 376.62N and F(F)=798.7N.

Which implies torque on each disc in the rear= T(R)=F(R)*Radius = 376.62*0.06 = 22.6N and that on the fron(with radius of the disc=0.08 m) T(F)=63.9N

Finally force per wheel in the rear becomes F(Rw) =T(R)/Radius of the wheel (R(w)) = 22.6/0.292 = 77.36Nand also F(Rr) = 218.72N.

Thus, net deceleration Acc=[2*F(Rw)+2*F(Rr)]/Weight othe vehicle(W) = 2(77.36+218.72)/3500 = 16.9m/s^2.

And, Stopping distance D(s) = V^2/2*a = (14*14)/2*16.9= 2.89m.

C.o.G & WEIGHT DISTRIBUTION:

C.o.G calculations were done by considering the originat the front end for X, at the chassis for Z and at thewheels for Y. The final value for Z was arrived at afteadding the ground clearance.


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Stability Analysis:

According to the National Highway & Traffic SafetyAdministration, most vehicle rollovers occur by trippingover low obstacles. For a Baja vehicle, this would alsobe the case. Then stability is obtained from the followinggraph.

fig xvi

Static Stability Factor (SSF) = T/2H where T= track widthand H= height of centre of gravity.

SSF=1324/(2x553)= 1.19

Using the graph, this gives our vehicle a four star rating.

FULL VEHICLE 3D VIEWS

fig xvii

fig xviii


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fig xix

VEHICLE TECHNICAL SPECIFICATIONS

CONCLUSION

This being Team Stratos first attempt at Baja SAE , outeams objective was to design and build a vehicle thatcan complete all competition events without failure. Aldesigns and calculations were done to realize this aim.

Reliability and safety were considered paramountkeeping the nature of the end-user in mind. Finally, a

high level of manufacturability was incorporated toensure feasibility for mass-production.REFERENCES

1. Chassis Engineering by Herb Adams2. Automotive Mechanics by Crouse Anglin3. Race Car Vehicle Dynamics by Millikens & MillikensCONTACT

Manish O. Team Captain 91-9844421914

Mokshith S.N Design Head 91-9611666646

Karthik N Marketing Head 91-9036227798

car #8_bangalore institute of technology_team stratos

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