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Chassis

Introduction

Purpose and GoalsThe chassis is the backbone of the Mini-Baja; it must support all the car’s

subassemblies as well as protect the driver. The chassis design is crucial to the success of

the project because if the chassis fails, that puts the Baja and the driver at tremendous

risk. The goal of the 2008 Mini Baja frame will be to protect the driver, offer sturdy

mounting for all subsystems, maintain all SAE rules and regulations, and still be

lightweight.

BackgroundChassis design in the SAE Mini Baja competition is critical for a winning car.

These cars are all powered by 10 horsepower Briggs and Stratton engines. To extract

maximum acceleration from this engine a lightweight chassis is necessary. At the same

time the chassis must undergo the rigors of off-road racing. To analyze a structure that

will undergo such loads, finite element analysis (FEA) is often a viable solution. FEA

breaks the structure into smaller elements and analyzes each element as a body and can

calculate the stress, deflection and other reactions of any structure. A Transient FEA will

be the main simulation done to optimize the weight and strength of the 2008 Mini Baja

Chassis.

From here forward a few assumptions have been made to aid design and analysis.

The total weight of the Mini Baja car, without a driver, is estimated to be 400 lbs, with

the lightest car weighing 318lbs. [] This is the average weight of the most competitive

school’s cars from 2007. The driver will be referenced as a 6 foot 3 inch tall male

weighing 250lbs, as per the SAE rules. []

Design Objectives

To build a chassis to meet all of the previously mentioned goals the frame must:

• Endure the maximum dynamic load according to SAE Technical paper 2006-01-

3626[] with a factor of safety of 1.5

• Keep a driver alive during a 7.9g front impact

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• Keep a driver alive during a 7.9g side impact

• Keep a driver alive during a 7.9g Roll over situation

• Abide by all SAE rules and Regulations

• Weigh less than 80lbs

• Have mounting structures for all subsystems that will withstand the loads

produced by those subsystems.

SAE Rules and RegulationsAll SAE rules and regulations can be found in Appendix A. The Frame rules are

specifically in “SECTION 3 ROLL CAGE, SYSTEMS & DRIVER’S EQUIPMENT.”[]

Some highlights of that section are:

• “The driver’s helmet to be 15.24 cm (6 in) away from the straightedge applied to

any two points on the cockpit of the car, excluding the driver’s seat and the rear

driver safety supports.”[]

• “The driver’s torso, knees, shoulders, elbows, hands, and arms must have a

minimum of 7.62 cm (3 in) of clearance from the envelope created by the

structure of the car.”[]

• Fit a 95% Male driver, while maintaining all constraints above.

• The LBD, LFS, SIM, FAB, and FLC must be at minimum 0.035 wall thickness

tubing with a minimum outside diameter (O.D.) of 1 inch.

• The RRH, RHO, FBM, and LC must be “(A) Circular steel tubing with an outside

diameter of 2.5 cm (1 inch) and a wall thickness of 3.05 mm (.120 inch) and a

carbon content of at least 0.18%”, or “(B) Steel members with at least equal

bending stiffness and bending strength to 1018 steel having a circular cross

section with a 2.5 cm (1 inch) outer diameter and a wall thickness of 3.05 mm

(.120 inch).”[]

Figure 2 below displays the location of each frame member referred to above.

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Figure 2: Frame Members

Maximum Dynamic LoadThe loading conditions used to analyze the Chassis are just as important as the

analysis itself. The more accurate the load the smaller the factor of safety can be used.

The 2008 Mini Baja Chassis team purchased SAE technical paper 2006-01-3626[],

Structural Considerations of a Baja SAE Frame. In this paper Auburn University

students used load cells to measure the amount of force translated into a Mini Baja Frame

during two loading conditions. These load cells, shown in Figure 3, gave a load data on a

time scale shown in Figure 3 for a “single wheel vertical impact” in Figure 4.

Figure 3: Left - Load Cell [], Right - Vertical Impact Graph []

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Figure 4: Vertical Impact Demonstration []

This data can be directly used on the frame during a transient FEA analysis. Because the

material that will be used for analysis, chromoly steel, is a well-known material and the

loads and stresses that are being used for the shock force analysis are from actual test

cases a factor of safety of 1.5 is used for the analysis of the frame. (See Appendix B for

factor of safety justification)

7.9G Impact Using data from “The Motor Insurance Repair Research Centre” [] the estimated

maximum g-force that the Mini Baja car will see is 7.9g’s. The Motor Insurance Repair

Research Centre did rear impact tests for vehicles of different masses. To estimate the

maximum g-force on the Mini Baja car the g-force of the closest in weight car will be

used. The mass and g-force of each car used in the impact test is shown in Table 1. []

Table 1: Left - Impact Acceleration [], Right - Vehicle Mass []

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The mass of the Mini Baja car is 20.2 slugs, therefore the g-force that will be used for

analysis is 7.9g’s(the car with the closest mass(OW3718)). These values are highlighted

in Table 2 below.

Delta V MPH pulse(ms) g-force Mass of Car(slugg)6.337986158 92 3 81.540901546.835083112 68 4.4 99.35656078.077825496 84 4.4 134.64527028.264236854 80 4.9 99.013951878.947745165 72 5.7 92.29881889.258430761 129 3.3 102.302996610.31476179 130 3.6 102.302996610.62544738 65 7.9 69.2069836610.62544738 103 4.9 94.8341241410.6875845 89 5.5 96.2730812310.6875845 74 6.6 91.7506446710.6875845 116 4.1 99.3565607

11.24681858 65 7.9 123.339178811.43322993 82 6.4 119.913090511.43322993 93 5.6 98.6028212712.05460112 93 5.6 100.1103001

Table 2: Vehicle Mass and Acceleration []

Frame WeightTo calculate the maximum frame weight, the estimated values of each subsystem were

added up and subtracted from an estimated total vehicle weight of 400lbs. This resulted

in a frame weight of 80lbs as shown in Table 3.

Compon ent Weigh t(lb)

Tir es 50

Suspe nsi on 50

Stee ri ng 15

Engin e 60

Transm issio n 60

Re ar Housi ng 40

Bo dy Panel s 10

Skid Plate 10

Ele ctro ni cs 5

Br akes 20

Total 320

Tar ge t 400

Frame we ight 80

Table 3: Estimated Vehicle Weight

Design and Analysis

Material SelectionTo build the Mini Baja frame steel must be used according to the rules. There are

many different types of steel available to the public. The frame of the Mini Baja will be

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made from tubular sections. Tubular sections offer superior loading capabilities per

pound when compared to solid sections or square sections.

The material selection criteria are very similar, so the suspension material

selection chart will be used to choice a material for the chassis. According to that

material selection chart (Table 16 on page 69) 4130 steel is the preferred material for use

in the Mini Baja frame.

Roll Cage TubingAccording to Roll Cage Material Specifications 31.5 in the SAE Rules [Appendix

A] the roll cage must “(A) Circular steel tubing with an outside diameter of 2.5 cm (1

inch) and a wall thickness of 3.05 mm (.120 inch) and a carbon content of at least

0.18%”, or “(B) Steel members with at least equal bending stiffness and bending strength

to 1018 steel having a circular cross section with a 2.5 cm (1 inch) outer diameter and a

wall thickness of 3.05 mm (.120 inch).”[] From the material selection chart 4130 alloy

steel was chosen as the material to be used for the entire frame. There is a note in the

rules about the use of alloy steel: “NOTE: The use of alloy steel does not allow the wall

thickness to be thinner than 1.57 mm (.062 inch).” To allow the use of this alloy steel an

equivalency calculation must be made to that of 1018 steel. This calculation is

demonstrated below in Equation 1.

“E = The modulus of elasticity” []

“I = The second moment of area for the cross section about the axis giving the lowest

value” []

“Sy = The yield strength of material in units of force per unit area” []

“c = The distance from the neutral axis to the extreme fiber” []

ID = Inside Diameter

stiffB = Bending Stiffness

strengthB = Bending Strength

For 1018 Steel:

E =29700ksi

I = ( )44

64IDc −×

π

= ( ) 444 in0.0335558375.0164

=−×

π

Sy =53.7ksi

24

c =1inch

( )( ) 71801.94805

103355583.0107.53

996608.1403355583.010297003

3

=××==

=××==

cSyIB

EIB

strength

stiff

Equation 1: Bending Stiffness and Strength

In Table 4 below the bending stiffness and strength were calculated for available sections

of 4130 steel tubing. The available sections were found using a popular metal vendor. []OD(in) ID(in) Moment of Inertia(in 4̂) bending stiffness(lb*in 2̂) bending strength(lb*in) Weight(lb/ft)

1 0.75 0.03355583 996608.14 2117.372856 1.1710286621.125 0.959 0.037109744 1102159.40 2081.444313 0.925966491.25 1.134 0.038667283 1148418.30 1951.924434 0.7402078861.25 1.12 0.042602298 1265288.25 2150.564008 0.8246718411.25 1.084 0.052064518 1546316.18 2628.216858 1.0370469231.25 1.06 0.057870556 1718755.52 2921.305677 1.1747759531.25 1.01 0.068761719 2042223.06 3471.091583 1.4518078771.5 1.37 0.075582124 2244789.09 3179.488024 0.9986532431.5 1.31 0.103942577 3087094.54 4372.517738 1.4290564631.5 1.26 0.124781421 3706008.21 5249.138454 1.77300431

1.625 1.509 0.08775855 2606428.95 3407.732022 0.97307531.625 1.459 0.119852623 3559622.91 4653.969554 1.3702882221.625 1.435 0.134130926 3983688.51 5208.407041 1.5561967171.625 1.385 0.161660136 4801306.03 6277.387421 1.933602526

1.5 1.43 0.043240292 1284236.66 1818.974939 0.5489782371.5 1.402 0.058850911 1747872.06 2475.661658 0.7612248391.5 1.37 0.075582124 2244789.09 3179.488024 0.998653243

Table 4: Bending Stiffness and Strength of Available Tubing

The lightest section is in bold. This is the 1.25 O.D. tubing with a 0.065 thickness wall.

This is the lightest usable tubing that is still within all of the SAE restrictions.

The other members of the roll cage(frame) will use at a minimum 1.00 inch O.D.

tubing with a 0.035 wall thickness as per the minimum requirements in section 31.2.1[].

Design

The design of the chassis has to incorporate two major things, driver comfort and

safety and subsystem mounting. The driver safety for the design section is simply

satisfying all of the SAE frame rules.

Driver Comfort and safetyTo verify that a driver of 6 foot 3 inch, 250lbs would fit comfortably into the

frame, a model of that person was drawn. The driver was then placed in the frame in the

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driving position as in Figure 5. While this driver was in the frame measurements were

made to make certain all of the SAE safety rules were satisfied.

Figure 5: 95% Male in Mini Baja Frame

Subsystem MountingTo mount the subsystems that subsystem was first placed in space in relation to

the frame with mounting tabs at the mounting location. Finally piping was routed to the

mounting tabs of that subsystem. This two step process is demonstrated with the

engine/gearbox assembly in Figure 6.

Figure 6: Two step subassembly mounting process step 1 on the left and step 2 on the right

Initial DesignUsing the rules, subsystem dimensions, and a 95% male as the driving factors a

frame was designed using the minimum frame tubing specified in the Roll Cage tubing

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section above. The total weight of the initial frame design is 49.875lbs. This frame is

displayed below in the left of Figure 7 without subsystems, and the right of Figure 7 with

subsystems.

Figure 7: Initial Frame without subsystems (left) and with subsystems (right)

Analysis

Shock Load

As stated before, a transient FEA analysis is preformed on the frame using the

force vs. time chart shown previously in Figure 3 on page 21. This force is broken up into

X and Y values because the shock is not mounted vertically. The maximum and

minimum angle of the shock (theta) in relation to the frame is 20.71 and 23.07 degrees

respectively. The calculation of the X and Y forces are shown in Equation 2, and the

resulting forces are listed in Table 5. This force will be applied to the front and rear

shock mount points individually.

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Figure 8: Shock angle in relation to the frame

)cos()sin(

θθ

FFFF

Y

X

==

Equation 2: Calculation of X and Y components of the Shock Force

TIME(s)Load(lb)TIME(s)Load(lb) TIME(s)Load(lb)

0.05 112.5 0.05 44.0836875 0.05 105.230475

0.075 112.5 0.075 44.0836875 0.075 105.230475

0.1 400 0.1 156.742 0.1 374.1528

0.125 1280 0.125 501.5744 0.125 1197.28896

0.15 800 0.15 313.484 0.15 748.3056

0.175 1460 0.175 572.1083 0.175 1365.65772

0.2 390 0.2 152.82345 0.2 364.79898

0.225 -450 0.225 -176.33475 0.225 -420.9219

0.25 150 0.25 58.77825 0.25 140.3073

X Force Y ForceTotal Force

Table 5: Shock Force X and Y Components

For the FEA analysis the mass of the driver and engine/transmission are also

modeled in the FEA program. To satisfy the SAE rules section 20.2 vehicle

configurations the driver is assumed to weigh 250lb. [] The engine and transmission was

weighed and found to be approximately 100lbs. The density of the frame tubing

(0.284lb/in^3) is also modeled with acceleration due to gravity taken into account.

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The order of frame tubing used is listed below in Table 6. These are all readily

available sections according to our online retailer. [] They are listed in order of increasing

weight pre foot. If a section of tube needs to be larger to reduce stress the next size up

from Table 6 is used. This will insure that the lowest weight frame is created.

OD THICKNESS Wieght/FT

Cross Section

Reference Number

1 0.035 0.359 1

1 0.049 0.5042 2

1 0.058 0.5912 3

1.125 0.058 0.6579 4

1.25 0.058 0.748 5

Table 6: Frame tubing sections and reference numbers

Front Shock LoadThe Initial frame was analyzed using the shock force at the front shock mount as

shown in Figure 9.

Figure 9: Front Shock Force

The results are shown in Table 7, with the graph of maximum von-mises stress shown in

Figure 10.

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Init ial CASE Init ial Test with Minimum Tubing Thicknesses MAX Stress = 42066.66667

Time Maximum Von Misses Stress Location Location Reference # FAIL?

0 2337 Cross Bar 1 PASS

0.05 4614 SIM Triangle Support 2 PASS

0.075 4809 Cross Bar 1 PASS



0.15 44062 SIM Triangle Support 2 FAIL

0.175 54093 Cross Bar 1 FAIL

0.2 52720 Cross Bar 1 FAIL

0.225 49861 SIM at RRH 3 FAIL

0.25 38674 SIM at RRH 3 PASS

Table 7: Maximum Von-Mises Stress for Front Shock Force

Figure 10: Time vs. Stress for Front Shock Force

To change the stress at the times that the frame fails the tubing sections were modified or

braces were added. The first time optimized was at 0.15s. At time = 0.150sec the

maximum von mises stress was 44062psi and is located at the SIM Triangle Support as

shown in the left of Figure 11. To decrease the stress value in the SIM the SIM tubing

size was moved up to tubing cross-section 2 from cross-section 1. This reduced the

maximum stress to 35400psi as shown in the right of Figure 11.

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Figure 11: Left - Initial Von-Mises Stress at time 0.15sec, Right - Stress after increased SIM size

At time = 0.175sec the maximum von mises stress was 54093psi and is located at the

cross bar that connects the left and right front shock mounts as shown in the left of Figure

12. To decrease the stress value in the cross bar the tubing section was moved up to

cross-section 2 from cross-section 1. This reduced the maximum von mises stress to

42321psi as shown in the left of Figure 12. This is still above the maximum stress of

42066psi, so to further reduce the stress the cross bar section was increased to cross-

section 3, which reduced the maximum von mises stress to 38372psi as shown in Figure

13.

Figure 12: Left - Initial Von-mises Stress at time 0.175sec, Right - first revision at time 0.175 sec

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Figure 13: Final revision at time 0.175

At time = 0.200sec the maximum von mises stress was 52720psi at Cross Bar as shown

in the left of Figure 14. Due to previous changes in the Cross bar and SIM section size

the maximum von mises stress was reduced to 42852psi and moved to the SIM at the

RRH as shown in Figure 14. This is still above the maximum 42066psi, so a small

support of cross-section 1 tubing was added between the RRH and SIM. This reduced

the maximum von mises stress to 39818psi as shown in Figure 15.

Figure 14: Left - Initial Von-mises stress at time 0.200sec, Right - first revision at time 0.200sec

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Figure 15: Final revision at time 0.200sec

At time = 0.225sec the maximum von mises stress was 49861psi and is located at the

SIM and RRH as shown in the left of Figure 16. Due to previous changes in the frame’s

tubing sections the maximum stress reduced to 40656psi as shown in the right of Figure

16.

Figure 16: Left - Initial Von-mises stress at time 0.225 sec, Right - Final revision at time 0.225sec

The changes to the frame to withstand the front shock load resulted in a gain of 8.381 lbs

giving a total weight of 58.256lbs. The new stress vs. time graph is shown below in

Figure 17 and the data in Table 8.

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Second Test with Changes at Maximum Values MAX Stress = 42066.66667


0 2184 Cross Bar 1 PASS







0.2 39818 Bottom Spot 4 PASS

0.225 40656 Bottom Spot 4 PASS

0.25 33462 Front Triangle 5 PASS

Table 8: Maximum Von-mises stress, time, and location after front shock force optimization

Figure 17: Time vs. stress of Frame after front shock force optimization

Rear Shock LoadThe frame was analyzed using the shock force at the rear shock mount as shown in Figure

18.

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Figure 18: Rear Shock Force

The results are shown in Table 9, with the graph of maximum von-mises stress shown in

Figure 19.CASE 1 Init ial Test with Minimum Tubing Thicknesses MAX Stress = 42066.66667

Time Maximum Von Misses Stress Locat ion Locat ion Reference # FAIL?

0 10402 FAB at RRH 1 PASS

0.05 18831 FAB at RRH 1 PASS



0.125 143664 FAB at RRH 1 FAIL





0.25 61185 Middle of FAB 2 FAIL

Table 9: Initial Von-mises stress of rear shock load

Figure 19: Time vs. Stress Graph of Initial rear shock load

35

The maximum von mises stress at 0.125sec is 143664psi, and is located on the FAB at

the RRH as shown in the left of Figure 20. The FAB is in need of a bracing bar to

transmit the force from the shock. A bracing bar of cross-section 1 was added at shock

point to the RRH, this lowered stress to 69736psi and moved maximum stress point to the

FAB at the shock point. This is shown in the right of Figure 20. The tubing section of

the FAB was increased until the maximum stress went down to 38282psi with cross-

section 4. This is shown in Figure 21.

Figure 20: Left - Initial Von-mises stress at time 0.125sec, Right - first revision at time 0.125sec

36


At time = 0.150sec the maximum von mises stress was 149620psi and was located at the

RRH on the FAB as shown in the left of Figure 22. Due to previous changes in the

frame’s tubing sections the maximum stress reduced to 39854psi as shown in the right of

Figure 22.

Figure 22: Left - Initial Von-mises stress at time 0.150sec, Right - Final revision at time 0.150sec

At time = 0.175sec the maximum von mises stress was 120479psi and was located at the

RRH on the FAB as shown in the left of Figure 23. Due to previous changes in the

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frame’s tubing sections the maximum stress reduced to 52332psi as shown in the right of

Figure 23. The FAB tubing was increased to cross-section 5 to reduce the stress to 41165

as shown in Figure 24.

Figure 23: Left - Initial Von-mises stress at time 0.175sec, Right – first revision at time 0.175sec


At times 0.200, 0.225, 0.250sec the maximum stresses were reduced to 27442, 21503,

and 30276psi respectively due to previous changes. These results are displayed in Figure

25 and Figure 26 respectively.

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Figure 25: Left - Final Revisions at time 0.200sec, Right - Final revision at time 0.225sec


The changes to the frame to withstand the rear shock load resulted in a gain of 6.158 lbs

giving a total weight of 64.414lbs. The new stress vs. time graph is shown below in

Figure 27 and the data in Table 10.

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CASE 2 Second Test with Changes at Maximum Values MAX Stress = 42066.66667


0 2614 Shock mount on FAB 3 PASS

0.05 4458 Shock mount on FAB 3 PASS







0.225 21503 FAB at LC 4 PASS

0.25 30276 FAB at LC 4 PASS

Table 10: Maximum Von-mises stress after rear shock optimization

Figure 27: Time vs. Stress after rear shock load optimization

Subsystem SupportThe frame is analyzed at the mounting points of the suspension. The suspension

sub system translates the greatest reaction forces of all the mounted systems. The

reaction forces gathered from the suspension sub team’s FEA will be applied at the

mounting points. Table 11 summarizes those reaction forces and Figure 29 and Figure 30

displays the stress results. The rear suspension forces are added together and applied at

the same time because both the upper and lower arms are connected to the same mount as

shown in Figure 28. The forces and moments were applied dynamically to the frame.

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Reaction Moments(lb*in)

A-Arm Mounting Area X Y Z MY

Front -568 0 0 0

Rear -48 0 0 0

Front -116.715 903.95 78.38 0

Rear -116.715 903.95 78.38 0

Rear Upper Single Mount -616 0 0 1560

Front -48.5895 338.515 32.115 0

Rear -48.5895 338.515 32.115 0

Front -48.5895 338.515 32.115 0

Rear -664.5895 1242.465 110.495 1560Rear Total

Reaction Forces(lb)

Rear Lower

Front Lower

Front Upper

Table 11: Suspension Reaction Forces

Figure 28: Rear Suspension Mount

Figure 29: (Left) Front Upper A-arm Stress, (Right) Front Lower A-arm Stress with support

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Figure 30: (Left) Initial Rear Suspension Stress, (Right) Final Rear Suspension Stress

The stress values were initially above the max allowable 42,067psi for the front

lower analysis, so a bar of cross-section 1 was added as shown in right of Figure 29. The

rear suspension section was changed from section 1 to section 2 to stay below the

allowable stress.

Impact LoadingTo calculate the forces used to analyze the 7.9g impact and newton’s second law

is used. The force calculation is shown below in Equation 3.

( ) ( ) lbf

ftfta

slugsmmaf

476.513838.2542.20sec

38.254sec

2.329.7

2.20

=×=

=×=

==

Equation 3: Newton’s Second law for Impact Force

The 5113.038lb force will be used in a transient FEA. The force will be ramped over a

pulse of 0.065sec. [] The impact analysis is done to verify that no frame member will fail

during the impact and the driver will therefore remain safe. The ultimate tensile strength

of the chromoly steel being used is 92,700psi. As long as the stress remains below this

value the frame will be considered safe enough. The displacement of the pipes will be

also be monitored to verify that the driver will not be harmed by any protruding bars

during the impact. SAE rules [] state that the driver’s arms etc must be at minimum three

inches from all structural members of the frame. Therefore any displacement values over

three inches will be deemed harmful to the driver.

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Front & Side Impact

Only one modification was made to the frame during Front and Side impact

analysis. During the side impact force the part of the LFS that meets the RRH needed

extra bracing of 1 inch O.D. and 0.035 wall thickness to keep the stress below the

ultimate tensile of 92,700psi. The front and side impacted are summarized below in

Table 12. Maximum Vonmises Stress (psi) Time (sec) Maximum Displacement (in) Setup Figure Stress Figure

Front Impact 90717 0.00345 0.041166 Figure 28 Figure 28

Side Impact 105770 0.069 0.505313 Figure 29 Figure 29

Side Impact after Gussett Revision 79985 0.069 0.510962 Figure 30 Figure 30

Table 12: Side and Front Impact Summary

Figure 31: Left - Front Impact Force, Right – Front Impact Stress

Figure 32: Left - Side Impact Force, Right - Side Impact Stress

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Figure 33: Left - Supporting Member added to LFS at RRH, Right - Revised Side Impact Stress

Roll Over

The force used for the roll over analysis is dependent on the height of the drop.

The purpose of the roll over analysis is simply to rate the frame for a certain drop height.

The starting point will be a ten for drop. The force used to anlyze the frame at that height

is gathered using data from “The Motor Insurance Repair Research Centre.” [] A impact

velocity can be calculated using Equation 4. That impact velocity is then used to

interpolate a impact acceleration and pulse using data from “The Motor Insurance Repair

Research Centre.” []

PE = Potential Energy

KE =Kinetic Energy

h =drop heightv =impact velocity

mghmv

mvKE

mghPEKEPE

=

=

==

2

2

21

21

Equation 4: Impact Velocity

For a ten foot drop the impact velocity is calculated to be 25.4 ft/s and impact

acceleration and pulse is interpolated as 6.8g’s and 70 miliseconds respectively. Using

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Equation 3 the impact force is calculated as 4423lb. The resulting stress is shown in

Figure 34.

Figure 34: (Left) Roll Over Force, (Right) Roll Over Stress for Ten Foot Drop

The stress values for the ten foot drop is 102,840psi which is above the UTS of

92,700psi. The frame is next analyzed at a eight foot drop. For a eight foot drop the

impact velocity is calculated to be 22.7 ft/s and impact acceleration and pulse is

interpolated as 6.3g’s and 72.5 miliseconds respectively. Using Equation 3 the impact

force is calculated as 4120lb. The resulting stress is shown in Figure 35.

Figure 35: Eight Foot Drop Stress

The stress value for the eight foot drop is 80,054psi which is below the UTS of

92,700psi. So our frame will be rated for a maximum roll over drop of eight feet. During

the Mini Baja race the car should not have to undergo a roll over drop over five feet. []

Design for ManufacturingDuring initial design, design for manufacturing was considered. The Florida Tech

Machine shop has all of the necessary tools to cut, bend, cope, and weld chromoly tubing.

The shop only has a limited size of dies for the tube bender. Therefore tubing bend

45

radius is limited by the tubing outside diameter as per Table 13. The initial frame design

contained these constraints. There are no necessary revisions to the frame due to

manufacturing.Tubing O.D. Bend Radius

0.5 2.5

1 3

1.25 3.5

Table 13: Available Bend Radii

Fabrication Process

To build the chassis chromoly tubing was purchased in eight foot sections then,

cut, bent, coped and welded in the appropriate spot on the car.

To cut and bend the tubing a chop saw and tube bender were provided in the

Florida Tech Machine shop. The tubes were then coped to fit a mating tube using a tube

notcher as shown in Figure 36 below.

Figure 36: Left to Right - Tube Bender, Chop Saw, and Tube Notcher

Detailed Drawings

Detail Drawings for the frame can be found in Appendix H.

46

BudgetDescription Price QT Total Vendor

TUBINGChromoly tubing (1.25x0.065) $197.40 48ft $197.40 onlinemetals.com

Chromoly tubing (1.00x0.035) $91.68 32ft $91.68 onlinemetals.com




Chromoly square tubing (0.50x0.035) $1.61 2ft $1.61 onlinemetals.com

TABSFront suspension - 11 Guage Radious tabs - PN2514 $2.00 20 $40.00 Tabzon Chassis Components

Rear suspension - Alloy Steel Sheet 4130 ANNEALED 0.1 thickness (12''x12'' Section) $20.85 2 $41.70 onlinemetals.com

Transmission/Engine - 11 Guage Radious tabs - PN2514 $2.00 5 $10.00 onlinemetals.com

Body Panels - Alloy Steel Sheet 4130 ANNEALED 0.05 thickness (12''x12'' Section) $9.29 1 $9.29 onlinemetals.com

Various - Alloy Steel Sheet 4130 ANNEALED 0.05 thickness (12''x12'' Section) $9.29 1 $9.29 onlinemetals.com

TOTAL $481.51

Table 14: Chassis Budget

Manufacturing and assembly of the frame will be done by the students using

Florida Tech resources (welder, welding rod, coping tool, and mill). The school does not

require payment for these resources; therefore they are not listed above in the budget.

The complete budget with manufacturing, assembly and retail part cost can be found in

Appendix I.

Design Changes

During the spring 2008 Spring semester minimal design changes were done to the

chassis. Due to changes in the transmission type the rear end of the car had to be

redesigned. The dame FEA analysis was performed on the chassis to verify the changes

did not modify the results. The old and new rear end is shown Figure 37.

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Figure 37: Chassis Rear End Before (left) and after (right) transmission reselection

Scheduling

Plan for CompletionAs of now the car is in racing condition, and has passed initial testing. Further

testing will be done to prove the chassis has satisfied all nondestructive design

constraints.

Gantt ChartThe Gantt Chart for the chassis section can be found in Appendix D.

Conclusions

After shock load, and impact analysis on the frame it is ready to endure a Mini

Baja race and fulfill all of its goals. Weighing in at a manufactured total of 80lbs (extra

weight added due to welding) the frame will help the overall performance of the car

during competition. The chassis is completed and has gone through initial testing. This

includes low and high speed testing over rough terrain and small (1 foot) jumps. There

are plans for high speed extremely rough terrain and larger (4 foot) jumps testing. As of

now the chassis is ready for the SAE competitions. The final frame is displayed in Figure

38.

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Figure 38: Final Frame after Complete Analysis

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