MET2013 Hinojosa Ricardo

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2013 Bearcats Baja Front Suspension System

A Baccalaureate thesis submitted to theSchool of Dynamic Systems

College of Engineering and Applied ScienceUniversity of Cincinnati

in partial fulfillment of therequirements for the degree of

Bachelor of Science

in Mechanical Engineering Technology

by:

Ricardo Hinojosa

April 2013


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2013 Bearcats Baja Front Suspension System

Ricardo HinojosaFront Suspension Lead

Copyright 2007 SAE International

ABSTRACT

Baja SAE is an annual intercollegiate competition in

which teams race a single seat all-terrain vehiclepowered by a ten horsepower Briggs and Strattonengine. Each year several Mechanical EngineeringTechnology Students at the University of Cincinnatidesign and build the vehicle not only for the competitionbut also as a required senior design. As the lead for thefront suspension, I was faced with the challenge ofdesigning a 100% new front suspension for this years100% new car.

INTRODUCTION

The suspension system on an off-road vehicle servestwo main purposes; to maintain tire contact with theground while the vehicle traversers uneven terrain and toprovide a comfortable ride for the occupants of thevehicle. This document will highlight the logic andreasoning behind the front suspension design for the2013 University of CincinnatisBaja SAE vehicle.

Project Statement: To design, optimize, and fabricate afront suspension system that will allow this years BajaSAE Vehicle to successfully navigate the suspension

B1.2.2The vehicle must have adequate groundclearance and traction

B14.4Thread ExposureThreaded fasteners used must have at least two (2)threads showing past the nut.

B14.6Unmarked Fasteners / Shop ManufacturedFastenersAny threaded fastener (threaded rod, eye bolts, titaniumbolts, etc.) That is unmarked, or does not have anymarkings as listed, must be documented by either:(A) A purchase receipt and manufacturersdocumentation indicating that the fastener meets orexceeds Grade 5 standards for that size. OR(B) Equivalency calculations with a purchase receipt ortest data showing that the fastener exceeds the strengthfor a Grade 5 fastener of the same size.

CUSTOMER REQUIRMENTS

(For entire vehicle, not just suspension)

Track Width - Front: 56" (2012, 56")

Track Width - Rear: 54" (2012, 64")


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Design Research

Figure 1- Degrees of Freedom

Figure 2- Double wish bone suspension

Double Wish Bone Suspension

Advantages: Adjustability

C t

Figure 3- Twin traction beam

Twin Traction Beam SuspensionAdvantages:

Strong

Simple

Lots of travel

Disadvantages: Heavy

Fixed (nonadjustable) geometry

Size (long radius arms running from the beamsback to the driver)

DesignWeighing the pros and cons of each design, the decisionwas made to go with a double wish bone setup. Thiswas for a couple of reasons; the first of which wasadjustability. The twin traction beam setup offers verylittle in the terms or caster and camber adjustment.While not a deal-breaker, the need for adjustability couldarise during testing phase. Another reason was weight.The twin traction beam system is not only based around


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Initial Design

An Initial design was created as a starting point foroptimization (Figure 4). As the spindles selected to beused are OE Polaris units off a 2005 Predator 500 ATV,the only parts that needed to be designed were the

upper and lower suspension arms. There were costs andbenefits to this decision. Firstly, as the geometry of thespindle is already defined, suspension geometry had tobe based off of dimensions that are already set in stone.Steering geometry and initial suspension setup werebased off the limitations of those predeterminedparameters. On the other hand, choosing a pre-manufactured unit allowed the use of otheraccompanying components, such as brake calipers,

brake rotors, wheel bearings, and hubs. This not onlysaved time, but also saved a considerable amount ofmoney. These benefits far outweighed the cost of losingthe design flexibility of being able to choose thegoverning dimensions of the spindle unit.

The first design incorporated aftermarket ATV ball joints.Later designs used OE Polaris Ball joints. This was dueto a design oversight of loading rod end in bending

(threads in bending causing high stress concentrationsand premature failure).

This initial design was a cooperative effort with the framedesign lead. The suspension mounting points and frontend fame design were done in unison in order tomaximize driver leg room as well as optimize front endframe geometry. The numbers that came from this

collaboration are as follows:

Front frame width: 14 (center of tubes)

Mounting bar vertical distance: 5.9 (center oftube)

Every design change after the initial concept design wasa product of optimization.

One parameter that was optimized from the lessonslearned from the previous years vehicle was theplacement of the spindle with relation to the very front ofthe frame. This parameter is especially important as itdictates what part of the vehicle makes first contact withlarge obstacles. After observing many online videos ofcompetitions from the past, many vehicles had thewheels place behind the very front of the vehicle. This isnot an optimal setup, as it means on tall obstacles the

first portion of the vehicle to make contact is the frame.From research, a majority of teams that successfullycleared large obstacle had the tires make first contact.Having the tires placed ahead of the vehicle permitsthem to absorb any frontal impacts (when crossing a log,for instance) and allows the suspension to absorb thebrunt of the impact. This also protects the driver fromharsh impacts and sudden decelerations as well asprevents structural harm to the lower portion of theframe.

Optimization

Before any design optimization could be completed, a


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Scenario 1: hitting a tree at 25mph (36.7 fps)

Scenario 2: 5 foot jump landing nose first

Scenario 3: wheel hitting tree with steering lockedand brakes 100% locked

These scenarios were the basis of optimization for thesuspension control arms.

Scenario 1: Impacting a tree at 25mph (36.7 fps)

The first scenarios loading conditions were based ondata published by The Motor Insurance Repair ResearchCentre [2]. The data shows impulse time and Gs forces

caused by crashes ranging from 11-32 mph

Using the projected vehicle/driver weight of 550 lbm, theimpact force was calculated base of a load of 4.5 gs.

Figure 6- Lower arm design #8

Figure 7- Lower arm side view

Black arrows point toareas of max stress

Black arrow point toarea of max stress


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Figure 8- Progressive Air Spring Curve

At 5.5 inches of travel (very close to what the chosenunits have) the air spring requires 1400 lbf (when innerair pressure is set to 70 psi) in order to achieve fullcompression.

The force was calculated using the mass of the vehiclebeing dropped 5 feet. The calculation is as follows:

(Total load is distributed between the two shocks for1500 lbf per side)

The calculated force to fully compress the shock wasvery close to that show in Figure 8. This calculated force

The results show the max stress in this scenario wasapproximately 82.1 ksi. Using the iso-clipping feature inSolidworks, the areas of maximum stress wereexamined. Through this analysis, it was observed thatjust 0.02% of element volume was above 70 ksi. Thisphenomenon is a modeling singularity. The intersections

of multiple tubes converge to a single point, and its atthis point is where the max stress is seen. Unfortunately,the computer used to run the FEA was not powerfulenough to analyze a mesh fine enough to account forthis; the actual stress is much lower.

The current factor of safety for the lower control arm is1.2, with the understanding that the real life end productwill be stronger than what the model is showing due to

the addition of weld fillets.

Another important aspect of this loading condition is thereaction forces into the frame and mounting hardware.FEA was run with the control arm mounting pointsmounted rigidly. As this cannot be assumed in real life,the reaction forces into the frame needed to beanalyzed. Mounting tabs and bolted connectionsunderwent optimization using the results from FEA.

Figure 10 - Reaction Forces

Shown above are the reaction forces caused by the


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Metric sized bolts were used instead of English due tothe fact that the shocks required metric hardware of thesame size. If the need ever arose to replace the shock orcontrol arm, only one size wrench could remove all bolts.This would be extremely beneficial in the worst casescenario that the shock and control arm had to be

replaced at the same time during a race

Scenario 3: wheel hitting tree with steering lockedand brakes 100% locked

One of the major factors of this Scenario is that thebrakes are 100% locked. The calculated torquegenerated by the brakes, calculated by brake systemlead came out to be 216 lb-ft. The resultant force at the

upper and lower ball joint came out to be 320 lbf.

Figure 11- Brake Torque FBD

This force was applied to the model of the upper armshown in Figure 12. The braking torque calculated is100% worst case. This torque was calculated using thehighest coefficient of friction for the brake pad, and theb ki f ( h l ) d ll f h hi l i h

The results show a max stress of 39.8 ksi (figure 13),well below the 100.2 ksi yield strength of the material.This results in a factor of safety of 2.5.

Figure 13- upper arm FEA

Shock choice

The overall light weight theme for this years vehicleheavily influenced shock choice. With several greatoptions to choose from, the final selection was the FoxFloat R air shock. With rebound and spring preloadadjustability, these shocks are more than adequate tohandle the job of suspending this years vehicle.

There were a few reasons why this shock was chosen.First was weight. Fox claims a weight of between 2 -2.25

lbs (depending on application) and this is far less thanthe fox podium coil over shock offered through thePolaris sponsorship program. The shocks also offereddifferent stroke lengths to accommodate differentdesigns Based off of space limitations a shock with 5 3


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yield strength applied to the Solidworks models was thelowest of the three. This value was 100.2 ksi.

As the tubes required to make up the lower and uppercontrol arms were profiled to a tolerance of +/- 0.005 [4],Assembly was straight forward. A simple jig made from

extruded aluminum T-slot was designed to make finalwielding an extremely easy and more importantlyrepeatable process. The only tools required to ensure anaccurate final product were two hex keys, and a digitalangle finder.

Figure 14 - Model of Jig

diameters had to be bored manually via manual lathe. Aspecial boring bar with carbide inserts was purchasedspecifically for this job. After a few trial runs of the part,the required diameter was bored with a tolerance of +/-0.0005.

Results and Proof of DesignAfter manufacturing, final painting, and assembly, theoverall system weighed in at 54 lbs; this is 4 lbs overbudget. While this was not detrimental to the system sperformance, the extra weight pushed the overall vehicleweight up. Refining the loading conditions for FEA basedoff of this years performance will guarantee that thisweight goal is met in the next iteration of the suspension

system.

Approximately 20 hours of testing was performed on theentire vehicle before competition at the Baja SAE eventhosted at Tennessee Tech. While testing, no parts brokeor wore out. However, testing the suspensioncomponents to their theoretical maximum limits was notwise due to the lack of time and spare parts. Nothingduring testing ever suggested that the suspension andits components would fail during its trial by fire during

competition and this proved to be correct.

The original problem statement stated at the beginningof this paper was to design and build a front suspensionsystem that would allow the 2013 Baja SAE vehicle tosuccessfully navigate the suspension and tractioncourse at competition. Not only was this goal meet, itwas surpassed by allowing the vehicle to place 17th outof 63 other vehicles that attempted the course. Of those

63 vehicles, only 31 actually finished the course. Someof the obstacles encountered very closely approximatedthe worst case scenarios used to optimize thesuspension design. One such obstacle was this logcrossing


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Figure 17 - Log Hit Results

The front end of the vehicle in the above photo is

approximately 6 feet above the ground. After the run wasover, the entire front end was inspected for damage; novisible damage could be seen. The vehicle then went onto successfully complete the 4 hour endurance racewithout a single problem, placing 13

thout of 100.

CONCLUSION

The original goal of designing and manufacturing a front

end suspension system to allow the Baja vehicle tosuccessfully navigate was met and exceeded. As statedin the previous section, our 17

th place finish in the

suspension and traction course was quite anaccomplishment. Only half the vehicles at thecompetition made the attempt and half of the vehiclesthat did attempt the course didnteven finish. The use ofFEA and real life off-road racing experience helpedcreate realistic worst case loading conditions that

enabled the system to survive the worst that thesuspension course could throw at the vehicle.

RECOMMENDATIONS FOR FUTURE BAJA SAETEAMS


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Works Cited

[1] SAE International, "2013 Collegiate Design Series," SAE International, 2013.

[2] A. Linder, M. Avery, M. Krafft and A. Kullgren, "Change Of Velocity and Pulse Characteristics in Rear Impacts: RealWorld and Vehicle Tests Data," The Motor Insurance Repair Research Centre, Thatcham.

[3] American Fastener, "ASTM, SAE AND ISO GRADE MARKINGS AND MECHANICAL PROPERTIES FOR STEELFASTENERS," [Online]. Available: http://www.americanfastener.com/technical/grade_markings_steel.asp. [Accessed26 April 2013].

[4] VR3 Engineering, Ltd., "Profilling Process," [Online]. Available: http://www.cartesiantube.com/profilingprocess.html.[Accessed 26 April 2013].

[5] A. Staniforth, Competition Car Suspension, Sparkford: Haynes Publishing, 2006.

[6] W. F. Milliken and D. L. Milliken, Race Car Vehicle Dynamics, Warrendale: Thirteenth printing, 1995.

[7] Matweb, LLC, "AISI 4130 normalized at 870 C," MatWeb, 2012. [Online]. Available:http://matweb.com/search/DataSheet.aspx?MatGUID=666dfca4fbc74e669b771f35d70a0796&ckck=1. [Accessed 2November 2012].

[8] D. C. Giancoli, Physics, Upper Saddle River: Pearson Prentice Hall, 2005.

CONTACT

Ricardo Hinojosa

Mechanical Engineering [email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Appendix A

Definitions, Acronyms, Abbreviations

: foot

: Inches

Lb: pound

Lbf: pound force

Lbm: pound mass

Slugs: English unit of mass

FPS: feet per second

MPH: miles per hour

psi: pounds per square Inch

ksi: Kips per square inch

Kip: 1000 pounds

Normal Turns: Required steering angle in radians [7]

l/R: Where l= wheel baseR= radius of turn

Ackerman Steering Angle: The geometric steer angle required for a car of wheel base, l, to track a turn of Radius, R, atlow speeds where the external forces due to acceleration are negligible. [7]

Roll center height- found by projecting a line from the center of the tire-ground contact patch through the front view instant


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Appendix B

Recreational Vehicle Front SuspensionCUSTOMER SURVEY

In order to improve our products, Company XYZ would like customer input about a NEW up and coming front

suspension system. This survey is measuring the importance of selected features that are important to the designprocess.

How important is each feature to you for the design of an Off-road front suspension?Please circle the appropriate answer. 1 = low importance 5 = high importance

Suspension Travel 1 2 3 4 5 N/ARide comfort 1 2 3 4 5 N/AAdjustable 1 2 3 4 5 N/A

Ground Clearance 1 2 3 4 5 N/ADurability 1 2 3 4 5 N/AMaintenance 1 2 3 4 5 N/AReplacement parts cost 1 2 3 4 5 N/A

How satisfied are you with the current Off-road front suspension?Please circle the appropriate answer. 1 = very Unsatisfied 5 = very satisfied

Suspension Travel 1 2 3 4 5 N/ARide comfort 1 2 3 4 5 N/A

Adjustable 1 2 3 4 5 N/AGround Clearance 1 2 3 4 5 N/ADurability 1 2 3 4 5 N/AMaintenance 1 2 3 4 5 N/AReplacement parts cost 1 2 3 4 5 N/A

How much would you be willing to pay for a complete Off-road front suspension?

$50-$100 $100-$200 $200-$500 $500-$1000 $1000-$2000


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Appendix CBUDGET

MONEY

FRONT SUSPENSION: $2500

WEIGHT

FRONT SUSPENSION: 50 LBS


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Appendix D

Project tracker

Rick Front suspension

HOURS 15-Oct 22-Oct 28-Oct 4-Nov 1 1-Nov 14-Nov 19-Nov 26-Nov 3-Dec 10-Dec 17-Dec 24-Dec 1-Jan

20

15

5

5

15

20

200

General Design

Upper C/A optimization

Lower C/A optimization

gather aftermarket part info

price quotes

order/gather parts

ITEM

MFG!

design fixtures for MFG

Create Machining Drawings

Finalize solid models

Front suspension

HOURS 14-Nov 16-Nov 19-Nov 23-Nov 26-Nov 30-Nov 3-Dec 7-Dec 10-Dec 14-Dec 17-Dec 21-Dec 24-Dec 28-Dec 1- Jan20

75

75

50

fab upper a-arm

Integrate into entire system

Design Fixtures

Rick

ITEM

fab lower a-arm


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Appendix E

Manufacturing DrawingsBall Joint Cup


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16

Threaded Insert for Upper Control Arm


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17

Delrin Bushing for Lower Control Arm


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18

Lower Control Arm


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19

Lower Control Arm Tube Size


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21

Upper Control Arm Tube Size


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22

Gusset Plate


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24

Lower Arm Mounting Tab


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25

Upper Control Arm Mounting Tab


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26

Upper Shock Mounting Tab

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