University of Wisconsin Platteville_Baja Wisconsin

7/31/2019 University of Wisconsin Platteville_Baja Wisconsin

1/16

University of Wisconsin-Platteville

Society of Automotive EngineersMini Baja Team

2009 Design ReportSAE Mini Baja Wisconsin Competition, June 11-14, 2009


2/16

1

Vehicle Number 22

UW-Platteville2009MiniBaja Team Design Report

Brendan BehrensTeam Captain

Kyle Droessler

Co-Captain

Copyright 2007 SAE International

ABSTRACT

The Society of Automotive Engineers sponsorscompetitions that challenge aspiring engineers to createa miniature off-road vehicle. The SAE Mini Bajacompetition objective is to design and fabricate aprototype vehicle that could be manufactured for

consumer sale. The University of Wisconsin Platteville(UWP) has accepted the challenge to participate in thiscompetition. An aspect of this competition is to composea design documentation package that creates anoverview of the vehicles construction elements. TheUWP Mini Baja team has created this report todescribe their design.

INTRODUCTION

The purpose of designing and manufacturing a MiniBaja car was to create a prototype recreational off-roadvehicle that could provide a fun, safe, and reliableexperience for a weekend off-road vehicle enthusiast. Inorder to accomplish this task, different design aspects ofa Mini Baja vehicle were analyzed, and certainelements of the car were chosen for specific focus.There are many facets to an off-road vehicle, such asthe chassis, suspension, steering, drive-train, andbraking, all of which require thorough designconcentration. The points of the car that the Universityof Wisconsin Platteville decided to specifically focus onwere the chassis, drive-train, and suspension. The mosttime and effort went into designing and implementingthese components of the vehicle because it was felt thatthey most dramatically effect the off-road drivingexperience. During the entire design process, consumer

interest through innovative, inexpensive, and effectivemethods was always the primary goal.

FRAME DESIGN

OBJECTIVE

The objective of the chassis is to encapsulate allcomponents of the car, including a driver, efficiently andsafely. Principal aspects of the chassis focused onduring the design and implementation included driversafety, suspension and drive-train integration, structural

rigidity, weight, and operator ergonomics. The numberone priority in the chassis design was driver safety. Withthe help of the 2009 Baja SAE Competition Rules andFinite Element Analysis (FEA), design assurance wasable to take place.

DESIGN

The main components of the frame are broken into twogroups: the chassis and the roll cage. The roll cage ismade up of the RRH, RHO, FBM, LC. The chassis ismade up of LBD, LFS, SIM, FAB, and FLC (See page11, the Acronym list, for member clarification).

Material

1020 DOM was chosen for the roll cage because of itshigh toughness and ductility. A very tough material isimportant in a roll cage because in the event of impact,such as a rollover, the roll cage needs to absorb asmuch energy as possible to prevent the roll cagematerial from fracturing. 4130 was chosen for the

chassis because it has structural properties that providea low weight to strength ratio. 4130 is a chromiummolybdenum alloy steel that has controllable properties.

Attributes to 4130 include corrosion resistance and theability to maintain a Bainite micro-structure after welding.This prevents the area around a weld from becomingoverly brittle.

1 inch diameter tube with a thicker wall was used insteadof 1.5 inch diameter tube with a thinner wall formanufacturability purposes. Although the thinner wall,1.5 inch diameter tube would be slightly lighter than thethicker wall, 1 inch diameter tube, it would have beenmore material and more difficult to weld.

Safety

Roll cage safety features were first implemented inaccordance with the 2009 Baja SAE CompetitionRules, which served as a baseline. The first primarysafety standard focused on during design wasmaintaining a minimum of 6 inches vertical distance fromthe drivers head to the bottom of the RHO and a 3 inchclearance between the rest of the body and the vehicleroll cage. These dimensions created a roll cageenvelope that was safe for the driver. After the roll cage


3/16

2

envelope was created, the next aspect addressed duringbaseline design was roll cage structural integrity. Rollcage structural integrity guidelines can be found in the2009 Baja SAE Competition Rules section 3, RollCage, Systems, and Drivers Equipment. All 2009 BajaSAE Competition Rules guidelines were implementedthroughout the entire frame.

Once the baseline requirements were met, other safetydesign points were implemented. The chassis was

additionally designed to give the occupant extra spaceand protection with curved vertical supports and extralateral bridge supports, which can be seen in Figure 1.These supports tie the right and left sides of the cartogether, increasing structural integrity and reducing thechance of driver ejection during roll-overs. To furtherimprove the roll cage safety and verify its structuralintegrity, finite element analysis was completed on theroll cage.

Roll Cage FEA Safety Analysis

Simulated loads within a computer program were placedon a wire frame model of the roll cage at critical points to

simulate the amount of force that the vehicle wouldundergo from its own weight and a driver in the event ofa rollover.

To conduct a finite element analysis of the chassis, anexisting chassis design was uploaded from the computerprogram SolidWorks to a finite element analysisprogram known as Algor. The loading performed bythe Algor FEA software modeled an end over endrollover. Different loads at various angles were appliedat points on the top of the roll cage to simulate thatscenario, as seen in Figures 2 through 7. The weight ofthe vehicle itself was assumed to be 450 pounds. Then200 more pounds were added to the vehicle weight to

simulate the weight of a driver. The combined valueswere used to model the loads exerted on the roll cage.The results show that with the total load of 650 pounds,distributed across the top of the roll cage, the frame willnot fail. The maximum stress during the simulation wasfound to be about one half the value of the roll cagesmaterial yield strength. The maximum stresses anddisplacements are shown in Table 1.

Location of Load Relative to Roof of the Chassis

135 Deg 90 Deg 45 Deg

Max Stress (psi) 25580.9 8644.76 11923

Max Displacement (in) 0.142465 0.051976 0.111683

Table 1: FEA safely results

In order to simulate a worst case scenario, the yieldstresses of the two different materials on the Mini Bajacar roll cage were found. Determining the yield strengthof the roll cage is an important aspect, because once thematerials begin to yield, the roll cage will lose much of itsstructural integrity. The 1020 steel had a yield strengthof 47,863 psi, and the 4130 Chromoly had a yieldstrength of 170,000 psi. A loading configuration thatwould produce the highest stresses with the smallestload was then determined. Applying a distributed load

across the front cross member at 135 degrees withrespect to the top of the roll cage seemed to achieve thehighest stress with the smallest load, and would besimilar to a forward flip landing on the front of the rollcage. After some trial and error, a maximum distributedload of 80 lbs/in (1215.20 lbs total) was determined tocause a stress of approximately 47,800 psi in one of the1020 steel members. This means that it would takeapproximately 1215 lbs of load on the weakest memberof the roll cage to cause a failure in the roll cage, asseen in Figure 8.

The results from these simulations are accurate for thetype and amount of loading that was applied to theknown material and geometry. However, these loadingscenarios generally do not exactly represent an actualrollover crash. To accurately depict a rollover incident,dynamic loading would have to be used to simulate thetypes of impact loading that would occur during an actuarollover. It would be very difficult to accurately modelthis event without known data gathered from an actualrollover. This data could be gathered using straingauges attached to the frame of the vehicle. The results

gathered from the FEA illustrate that the frametheoretically will not fail in a rollover until there isapproximately 1215 lbs of force on the weakest memberof the roll cage. The FEA results show a design thatmeets the expectations set for this chassis. With thedata collected from the FEA simulations, the roll cagewas found to have a theoretical factor of safety ofapproximately 1.87.

Safety Harness

A five point racing harness attached to the most rigidmembers of the roll cage was utilized to ensure themaximum amount of driver safety restraint. Attachingthe seat belts to the most rigid and structural chassiscomponents guarantees reliability of the seat belt underthe extreme forces possible in a collision. Using a quick-release lever style seat belt clasp gives the driver theability to get out of the vehicle in a safe amount of time inthe event of an accident. SAE requires that a driver beable to evacuate a Mini Baja car in less than fiveseconds. The safety restraints provided in the car will besufficient for keeping a driver safe in the event of acollision, while still allowing the driver to escape in therequired amount of time.

Suspension and Drive-Train Integration

Integrating the suspension and drive-train components

into the chassis was a crucial part of making an effectiveoff-road vehicle. To complete the goal of integratingthose components efficiently and effectively, all thecomponents were solid modeled in the computer aidedmodeling program SolidWorks. After solid modelingwas complete, all the components restrictions andrequirements were considered. A few key drive-trainrequirements to be included in the chassis designconsisted of the distance the primary and secondaryclutches needed to be apart and keeping the center ofgravity of the vehicle as low as possible. A fewimportant suspension requirements considered during


4/16

3

the design of the chassis consisted of the angle at whichthe shocks needed to be mounted, the distance the A-arms needed to be mounted apart, and the anti-diveangle in which the front and rear A-arms needed to bemounted. Once all requirements were compiled, thesuspension and drive-train were integrated into thechassis design.

Another aspect of the chassis that was consideredduring the integration of the suspension was chassis

deflection due to forces exerted through the suspension.To accurately minimize deflection in the chassis, FEAanalysis was conducted and light weight Chromolytubular members were added where the deflection wasgreatest. The simulated loads conducted through FEAhelped determine where and how additional membersshould be added to the chassis. The method forsimulating loads on the suspension points using FEAwas similar to that of the rollover analysis, as previouslydescribed in the chassis safety section.

Impact loading was simulated on the shock mountingpoints, at the angle in which the shocks were going to bemounted, until a member in the chassis reached its yield

stress. Additional members were added to create thebest combination of weight addition and structuralrigidity. Each rear shock mount was able to withstand800 lbf of loading before yielding, as seen in Figures 9and 10. Each front shock mount was able to withstand485 lbf of loading before yielding, as seen in Figures 11and 12. Based on the information presented above, thestrength of these mounting points will be enough towithstand the forces exerted on them in extreme off-roadconditions. Withstanding forces that simulate extremeconditions ensures rigidity and reliability in normal off-road conditions. Also, as a result of steel having aninfinite fatigue life, these tests were able to verify that

under normal loading, the fatigue limit of the material willnot be exceeded. It is well known that 90% of allmaterial failures are due to fatigue, which is why it is soimportant that the stresses exerted on the chassissuspension mounts do not exceed their fatigue limit.

Structural Rigidity

Overall frame structural rigidity is important to enhancethe capabilities of an off-road vehicle. To measure theoverall frame rigidity, torsional rigidity analysis wasconducted through FEA. The objective of the torsionalrigidity analysis was to manipulate the chassis designwithin the FEA software to increase the amount of torque

per degree of chassis deflection. By theoreticallyincreasing this value, the actual vehicle could have theability to be more torsionally rigid, making it able towithstand more intensive terrain without failure. Toachieve this analysis, a simulated torque of 70 ft-lbf wasplaced on the back of the car, while the front of the carremained fixed, as seen in Figure 13. With the degree ofrotation data collected from the FEA software, the torquewas divided by the degree of rotation, creating atorsional rigidity value for the frame. The angle rotatedunder the 70 ft-lbf of torque was found to be 0.126degrees, as seen in Figure 14. The UWP Mini Baja car

frame has a 555 ft-lbf/degree theoretical torsional rigidityrating. It has been concluded that this meetsexpectation, and shows that the vehicles frame isstructurally suitable for the terrain it has to withstand.

Weight

Keeping the frame as light as possible was a top priority.When power is limited, vehicle weight is a large factor invehicle performance. The frame is one of the largestand heaviest components of the car, and which is whyspecial attention was placed on the vehicles frameweight. The strategy utilized to minimize weightconsisted of determining defined goals for the chassisand employing the correct material in the best places toaccomplish those goals. Once baseline safety designrequirements were met, FEA aided the material decisionmaking process. FEA specifically helped determinewhether a member was under high or low stresses, inthe scenarios discussed previously, making the chassisdesign process efficient and effective. Low stresschassis members were made out of 0.049 inch wallthinness 4130 Chromoly, and higher stress chassismembers were made from 0.065 inch wall thickness

4130 Chromoly. Chromoly was chosen because of itsweight reduction capability and beneficial materialproperties, as was stated previously. Throughaccurately determining stresses on the chassis indifferent scenarios, weight reduction was able to bemaximized through material selection and placement.The final weight of the chassis was measured to be 85pounds.

Operator Ergonomics

The ergonomics of a cockpit can noticeably affect thequality of an off-road vehicle driving experience. This iswhy operator ergonomics was a factor that was

considered in the design of the frame cockpit. Thecockpit, consisting of the area in the roll cage where thedriver sits to operate the vehicle, was designed formaximum comfort and ease of vehicle entrance and exit.The first aspect of the chassis that was designed aroundergonomics was the firewall angle. The angle of thefirewall, which inherently limits the amount an operatorcan lean back while driving, was set to 19 degrees,which is just less than the maximum angle required bythe 2009 Mini Baja SAE Competition Rules. Lettingthe driver lean further back gives a more relaxed positionto drive the car. As the rollover FEA analysis shows,there were no detrimental effects to structural integrity

found in leaning the firewall back for ergonomicpurposes. The next ergonomic improvement made tothe chassis was side wall height. While still remainingwithin the 2009 Mini Baja SAE Competition Rules, theside wall height was set low enough to create easyentrance and exit, while still letting the driver remainsafely encapsulated in the vehicle. The last specificergonomic consideration made during chassis designwas the decision to position the steering support in away that makes it easy for people of all sizes tocomfortably sit in the vehicle, while still being able toeffectively support the steering column and house the


5/16

4

dashboard. The steering support remains out of the wayof drivers knees and additionally makes it easier to enterand exit the vehicle.

Manufacturability

All design work for the UWP Mini Baja frame was doneusing SolidWorks. Using this program to produce athree dimensional model allowed easy revision of pre-build designs, and gave design team members a visualpicture of what the frame would look like. After thedesign of the frame was finalized, a list of requiredsupport members was created and exported to Bend-Tech, allowing easy bending and fitting of varioustubular frame components.

Tube Bending

To increase manufacturability, many bends were usedas opposed to miters. By implementing bends into thedesign of the frame, the number of cuts and welds weredecreased. Decreasing the number of cuts and weldslowers the production cost and increases overall chassisstrength. For example, by using more bends, a CNCtubing bender could be used during manufacturing, in

place of hand welded miter joints, reducing man-hoursand production costs. All bends were designed to bemade using a tube bender fitted with a 9-inch diameterdie, which would eliminate costly tooling changes fromthe manufacturing process.

Mounts

All suspension mounts for the chassis were cut from0.1875 inch cold rolled plate steel, using a CNC lasercutter. The 0.1875 inch cold rolled plate steel waschosen to give all mounts sufficient strength anddurability while still allowing the chassis to remain light.Common materials throughout the manufacturing

process eliminate costly and unique inventories,therefore lower the production cost.

Welding

All welds on the UWP Mini Baja vehicle were madeusing a gas tungsten arc welding (GTAW) process. Allwelds used 1/16 inch 2% thoriated tungsten ER, 70-S2filler rod, and pure argon shielding gas. The GTAWprocess was selected because it provided the bestcontrol of heat affected zones while also reducinginternal stress in the frame. ER 70-S2 filler rod wasselected it order to allow the weld to flex slightly withoutcracking. Also ER 70-S2 has sufficient oxidizers to

make welding easier. Pure argon was used to increasearc control. Before any joints were welded, allconnected members were purged with pure argon toprevent scaling and oxidization on inner surfaces whichwould reduce the strength of the welds. All joints wereground and de-burred inside and outside of the joint priorto welding to ensure there would be no contaminationduring the process.

BODY AND COMPOSITES

OBJECTIVES

The purpose of the body is to prevent debris fromentering the vehicle, with the intent of protecting thedriver and the vehicles components. The seat wasdesigned to support the driver comfortably and safelywhile they are operating the vehicle.

DESIGN

The UWP Mini Baja cars body was kept very lightthrough the use of HDPE plastic and fiberglass.

Body Panels

The body panels were made out of .080 inch thick HDPE(High-Density Polyethylene) plastic. HDPE plastic is avery light material that has desirable properties for abody panel. HDPE Plastic has a tensile strength of4,600psi, shear strength of 3,380 psi, and it takes 4,570psi to cause a 10% deflection in the material. Theseproperties also make the body panels more highlypuncture resistant. The HDPE panels provide theproperties necessary to protect the driver and vehicle

components from rocks and other debris. When thepanels were integrated into the car, the panels wererecessed into the chassis to provide visibility to thechassis members, making the car aesthetically pleasing.Dzus clips are utilized to affix the body panels to thevehicle. Dzus clips allow for the effortless removal of allbody panels, providing access to all parts of the car.

Hood and Dashboard

The hood and dashboard of the car is made of E glass-mat and polyester resin. E glass-mat is used because itis relatively inexpensive and provides the necessaryproperties to create an optimal hood and dashboard for

the vehicle. E glass-mat has very good strength in alldirections, compared to a uni or bi directional fabric. Eglass mat has short and very strong fibers. Using theequation in Figure 15, the hood and dashboard wascalculated to have 66,000 psi of tensile strength. Thisstrength ensures the durability of the panels in all off-road conditions. The hood and dashboard, like the bodypanels, are held on by Dzus clips, which allow for easyaccess to all of the components in the front of the car.

Seat

The seat in this car was also designed to be very lightweight. This was achieved by making a small seat out o

fiberglass and having a detached headrest mounted onthe fire wall. Many teams use a full size racing seat,made of aluminum. Aluminum racing seats give thedriver very good support but, they are very heavy. Thefiberglass seat was designed to provide the same, if notbetter, support than an aluminum racing seat, whilebeing substantially lighter. This was done by creatinglumbar support in the seat and shaping the seat to begenerally ergonomic for people of all sizes. The seat,like the hood, was made using E glass-mat andpolyester resin and has the same properties andstrength as the hood. Mounting bolts on a plate were


6/16

5

fiber-glassed into the seat for easy mounting into thechassis. Ribs and creasing on the edges of the seatwere utilized to make the seat mechanically strongenough to support all drivers able to operate the vehicle.The seat implemented in the UWP Mini Baja carprovides a good combination of weight reduction andergonomics.

SUSPENSION DESIGN

OBJECTIVE

The objective of the suspension is to improve thestability and comfort of the vehicle through a variety ofterrain. The main focus of the UWP Mini Baja carssuspension was to create an overall good performingsuspension system that could perform in all terrainsequally.

DESIGN

Overall Suspension

The static ride of the vehicle was designed to be 13inches high. Once a driver is positioned in the vehiclefor operation, the suspension will sit at an optimal 12

inch ride height. This height was chosen for acombination of desirable ground clearance whilemaintaining a low center of gravity. This combinationwas necessary to keep this off-road vehicle versatile inall terrain. The ground clearance gives the vehicle theability to overcome high rocks, hills, and bumps. Thelower center of gravity will give it an ability to handlebetter in tight maneuvering situations at high rates ofspeed.

Front Suspension

The front A-arms were designed to be as long aspossible to get a suspension ratio of 2:1, improve

suspension response, and to have the greatest vehiclestability. These A-arms give the vehicle a front trackwidth of 56 inches. The suspension ratio signifies thenumber of inches the wheel travels vertically comparedto the number of inches the shock compresses. The 2:1ratio was chosen because it gave the best combinationof a soft and stiff ride. The ratio is able to do thisthrough shock efficiency. As the suspension ratio getscloser to 1:1 the more effective the shock, creating astiffer ride. As the suspension ratio surpasses 2:1, theshock effectiveness gets exponentially smaller, givingthe A-arms the ability to move more freely, creating asofter ride. Another aspect of the front suspension that

affects the shock effectiveness and ride comfort is theshock angle. The front shocks are mounted 30 degreesfrom vertical. As the shock angle becomes greater, theless effective the shock is and the softer the ride will be.The greater the shock angle, the greater the articulationcapability, to a certain point, and the stiffer the ride willbe. For the front suspension, a compromise betweenshock effectiveness, articulation height and ride comforthad to be made. Shock effectiveness was slightlycompromised so articulation height could be greater andthe ride comfort could be a combination of soft and stiff.The articulation height of the suspension was selected to

be 10 inches. To achieve this height and implement theshock as effectively as possible, the angle of the tangentline at the 10 inch articulation point determined theshock angle required. Greater articulation in the frontwas implemented to overcome aggressive terrain duringapproach. The 30 degrees of front shock angle alsogave a good combination of a stiff and soft ride. It isimportant to have that characteristic, because it enablesthe vehicle to handle better in rigorous maneuverabilitysituations, while still allowing the vehicle to be operatedcomfortably.

Adjustability

The front A-arms are very adjustable. An owner canadjust caster and camber very easily on the vehicle.The heim joints on the top A-arm, towards the inside ofthe car, make caster adjustments possible. A threaded

joint on the wheel end of the top arms gives an operatorthe opportunity to adjust camber. The arms weredesigned to have zero camber gain throughout themotion of the suspension cycle. This was designed bysetting the two A-arm planes from the frame joint to theknuckle joint parallel. Zero camber gain is a feature on

the vehicle that allows the most tire surface area to becontacting the ground in any suspension position.

A-arm Material and Structural Integrity

The front A-arms are constructed of 0.065 inch wallthickness, 1 inch diameter and 0.75 inch diameter 4130round Chromoly tube. This material was chosenbecause of its strength to weight properties. Finiteelement analysis was conducted on the A-arms,simulating the maximum loads an A-arm would ever seeUnder maximum loading, and with the addition of cross-sectional braces, the A-arms performed with minimumdeflection and no yielding, as seen in Figure 16.

Ground Clearance

The front lower arms were also designed for maximumground clearance. A bend three quarters of the distanceof the A-arm from the chassis creates extra groundclearance under the A-arms. The increased groundclearance in the front gives the vehicle an ability to travelon a wider variety of terrain that may be more intensive.

Shock Mounting

Shock towers were added to the chassis to achieve thecorrect shock angle, and increase the shock length forgreater shock travel possibility. An adjustable shock

tower brace was added to increase support of the shocktower under loading. FEA was conducted to verifyminimal deflection under loading by the shock, as statedin the chassis section under drive-train and suspensionintegration. The adjustability of the shock tower braceaids in the tensioning of the shock towers and theaccessibility to the front hood of the car, where variouscomponents are housed.

Anti-dive

An anti-dive angle of 10 degrees was set for the frontsuspension. This angle increases ease of handling and


7/16

6

improves comfort in aggressive terrain by making theshocks effective in all three axis. This angle can beseen in the chassis solid model pictured in Figure 1.

Rear Suspension

The rear suspension was more difficult to design, on thebasis that the drive-train components needed to beintegrated into it. The rear suspension was designed toaccommodate a slightly lesser track width than that ofthe front suspension. The rear track width was designedto be 55 inches wide and create a slight over-steer intight cornering situations, which allows for easiermaneuverability at higher speeds. Many of the sameprinciples were utilized in designing the rear suspensionas the front suspension. The objective and reason forkeeping the A-arms as long as possible was the samefor both the front and rear suspension. The 2:1 ratio wasalso maintained in the rear suspension as it was in thefront suspension, for the same reasons. A suspensiongeometric aspect that was different between the frontand the rear suspension was the shock angle. Rearsuspension articulation was not as much of a priority asit was with the front suspension. Having a high

articulation limit for the front was seen to be moreimportant than having it for the back. This was the case,because of drive-train CV joint limitations and the lessernecessity for articulation in the back. High frontsuspension articulation is more important than high rearsuspension articulation because the front suspensionneeds that extra articulation during an approach intorugged terrain, where as the rear does not. Since this isthe case, the shocks where mounted at their most effectangle, which is within 10 degrees of vertical. Themajority of the forces are acting in the vertical direction,and this is why the shock is most effective when it ismounted as close to vertical as possible. Setting the

rear shock vertical was also more beneficial in the rearbecause it allows the shock to dampen the extra weightin the rear to create a more comfortable ride. With thatrear suspension configuration, the same balance of astiff and soft ride was able to be created to match thefront.

Adjustability

The top rear A-arms allow for camber adjustment of therear tires. There is zero degree camber gain throughoutthe rear suspension travel, as was the case for the frontsuspension, for the same reasons. Caster adjustabilitywas concluded to be unnecessary in the rear.

A-arm Material and Structural Integrity

The rear A-arms are constructed of 0.065 inch wallthickness, 1 inch diameter 4130 round Chromoly tube.This was chosen for the same materials properties asdescribed in the front suspension section. FEA was alsoperformed on the rear arms, and proved them to becapable of handling the stresses exerted on them inextreme situations, as seen in Figure 17. As a result ofthe rear shock mounting location being offset, additionaltriangulation support was added to that section of the A-arms to minimize deflection and dramatically decrease

the possibility of yielding. Single link top A-arms wereable to be utilized after analysis found that half of theforces acting on the bottom A-arms are acting on the top

A-arms during normal suspension cycles. Once thisdiscovery was made, drive-train integration becameeasier and material weight was able to be saved.

Anti-dive

An anti-dive angle of 5 degrees was set for the rearsuspension. It is important to have a more aggressiveangle in the front than it is to have in the rear. This is thecase, because the front suspension needs to have moredampening along that axis to create a smoother ride andprevent diving after bumps and jumps. It was concludedthat it would be effective to give the rear suspension theability to react along that axis too, creating a morecomfortable, but still aggressive ride.

Shock Absorbers

The Fox Podium X shocks are both externally andinternally adjustable. The compression rate can beadjusted externally by a dial. The shocks can also beadjusted by changing the internal components. The

Podiums have a simple shim stack design. Changingthe diameter, thickness, and order of the shims willchange how the shock will react with compression andrebound rates. The setup can also be drasticallychanged by replacing the piston. The diameter of theholes on the piston and the size of the bleed grooves oneither the compression or rebound side of the piston canchange how the shock reacts. The shocks can also befine-tuned by adjusting the nitrogen pressure in theremote canister.

With the help of a shock dynamometer, the shocks wereset-up taking into consideration several factors, as seenin Figure 18. The front shock absorbers were setupdifferently than the rear shocks. Mini Baja cars seem tohave the tendency to nose dive off of bump and jumps,so to further prevent that phenomenon, the front shockswere setup with a stiffer high speed compression ratethan the rear shocks. The rebound rate was set so theshocks react fast enough that they are fully extendedbefore the next bump, but not so fast that the carbounces after landing a hard jump. Low speedcompression adjustment is also important for propershock setup. Through the low speed compressionadjustment, the shocks were setup to allow the vehicleto slightly roll while cornering, but not to the point wherethe vehicle rolls so much that it lifts the wheels off the

ground using the majority of travel through the low speedcompression adjustment. Also, by adjusting the lowspeed compression, the vehicle will be very controlledover small bumps, making for a smoother ride.

Refer to Figure 29 for a solid modeled image of thesuspension integrated into the vehicle.


8/16

7

STEERING SYSTEM DESIGN

OBJECTIVE

The steering system is designed to withstand the stressof safely maneuvering the vehicle through any type ofterrain.

DESIGN

Simplicity and safety were the main design specifications

for the vehicles steering system. A small, lightweightrack with a 12:1 ratio was chosen as the maincomponent of the assembly. The small size of this rackallows the geometry and joints of both the suspensionarms and tie rods to align perfectly and completelyeliminate bump steer. Custom stainless steel clevisesprovide a strong, corrosion resistant link between therack and the custom aluminum tie rods featuringopposing threads for easy adjustability. Lightweightaluminum rod was chosen for the tie rod material forease of manufacture, along with the fact that it will easilywithstand the strictly axial forces applied to it. Steeltubing was used for the steering column due to thetorsional loads it will need to withstand. A universal joint

provided easy redirection of the steering column as itextends from the rack, along with a safety feature. If thevehicle sustained a severe head-on collision, thesteering system would buckle instead of being driveninto the driver. The forward design of the bearing mountloop allows for easy entering and exiting of the vehicle.

A quick disconnect adapter for the steering wheel, whichalso allows for easy entrance and exit of the vehicle,completes the steering system.

Tie Rods

Intuitive analysis of a steering system shows that theforces exerted on tie rods produce almost strictly axial

forces on them. Since the tensile strength of alloyedaluminum can approach that of steel and buckling is notan issue in such a short rod, the weight saving, lowdensity of aluminum can be utilized in this case. Theuse of solid aluminum rod for the tie rods also introducesan ease of manufacture not available with steel tubing.Drilled and tapped holes on both ends of the tie rodsallow for easy incorporation of a heim joint and clevis oneither end. Furthermore, tapping one end of the tie rodwith a left hand thread allows the toe adjustment to becompleted by simply twisting the tie rod extending thetie rod with rotation in one direction and shortening itwith the other.

Steering Rack

The front suspension design incorporates a narrow frontend due to the long suspension arms. The longsuspension arms allow for a better suspension ratio. Bylimiting the spacing between the inside suspension arm

joints to 7 inches, the overall width of the car does notincrease with the longer suspension arms. The size ofthe steering rack is directly limited by this spacingbetween the suspension joints in order to overcomebump steer. Bump steer is the phenomenon wheresuspension travel can move the tie rods in or out,

causing uncontrollable turning of the wheels. Properlyaligning the suspension and tie rod joints intoparallelogram geometry, shown in Figure 19, cancompletely eliminate bump steer. The small rack fulfillsthis design specification, along with reducing weight andfreeing up space in the front end. The 12:1 ratioprovides very responsive steering, but does not makethe car too difficult to steer.

Clevises

The connection between the tie rods and rack iscompleted with custom, male clevises, shown in Figure20. These small parts allow for full implementation ofthe heim joints included with the rack, without interferingwith the rack mount. The use of stainless steel reducesthe likelihood of failure due to corrosion. The differingmetals in this area of the car (aluminum, steel, etc.)introduce differences in galvanic potential. This, alongwith the inevitable presence of electrolyte in the form ofmud, produces the perfect conditions for corrosion. Thecomplex geometry of these clevises also increases theprobability of stress corrosion cracking. The passive filmon stainless steel greatly reduces these risks of failure

much more effectively than painted steel ever could.

Steering Column

The use of aluminum was considered with the steeringcolumn just as it was for the tie rods; however, unlike thetie rods, the stress imposed upon the steering column isalmost strictly torsional. The modulus of elasticity foraluminum is approximately a third of that of steel. This,

combined with the equation for angular deflection ,produces an angular deflection in aluminumapproximately three times what it would be with steel.Since this sort of sloppiness is undesired in a steeringsystem, steel is used. The calculations in Figure 21confirm that the 0.75 inch steel shaft will withstand thestresses imposed upon it. The forward mounting loopfor the steering column provides extra knee room fordrivers of all sizes, along with providing room for easyexit of the vehicle in the event of an emergency.

DRIVE-TRAIN DESIGN

OBJECTIVE

The drive-train is a very important part of the Mini Bajacar, taking into consideration that all of the cars power istransferred through the drive-train system to the ground.The challenge is to harness the engines 10 horsepower

and distribute it to the ground in the most efficient way.The drive-train needs to be able to operate in the lowestand highest gear ratios while performing in all of thedifferent aspects of the competition.

DESIGN

The drive-train design focuses on being highly variablewhile also staying very light and easily serviced. Thedrive-train allows the car to be vary between the gearratios of 8.1:1 to 50.7:1. This gear ratio setup allows thecar to have a start up speed of 2.4 MPH and a top speed


9/16

8

of 33 MPH. The system includes a continuously variabletransmission (CVT), planetary gear box, and a chaindrive to a Polaris Outlaw rear housing with custom builtdrive axles.

Continuously Variable Transmission

The Comet 790 Series CVT has gear ratios from 0.54:1to 3.38:1. Utilizing this CVT gives the car manyadvantages that include a lightweight, simple, tunabletransmission setup. The primary clutch consists of acentrifugal clutch that automatically shifts gears ratiosunder varying engine speeds and torque loads. Theability to tune the CVT comes from changing weightsand springs in the primary clutch that will changeengagement RPMs and time to maximum ratio. Thisallows the car to go fast and have a high gear ratio whiletraveling over flat terrain and have a low gear ratio whiletraveling over rough or steep terrain. Figure 22illustrates the engine RPM to CVT ratio relationship.The CVT utilizes a 1 inch belt that requires a centerdistance of 9.41 inches, as seen in Figure 23.

Planetary Gearbox

The planetary gearbox that is used in the car ismanufactured by Zenith and offers a great way toachieve the desired 5:1 gear reduction in a compactpackage, as seen in Figure 24. The planetary gearboxhas many useful features, such as sealed bearings,lifetime lubrication, and a high energy density. Thesealed bearings for lifetime lubrication are a very nicefeature for the off-road enthusiast who wants to enjoytheir vehicle with little maintenance. The high energydensity allows the gearbox to withstand high torqueloads while maintaining a small size. Its versatilemounting brackets allow it to be mounted very low on thevehicle, in turn lowering the vehicles center of gravity.

To make the planetary gear box possible for ourapplication, a small amount of modification was requiredto the existing gearbox. The input required a customshaft that would fit the driven clutch in the Comet 790Series CVT and the output required a small modificationto the existing output shaft. Both were designed andmachined in-house at the University of Wisconsin-Platteville. Many different factors were taken intoconsideration during the design of the input and outputshafts. The input shaft required a change in size from inch at the exit of the CVT to 1- inch diameter at theinput of the planetary gearbox. The input shaft wasdesigned with a 2:1 taper from 1- inch to inch. A

taper was selected over a straight transition in order toreduce stress risers. The existing output shaft was alsomodified with a 2:1 taper as the input shaft reduced for a1.5 inch diameter shaft to a 1 diameter shaft.

Chain Drive

The output of the planetary gearbox consists of a 13tooth sprocket and hub that drive a chain which leads toa 39 tooth sprocket mounted on the rear housing, asseen in Figure 25. The sprocket on the planetary gearbox can easily be changed to accommodate differentgear ratios for variety driving conditions. The chain

system uses a metric 520 O-ring chain that islightweight, reliable, and able to withstand the appliedforces. The maximum constant load on the chain duringthe highest torque situation was calculated to be 2,700lbf. In addition to the forces generated during constanttorque loading, forces due to torque spikes needed to beincluded, which increases the maximum chain load toapproximately 3200 lbf. The manufacturer listed theyield strength of the chain to be 8,100 lbf which gives afactor of safety of 2.53 during maximum chain loading.Based on this, it was determined that metric 520 O-ringchain would be capable of withstanding the applied load.The design implements the use of a very short chainwhich minimizes the number of links and weight of thechain. Besides reducing weight, fewer links results inless stretch in the chain, because there are less links tobe stretched. Utilizing an O-ring chain significantlyincreases the life of the chain, because of the reducedmetal on metal wear that is common on a non O-ringchain. Using a chain in place of a belt also has thebenefits of a longer drive life, no drive slippage, and theability to withstand large torque loads.

Rear HousingThe rear housing consists of a Polaris Outlaw rearhousing. This is a very good choice because it providesa drive to the rear axles in a small and light package.The housing provides excellent way to transfer powerfrom the chain to the sprocket hub that drives the rearaxles. The rear housing pivots on the bottom, so thatchain tension can be adjusted by tilting the housingaway from the planetary gearbox. A bolt that goesthrough a plate in the frame is threaded into a cleviswhich makes it very easy to adjust the chain tension.The rear housing is sealed with rubber boots whichallows the rear housing to have lifetime lubrication,

resulting in very low maintenance.Rear Axles

To complete the drive-train, custom rear drive axleswere machined to accommodate the unique drive-trainsetup. These drive axles have a 1 inch diameter, andare made from 4330 alloy steel. The required diameterof the shaft was calculated to be 0.645 inch using the

equation By using a 1 inch shaft, a factor ofsafety of 1.55 was utilized.

Refer to Figure 29 for a solid modeled image ofthe drive-train integrated into the vehicle.

BRAKING DESIGN

OBJECTIVES

The purpose of the brakes is to stop the car safely andeffectively. In order to achieve maximum performancefrom the braking system, the brakes have been designedto lock up all four wheels, while minimizing the cost andweight.

DESIGN


10/16

9

The breaking system is mainly composed of componentsfrom Polaris and Wilwood. Two front brake calipersand one rear caliper, mounted to the rear housing, havethe ability to stop the vehicle effectively, and lock up allfour wheels. The front brakes are composed of 7 inchdiameter discs and dual 1.19 inch diameter pistoncalipers, made by Polaris. The rear brake consists of an8 inch diameter disc and 1.18 inch diameter pistoncalipers, also made by Polaris. Both front and rear brakecomponents are currently utilized on the 2008 PolarisOutlaw. Front brake disc with a dual piston design werechosen because of their superior braking ability,compared to single piston calipers. With the dual boredesign, braking can be more effective due to the fact thattwo pistons increase the surface area acting on thebrake pads. The Outlaw rear brake components werechosen because of their easy integration with the rearhousing and their ability to easily meet the braking needsof the car. Remote master cylinders from Wilwood werechosen to increase the flexibility of mounting locations,while also creating more space for the driver. Twomaster cylinders were chosen instead of one to ensurethat the braking system would still be able to perform,

even if one were to fail. Therefore, if either the front orrear brake system fails unexpectedly, braking power willstill be available to the driver. Steel brake lines run thelength of the car, and flexible braided lines are used atthe A-arms in the front for suspension travel and caliperpivoting.

Input Data

Master Cylinder Size (in) .625

Caliper Size Front (in) 1.19

Caliper Size Rear (in) 1.18

Pad Height Front (in) .929

Pad Height Rear (in) .787

Front Disc Radius (in) 3.5

Rear Disc Radius (in) 4Coefficient of Friction disc/pad .45

Weight of Vehicle (lb) 500

Wheelbase (in) 56

Table 2- Brake system data used to calculated systemspecifications

Through the use of a brake calculator and the input datalisted in Table 2, it was determined that the selectedcomponents would perform to the expectations needed.Ideally, the braking bias should be 50-50, but there willalways be a level of adjustment needed to optimizebrake performance. Being able to adjust the brakingbias is especially important for this car, because of

having two calipers in the front and one caliper in theback. The bias needed to optimize braking performancewas found to be almost exactly opposite of what wouldnormally be expected of a brake bias, which is normally60% in the front and 40% in the back. As a result ofhaving only one rear caliper, 38% of the braking powerneeded to be biased towards the front, and 62% ofbraking power needed to be biased to the back. Thepedal force needed to lock the brakes was calculated tobe 110 lbs, which is within the capability of any driver.From the calculations performed, it was found that atorque of 372.4 Nm and 182.33 Nm were required in the

front and back axles respectively to lock up the wheels.The braking components provide 468.58 Nm and 222.52Nm of braking effort in the front and in the backrespectively, which is more than the required amount tostop the vehicle. This creates an acceptable factor ofsafety for the braking system on the car. Braking forceto lock all four wheels verses the tire coefficient was alsocalculated and graphed for analysis, which can be seenin Figure 26. This graph is beneficial to illustrate theamount of force needed to lock up the tires on variousterrains with different tire types. Figure 27 shows thatwith the cars braking components, it will be able toeasily stop at any speed that it is capable of traveling.Table 3 below gives an overview of the braking systemspecifications that were calculated.

Braking Specifications

Front Brake Balance (%) 38

Rear Brake Balance (%) 62

Driver Force on Pedal (lb.) 110

Average Circuit pressure (psi) 900

Pedal Ratio 5:1

Table 3- Outline of braking specifications

ELECTRICAL DESIGNOBJECTIVES

The electronic system for the car was designed to fulfilltwo key purposes. First, the electronics system supportsthe mandatory safety equipment, specifically the brakelight and the kill switch circuit. Second, the electronicsprovide useful instrumentation, in particular atachometer.

DESIGN

The cars electrical system has been designed aroundthree main power buses, each with an independently

fused circuit. These buses are the safety lights bus, theunregulated power bus, and the regulated power bus.The safety lights bus is connected directly to the batterywhile the regulated and unregulated buses areconnected through a 40A relay. Every poweredelectrical component on the car is connected to one ofthese three buses. These buses are managed from acentrally located sealed enclosure located in thedashboard.

The safety lights bus powers the brake light. The brakelight is activated by a brake pressure switch located inthe rear brake line. The electronics are designed so thatwhen the kill switch is depressed, power is disabled on

both the regulated and unregulated buses, but the safetylight bus remains connected so that the brake can stillfunction. Because the kill switch closes the circuit whenactivated, the kill switch function is achieved by using apair of diodes to simultaneously ground out the enginesprimary coil and bypass the normally-open relay on theregulated and unregulated buses. One diode preventsthe engine from grounding through the relay and theother diode prevents battery current from flowing backinto the ignition coil. A 5, 10W resistor is placed inseries before the relay and kill switch pair to limit thecurrent from the battery to the two branch circuits. A


11/16

10

second 5, 10W resistor is placed in series with the killswitch. This is arranged so that when the kill switch ispressed, the relay, with an internal resistance of 78,has a significant portion of its supply current shuntedthrough the kill switch branch that has a resistance ofonly 5. This reduced current flow through the relaybranch and is enough to cause the relay to shut off,which then disconnects power to the regulated andunregulated buses, but still leaves the safety lights busconnected to the battery independent of the relay.

The component that is connected to the unregulated busis the tachometer. This bus was designed for expansionthough, which may include off the shelf instrumentation,power for accessories, and other similar components.

Anything connected to the unregulated bus is either notsensitive to electronic noise and voltage spikes in thepower supply, or contains protection circuitry. Theregulated bus contains the circuitry that conditions thesignal for the tachometer. The original signal for thetachometer was intended to be generated by themanufacturers signal sender. This signal sender isdesigned to connect to an external shaft from the

engine. Because designing a shaft for this applicationwould be expensive and difficult, a custom signalconditioning circuit has been devised. The ideal signalto the tachometer was found to oscillate at 8 kHz andpossess an amplitude of 60mV for the needle to read4000 RPM. Any signal input with an amplitude of 25mVand below is read as zero RPM on the meter, which is aproblem. This problem was solved through using anoptoisolator along with a few other techniques.

This electrical system provides a reliable and efficientway to manage all the electronic components on theUWP Mini Baja car. Refer to Figure 28 for the fullelectrical schematic of the UWP Mini Baja car.

CONSUMER INTEREST

The appeal of the finished product to a consumer isequally as important as all other aspects of the designprocess. A consumer must find the vehicle to bereasonably priced, aesthetically pleasing, exhilarating todrive, safe, and dependable in order to insure that thevehicle will be purchased. These consumer factors werecontinually considered throughout the design of theUWP Mini Baja vehicle.

To create a vehicle that was cost effective,manufacturing processes were closely monitoredthroughout the design process. For example, the

chassis was designed in SolidWorks and imported intoBend-Tech making the chassis easy to manufacturewith computer aided machinery, lowering productioncost. Utilizing similar processes throughout the rest ofthe vehicles design and manufacturing lowered theoverall price of the vehicle.

The Mini Baja car body was design to be aestheticallypleasing. A fiber-glass molded hood and dashboard withrecessed body panels that show off the chassis innerworkings make the car look clean and fun to drive.

To make the Mini Baja car the best performing car at thetrack, special attention was given to the drive-train andsuspension design. Those two components mostsignificantly affect a consumers attitude about theperformance of an off-road vehicle. The drive-train givesthe car a top speed of 33 mph, while still being able toprovide 600 ft-lbf of torque at the wheels. Thesuspension was highly analyzed through the designprocess and performs just as well. The suspension willsupport handling at high rates of speed, while still beingable to articulate aggressively for a wide variety ofterrains.

Paramount to all other requirements was the safety ofthe driver. Once baseline safety requirements were inaccordance with the 2009 Mini Baja SAE CompetitionRules, the design did not stop there. For example, FiniteElement Analysis verified the robust nature of the frame,further ensuring consumer safety while operating thisvehicle.

Extensive FEA, computer aided solid modeling,calculations, and testing throughout the design process,made design assurance possible. These types of

analysis verify reliability in most types of terrain.Designing the vehicle to be simple, yet sophisticated,was another way the UWP Mini Baja car promotesreliability. Another aspect of reliability this vehicleprovides is its capability to be easily maintained.Maintenance is able to be performed on all componentsof the vehicle easily, because the vehicle was fully solidmodeled before manufacturing took place, allowing forthe opportunity to eliminate component placementconstraints which would make it difficult to performscheduled maintenance like changing the oil or replacinga CVT belt.

CONCLUSION

Once all the design aspects have been combined intoone complete vehicle, the result is profound. Safe,reliable, fast, aggressive, and just plain fun to drive iswhat the UWP Mini Baja car is all about. With a focuson the drive-train and suspension design, the vehicle willbe able to handle any terrain that is put before it. Thedrive-train sports a respectable low end wheel torque of600 ft-lbf, while still having an exhilarating top speed of33 mph. The suspension creates a ride that iscomfortable, yet aggressive when handling corners athigh speeds. This vehicle only has a 10 HP engine, butas a result of effective design techniques it has the

ability to conquer the most difficult terrain. Throughweight reduction, increasing drive-train efficiency, andcalculating and tuning the suspension accurately, thepower restraint is a minimum factor in this vehicles off-road ability. Not only will the performance catch aconsumers interest, but features such as a comfortableseat, a sleek body design, practical electronics, and costeffectiveness will impress even consumers that are notavid off-road vehicle enthusiasts. The careful designand the technology that went into this vehicle will proveitself during manufacturing, in the show room, and ofcourse, at the track.


12/16

11

APPENDIX

Fig. 1 2009 UWP Mini Baja Chassis (SolidWorks)

Fig. 2 Distributed load at 135 degrees relative to the roofof the chassis (Algor)

Fig. 3 Result of 135 degree loading (Algor)





Fig. 8 Maximum loading until yield (Algor)


13/16

12

Fig. 9 Loading on rear shock mount (Algor)

Fig. 10 Result of loading 800 lbf (yield) on rear shockmount (Algor)

Fig. 11 Loading on front shock mount (Algor)

Fig. 12 Result of loading 485 lbf (yield) on front shockmount (Algor)

Fig. 13 Torsional rigidity loading (Algor)

Fig. 14 Result of torsional rigidity loading (Algor)

Fig. 15 Body calculation

Fig. 16 Maximum lower front A-arm loading (Algor)


14/16

13

Fig. 17 Maximum lower rear A-arm loading (Algor)

Fig. 18 Force verses velocity plot produced by shockdynomometer

Fig. 19 Steering diagram

Fig. 20 Steering Clevis (SolidWorks)

Fig. 21 Steering equations

Fig. 22 Speed verses RPM with CVT transmission

Fig. 23 CVT implemented on car


15/16

14

Fig. 24 Planetary gear box assembly

Fig. 25 Drive-train installed in the car

Fig. 26 Brake force to lock verses tire coefficient

Fig. 27 Brake distance verses speed

Fig. 28 Electrical schematic

Fig. 29 Solid model of UWP Mini Baja Car(SolidWorks )


16/16

ACKNOWLEDGMENTS

The University of Wisconsin Platteville Society ofAutomotive Engineers Mini Baja team would like to thankall faculty members in the Engineering Department fortheir participation and cooperation in the development ofthe 2009 Mini Baja car. We would like to extend aspecial thanks to Dr. David Kunz, John Abbing, AaronRath, Mitch Wellsandt, Roy Navarrete, Clyde Holverson,Lou Behrens, and Casey Schueller for crucial knowledgein their areas of expertise. We would also like to extendour gratitude to our sponsors: Anderson Welding andRepair, Briggs and Stratton, EH Baare, KD Auto Service,Kaiser's Contract Cleaning Specialists, Polaris, QualityDrive Systems, and Signs to Go.

REFERENCES

Davis, Ian. Braking Data File. London, England, 2005.

March 7, 2008.

Callister, W. D. Materials Science and Engineering. Sixth

Edition. New York, New York: John Wiley & Sons, Inc.2003.

Gere, James M. Mechanics of Materials. Sixth Edition.

Belomont, CA 94002: Bill Stenquist, 2004.

Hoa, D. G. Composite Materials Design and Application.

Second Edition. Boca Raton: CRC Press Taylor &

Francis Group, 2007.

Milliken, Doug F. and Milliken, William F..Race Car

Vehicle Dynamics. SAE International, December 1995.

Norton, Robert L. Design of Machinery. Fourth Edition.New York, New York 10020: McGraw-Hill Inc., 2008.

SAE International. "2009 Baja SAE Competition Rules."

September 27, 2009.

.

CONTACT

Brendan Behrens, Captain, [email protected] Engineering Student

Kyle Droessler, Co-Captain, [email protected]

Mechanical Engineering Student

Dr. David Kunz, Faculty Advisor, [email protected] Engineering Chairman

DEFINITIONS, ACRONYMS, ABBREVIATIONS

AISI: American Iron and Steel Institute

ATV: All Terrain Vehicle

Chromoly: Chromium-Molybdenum Steel

CNC: Computer Numerical Control

CVT: Continuously Variable Transmission

DOM: Drawn Over Mandrel

FAB: Fore/Aft Bracing Member

FBM: Front Bracing Member

FEA: Finite Element Analysis

FLC: Front Lateral Cross Member

GTAW: Gas Tungsten Arc Welding

HDPE: High Density Polyurethane

ID: Inside Diameter

LBD: Rear Roll Hoop Lateral Diagonal Bracing

Lbf: Pound Force

LC: Lateral Cross Member

LFS: Lower Frame Side Member

MPH: Miles Per Hour

OD: Outside Diameter

PSI: Pounds Per Square Inch

RHO: Roll Hoop Overhead Member

RPM: Revolution Per Minute

RRH: Rear Roll Hoop

SAE: Society of Automotive Engineers

SIM: Side Impact Member

UWP: University of Wisconsin Platteville

University of Wisconsin Platteville_Baja Wisconsin

Documents

Transcript of University of Wisconsin Platteville_Baja Wisconsin