2014 Bearcats Baja Rear Suspension System

Home , Custom wheel, Inboard brake, Weight transfer

A Baccalaureate thesis submitted to the Department of Mechanical and Materials Engineering College of Engineering and Applied Science University of Cincinnati

In partial fulfillment of the Requirements for the degree of Bachelor of Science In Mechanical Engineering Technology

By:

Zack Freije

April 2014

Thesis Advisor: Dean Allen Arthur 2014 Bearcats Baja – Rear Suspension System

Zack Freije Rear Suspension

ABSTRACT Each vehicle is evaluated in terms of ergonomics, functionality, and producibility. To this end, design Baja SAE is an annual intercollegiate engineering design reports must demonstrate both sound engineering competition organized by the Society of Automotive principles as well as economic feasibility for Engineers, and serves as a capstone project for senior production. Mechanical Engineering Technology students at the University of Cincinnati. The 2014 Bearcats Baja team An off-road vehicle’s suspension system serves three vehicle is a complete new build from the ground up. This primary functions: to maintain tire contact with the design freedom allows for drastic improvements over the ground while the vehicle navigates rough terrain, to 2013 rear suspension system performance. Extensive reduce impact forces transferred to the driver, and to research was conducted to determine the key provide optimal vehicle handling dynamics. (1) performance metrics for the system. Several design options were considered and a final design was selected RESEARCH based on system integration, manufacturability, system weight, cost, and design specifications required to meet Full compliance with all relevant Baja SAE rules is the performance metrics. Failure modes were required. The applicable rules have been supplied in established based on 2013 in-use performance results. Appendix B. (2) FEA optimization was performed side by side with the 2013 system to establish a baseline for improved stress A list of reference material used for concept and handling while optimizing weight. The final design was application of design are given in Additional Sources. produced with a 22% cost savings, 7% weight reduction, and was verified to within 0.5° for toe/camber angle and Great attention was placed on the design and within 1/16” for all setting dimensions relative to performance of the 2013 rear suspension system during established design specifications. competition. In addition, a careful survey of design choices was conducted for all the top 10 teams during INTRODUCTION competition in 2013. Time was spent discussing the merits of these choices with the competing teams to gain Baja SAE is an annual intercollegiate engineering design a better understanding of the engineering justification. competition run by the Society of Automotive Engineers (SAE). Teams of students from Universities around the Five primary design choices were identified for further world design and build small off-road vehicles powered study: by engines with identical specifications.  Single rear inboard brake – eliminates need for All teams must adhere to the rules and pass SAE’s heavy bearing carrier to support caliper, reduces technical and safety inspections. At each competition total vehicle weight. there are design evaluations and multiple dynamic events followed by a four-hour endurance race. Dynamic  Trailing arm integrated bearing carrier – a simple events for 2014 include maneuverability, rock crawl, and and lightweight bearing carrier can be welding into suspension & tracking. The endurance race carries the the trailing arm to reduce weight, complexity and single largest point total. These events require a hardware. suspension that is agile as well as robust.  Custom aluminum wheel hub – a simple and Furthermore, each team is competing to have its design lightweight hub can be manufactured for custom selected by a theoretical power sports manufacturing backspacing. Can eliminate rotor mounting bosses company with the stated intent of a 4,000 unit annual and hardware. production volume. Each team is responsible for all phases of the research and development process, from  Narrow rear chassis/Longer control arms – fundraising to designing, manufacturing and testing. Reduce length of heavy frame tubes. Increase 2

length of lateral links to provide better control over camber change rate.

 Minimize size of heim joints/Mount in double shear – Heim joints can be reduced in size if proper calculations are made to determine the worst case forces. Mount them in double shear to reduce bending moment on the threads.

Example pictures for these design cues are given in Appendix C. Figure 3 – Pitch Conditions

The primary levers for improving rear suspension 20-30% antisquat is preferred. This increases the normal performance are: force at the tires and improves acceleration. Antisquat also increases suspension stiffness, producing jacking forces similar to an anti-roll bar without reducing body  Toe Settings roll.  Camber Settings  Pitch Settings ROLL AXIS  Roll Axis

TOE SETTINGS

Figure 4 – Roll Center Height

Figure 1 – Toe Conditions Low roll center height is ideal as this leads to lateral acceleration weight transfer via body roll. Roll center Positive toe is preferred. For a rear wheel drive car, rear axis ideally slopes down towards the front of the vehicle toe-out leads to overseer which promotes a faster which allows for weight transfer to the front of the vehicle maneuverability response. Static toe or no change in toe during a turn. This allows more grip on the steering tires angle over the suspension travel is preferred. Dynamic and allows the rear end to break free and slide which or changing toe leads to bump steer. creates oversteer.

CAMBER SETTINGS DESIGN SELECTION - Early design concepts that were rejected include: dual a-arm, torsion bar, inboard shock, single tube trailing arm, toe control link. It was observed at competition that failure of any part of a dual a-arm setup would result in disengagement of the drive axle, rendering the vehicle immobile and costing valuable time being towed into the pits. The 2013 Bearcats vehicle sustained complete failure to one of the lateral links during an endurance race, but was able to complete a lap and drive into the pits under its own power. It was determined that torsion bars add weight and complexity Figure 2 – Camber Conditions and attempt to correct for problems with the initial suspension design. Toe control links added weight and Negative camber is preferred. Negative camber leads to complexity, but were unnecessary because the straight line stability. Static camber or no change in suspension could be designed such that optimal toe was camber angle over the suspension travel is preferred. achieved without them. The single tube trailing arm was a lead option until it was determined that SAE chassis PITCH SETTINGS rules made the desired mounting location impossible. It was also considered that a single tube would have

3 greater potential to fail due to buckling after impacting DESIGN SPECIFICATIONS - The following constitutes a stationary objects on course. complete list of the selected design specifications to achieve with this rear suspension system design. The selected design incorporates a semi-trailing arm and a pair of lateral links. This is similar to the suspension  Vertical Wheel Travel: 10.6” (269.2mm) used on an Integra Type R. This is the third year  Recessional Wheel Travel: 0.5” (12.7mm) Bearcats Baja has used and improved this general  Static Ground Clearance: 11” (279.4mm) design.  Jounce Ground Clearance: 4” (101.6mm)  Static Roll Center: 8” (203.2mm)  Motion Ratio: 0.5  Sag: 33% of total travel  Total Track Change: 4” (101.6mm)  Track Change in Roll: 0” (0mm)  Static Toe: 0 deg  Toe Change Rate: 0.5 deg/in  Static Camber: -0.5 deg  Camber Change Rate: 0.25 deg/in  Percent Anti Squat: 20%-30%  System Weight: 50 lbs (22.7 kg)

SYSTEM INTEGRATION – To accomplish the design specifications, both system integration and rules Figure 5 – Selected Design compliance were key factors. System integration required collaboration with braking, drivetrain, chassis, and front suspension systems.

To reduce the weight for the rear suspension, the brakes were moved inboard, allowing for elimination of the purchased bearing carrier. A new custom bearing carrier was integrated into the trailing link.

The Rear Roll Hoop was lengthened, lowering the drivetrain and vehicle CG, and optimizing the angle of the axles for increased power transfer and reduced plunge travel. In addition, the Rear Roll Hoop was made laterally narrow rearward of the gearbox to allow for longer trailing links which provide the desired camber change rate. Initial design concepts for trailing link geometry and mounting locations were rejected due to rules compliance for the Roll Hoop. Figure 6 – Axle Section View WEIGHT - The wheels and tires represent 61% of the DESIGN GOALS total system weight. However, weight was increased by 2% over last year for reliability. Previously, Douglas Blue PRIMARY OBJECTIVES Label (0.125” wall) were used, however all were bent during normal use. To improve this, ITP T9 Pro (0.190”  Improve handling response* wall) were selected which are considerably stronger and Reduce Weight  weigh 0.38 lbs more per corner. The same lightweight  Reduce Cost 23x7-10 Carlisle 489 tires were selected due to their performance at competition. Selecting a stiffer wheel will *Handling Response - Dynamic handling is subjective, mitigate the negative impact of the 2-ply tire. and prior to testing, is largely theoretical with respect to design. Each driver will have a handling preference and A reduction of 4 lbs (7% of total system weight) was each overall vehicle will respond differently to the same achieved by replacing the Polaris Sportsman 300 design. Course design and conditions also play a factor. bearing carrier with a custom carrier that was integrated Holistic handling response takes into account the into the trailing link. The new bearing carrier was interaction between the front and rear suspension designed around a Honda TRX420 which allowed for designs. additional weight reduction of related components. This

4 also allowed us to install inboard and outboard dust In order to optimize the tube size and wall thickness, two seals in order to improve the life of the bearings. graphs were used: C/I vs. Weight and Stress/Unit Length vs. Linear Weight (shown in the figures below). The lead Custom aluminum wheel hubs were designed which options were then tested in various combinations in FEA provided a potential 4.5 lbs (8% of total system weight) to achieve a desirable safety factor for the worst case reduction, however, Honda TRX420 hubs were loading conditions. The three primary failure modes purchased due to funding and manufacturing capability considered for the trailing arms are: 5’ vertical drop (2 shortfalls related to broaching the internal spline. This wheel landing), forward impact to an obstacle (tree or resulted in a 1.17 lb increase in weight. vehicle) at the vehicle top speed of 32mph, and a vertical drop, landing directly on the trailing arm tubes. Additional weight reduction was achieved through The lateral links were optimized in FEA for a full speed calculations and FEA to optimize weight and strength direct impact collision from another vehicle to simulate simultaneously. In addition, all mounting brackets and the failure mode experienced in the endurance hardware was sized and weight minimized using FEA competition last year. and hand calculations. Total weight for the system is 53 lbs, meaning a reduction of 4 lbs or 7% was achieved.

DESIGN OPTIMIZATION - In order to meet the design specifications, a SolidWorks 3D dynamic sketch was used. Sensors were placed on key components in order to quantify the impact of geometry or dimensional changes. Outputs include: wheel travel, ground clearance, track change, motion ratio, toe, camber, roll center and axle plunge depth. The final solid model was moved through the same range to verify the predicted outputs.

Figure 9 – Material Optimization

Figure 10 – Material Optimization

Figure 7 – 3D Design Optimization Sketch

Lower unit weight is desired as is a lower C/I value.

The material selected is AISI 4130 steel, stress relieved after welding and normalized to achieve a yield strength of 96 ksi. (3) For manufacturability, trailing arm tubes all have the same 1” OD. The lower members were FEA optimized to have a larger wall thickness (0.049”) for greater strength as needed due to direct impact with obstacles. This represents a 15% weight improvement per unit length over the 2013 design and a 17% lower

C/I value. The remaining tubes have a wall thickness of Figure 8 – Design Optimization Solid Model 0.035”. This represents a 38% improvement for weight and a 62% lower C/I value. The trailing arms are 5 designed to a safety factor of 1.9. The lateral links were FEA optimized to 0.875” OD and 0.049” wall. The lateral links are designed to a safety factor of 1.4.

All suspension pivot points were designed for less than 0.5° of misalignment, however, modest misalignment spacers were used to improve both manufacturability and serviceability. Lateral Links have opposing LH and RH heim joints with threaded inserts and jam nuts that allow for camber adjustment. The opposing thread types 1,700 lbf prevent the system from self-adjusting during use due to vibration. The heim joints are mounted in all cases in double shear with a bending safety factor of 1.9. Figure 13 – Vertical Drop to Stationary Object

All failure mode conditions were replicated using last year’s models for a direct delta comparison of max stress and stress concentration. In all cases, the new design is equal or better in handling stress as compared to last year.

The hardware is kept a consistent size for serviceability, and is designed to a safety factor of 4.8, calculated using the worst case resultant force found using FEA. 3,000 lbf

Figure 14 – Rear Impact to Lateral Link, Worst Case Position

1,700 lbf

Figure 11 – Two Wheel Vertical Drop, Five Foot Elevation

Figure 15 – Progressive Air Spring Curve

At 5.5 inches of travel, the air spring requires 1400 lbf (when inner air pressure is set to 70 psi) in order to achieve full compression. This reaction force was applied to the shock mount bracket with the trailing arm 2,700 lbf fixed in place. This did not produce the worst case scenario for stress response in FEA. Figure 12 – Forward Impact to Stationary Object Vehicle crash data was reviewed to establish a reasonable g-force at impact. An average value of 5 g was used to calculate the impact force. (4)

Forward Impact to Stationary Object – Initial calculations considered time of impact, however, no reliable testing data exists to validate this approach, so crash test data was used instead. This is to simulate hitting another vehicle or a stationary object such as a tree. The crash test data average of 5 g was used for this calculation.

Figure 16 – Impact Force Test Data

2 Wheel Vertical Drop (5 feet) – Initial calculations were performed assuming conservation of energy and using work and energy equations, given the available travel of the dampers. Forward momentum was neglected and Vertical Drop to Stationary Object – This is to simulate only vertical components were considered. This was hitting a log or rock with a sharp edge. The surface area compared to the impact force calculated using the 5 g the force is applied to is 1/8” wide. The same 1,700 lbf is deceleration from the crash test data. The largest impact used as in the 2 wheel drop scenario. force was used for FEA. Rod End Bending – Rod ends are most likely to fail in bending in the threaded region. To properly size a rod end, it is required to have an allowable load greater than 10 times the axial load. The worst case resultant force component on the control arm rod end is 811lbf. Based on this calculation, the 5/16” shank rod end is selected, with a safety factor of 1.9. (6)

(6)

Bolt Shear Calculations – All system bolts were sized based on shear strength calculations given the worst case loading condition resultant force. Double shear mounting condition is used in all locations. Grade 8 hardware is used in all locations with nylon locking nuts. 5/16” bolts selected provide a safety factor of 4.8.

Antisquat Calculations

Figure 17 – Double Shear

Figure 19 – Antisquat Geometry

Figure 20 – Antisquat Geometry

Figure 18 – Bolt Strength Formulas (7) 8

Roll Center Height and Roll Axis

Figure 24 – Trailing Arm Assembly Fixture and Weld Jig

Figure 21 – Rear View Roll Center Height

Figure 25 – Control Arm Mounting Bracket Attachment Figure 22 – Side View Roll Center Height The jig was made from available spare extruded The roll center height was successfully reduced and aluminum t-slot. Fixturing attachments were designed adjusted to work effectively with the front suspension for and fabricated in the shop. It allows for both RH and LH better overall vehicle dynamics. assemblies to be assembled, and has attachments for every step in the manufacturing process. MANUFACTURABILITY – A simple and flexible weld jig was designed and fabricated for the trailing arms using off the shelf aluminum structural framing components combined with machined bosses to position key components and act as heat sink. Tubes were bent and partially coped by Cartesian. (5) The remaining trimming and coping at assembly was performed in house using a custom alignment jig.

Figure 26 – Setting Up for Bearing Carrier Tube Cope

Figure 23 – Flexible Trailing Arm Fixture Design

Figure 27 – Custom Tube Inserts to Allow Clamping 9

Custom tube inserts were machined to prevent deforming the trailing arm tubes while clamped in place. They were tapped and a bolt inserted in the event they would be difficult to remove. An alignment jig was made to provide accurate hole cutting position. An overhead drill press on low speed with sufficient cutting oil was used to perform the necessary cope in the trailing arm tubes. Subsequent fitting of the bearing carrier was problem free, with a modest 0.5mm gap on either side.

Figure 30 – Shock Mounting Bracket

Figure 28 – Bearing Carrier Fitment Figure 31 – Control Arms All shock and control arm mounting brackets were pre- assembled from plasma cut 4130 1/16” steel sheet using RESULTS custom weld jigs to position the tube alignment reliefs. Frame mounting brackets followed the same process, DESIGN SPECIFICATIONS ACHIEVED each with a unique custom machined jig to hold the bolt centers in-line and position the bolt axis relative to the  Vertical Wheel Travel: 10.6” (269.2mm) bottom of the vehicle. The pre-assembled shock mount  Recessional Wheel Travel: 0.4” (10.2mm) brackets with tube copes are easily placed on the tubes  Static Ground Clearance: 11” (279.4mm) limiting them to two degrees of freedom. Additional jigs  Jounce Ground Clearance: 4” (101.6mm) were used to position the trailing arms according to the  Static Roll Center: 6.5” (165.1mm) SolidWorks design and align the shock mounting  Motion Ratio: 0.5 brackets to reduce the need for misalignment spacers.  Sag: 33% of total travel  Total Track Change: 3.8” (96.5mm)  Track Change in Roll: 0” (0mm)  Static Toe: +0.76 deg  Toe Change Rate: 0.27 deg/in (0.01 for 95% of use)  Static Camber: -0.6 deg  Camber Change Rate: 0.1 deg/in  Percent Anti Squat: 26%  System Weight: 53 lbs (24 kg)

Figure 29 – Frame Mounting Bracket

incorporated consideration for ease of manufacturing, assembly and use. The manufacturing and assembly process went smoothly and the low-cost, flexible jig was able to deliver the required dimensional accuracy for designed suspension performance. After assembly to the vehicle, the suspension met all of the design specifications within an exceptional margin of error. There are additional design improvements and weight savings that were unfeasible for this design iteration, but are described in the Recommendations section. Vehicle dynamic performance at competition will provide additional data points for ongoing design optimization to improve future vehicle executions.

RECOMMENDATIONS

Figure 32 – Validation and Testing of Design The easiest way to reduce weight and provide design Specifications flexibility for the rear suspension system is to make custom aluminum hubs. The COTS hubs used account SYSTEM WEIGHT for 11% of the total system weight. Material can be provided through sponsorship; however, machining 2013 components weighed back to back with 2014 capability is a challenge to be overcome. The greatest components on the same scale multiple times, with the challenge will be in broaching the necessary internal median values being used for comparison. spline. These will need to be adequately tested prior to competition to validate the FEA results. Consider a secondary COTS option in case testing does not validate the design.

SYSTEM COST (2 complete sets, plus spares)

Figure 33 – 2014 Honda Hub

The cost was reduced by designing for the elimination of components where possible and to fabricate parts rather than purchase them. Parts and material were reused where applicable. Material sponsorship was negotiated as were pricing discounts for purchased components. The requirement for material was also reduced by this design.

CONCLUSION Figure 34 – New Hub Design Concept

The selected rear suspension system design was able to With more investment in fixturing, dimensional accuracy deliver on all the design specifications. The system could be improved to the point that some of the heim provided both cost and weight savings. The suspension joints could be removed in favor of a Delrin or design was able to deliver these results while polyurethane bushing. This will provide additional weight outperforming the 2013 design in the failure modes savings. determined by competition performance. The design 11

The rear semi-trailing arm design can be further 5. "Cartesian Tube Profiling - Impossibly Accurate optimized for weight by converting to a single tube arm. Tube Profiling." Cartesian Tube Profiling. N.p., n.d. 1.25” diameter, 0.049” thick wall should be sufficient for Web. 23 Apr. 2014. the given loading conditions. The forward mounting location to the frame should be changed to a heim joint 6. "FK Rod Ends- Home Page." FK Rod Ends- Home and mounted in a configuration to allow for the single Page. N.p., n.d. Web. 23 Apr. 2014. tube design. This may require a custom hub offset or 7. "Fastener, Bolt and Screw Design Torque and Force custom wheel offset to ensure the shock clears the tire. Calculation - Engineers Edge." Fastener, Bolt and Screw Design Torque and Force Calculation - There is significant weight in the wheels and tires. There Engineers Edge. Engineers Edge, n.d. Web. 23 Apr. is additional work to be done to fully test a wide array of 2014. combinations (including air pressure) to optimize for all anticipated course conditions while reducing weight. This requires time investment after the vehicle is complete CONTACT and represents a substantial financial investment as well. I recommend either: 1. Douglas Blue Label (0.125” Zack Freije wall) combined with a 4 or 6 ply tire and sufficient air Rear Suspension pressure, or 2. Douglas Black Label (0.160” wall) [email protected] combined with a 2 ply tire such as the Carlisle 489. Keep in mind that there are course conditions in which low tire ADDITIONAL SOURCES pressure provide greater traction. Aird, Forbes. Race Car Chassis: Design and ACKNOWLEDGMENTS Construction. Osceola, WI: Motor International, 1997. Print. Thanks are due to the University of Cincinnati for their "BajaSAE Forums." BajaSAE Forums. N.p., n.d. Web. enduring support of the Bearcats SAE program. In 22 Apr. 2014. addition, the MET program provided a strong basis for success in this project due to the inherent combination of Dixon, John C. Suspension Geometry and Computation. theoretical and applied science. The faculty has been Chichester, U.K.: Wiley, 2009. Print. most gracious with their time and expertise in supporting Gillespie, T. D. Fundamentals of Vehicle Dynamics. our efforts. Most notably our faculty advisor: Dean Allen Warrendale, PA: Society of Automotive Engineers, 1992. Arthur. Print. Milliken, William F., and Douglas L. Milliken. Race Car Thanks are also due to our generous corporate Vehicle Dynamics. Warrendale, PA, U.S.A.: SAE sponsors, without whom we would not have been able to International, 1995. Print. finance the project. These include: Toyota, Gallatin Steel, Kaiser Aluminum, Solidworks Corp., VR3 Smith, Carroll. Engineer to Win. Osceola, WI: Motor Engineering, Polaris, Cincinnati Gearing Systems, International, 1984. Print. Cincinnati Steel Treating, Honda of Fairfield, Wilwood, Staniforth, Allan. Competition Car Suspension: Design, General Cable, Dana, Grainger, and Discount Tire. Construction, Tuning. Newbury Park, CA: Haynes North America, 1999. Print. REFERENCES

1. "Baja SAE." Wikipedia. Wikimedia Foundation, 18 Mar. 2014. Web. 22 Apr. 2014. 2. "2014_baja_rules_8-2103." Rules & Documents. SAE Collegiate Design Series, 2014. Web. 22 Apr. 2014. 3. "AISI 4130 Steel, Normalized at 870°C." ASM Material Data Sheet. N.p., n.d. Web. 23 Apr. 2014. 4. Linder, Astrid, and Matthew Avery. "CHANGE OF VELOCITY AND PULSE CHARACTERISTICS IN REAR IMPACTS: REAL WORLD AND VEHICLE TESTS DATA." National Highway Traffic Safety Administration. The Motor Insurance Repair Research Centre, n.d. Web. 22 Apr. 2014.

APPENDIX A COTS: Commercial Off-The-Shelf Roll center height: Found by projecting a line from the DEFINITIONS, ACRONYMS, ABBREVIATIONS center of the tire-ground contact patch through the front view instant center. [7] SAE: Society of Automotive Engineers Motion Ratio: The mechanical advantage (lever ratio) MET: Mechanical Engineering Technology that the wheel has over the spring in compressing it Track Width: Distance between the centerline of the Heim Joint: Rod end bearing. A mechanical articulating contact patches of the tires when looking at the vehicle joint consisting of a ball swivel with an axial opening in front view through which a bolt may pass which is pressed into a Wheelbase: Distance between the centerline of the casing with a threaded shaft attached contact patches of the tires when looking at the vehicle Control Arms: A pair of lateral suspension links that are in side view adjustable in length to affect changes to camber and toe Toe Angle: The angle, positive or negative, of the wheel angle from straight ahead. Toe to the outside of the vehicle is, CG: Center of gravity, specifically for the entire vehicle by convention, positive and results in less stability and driver Camber Angle: The angle between the vertical axis of ‘: Foot the wheels the vertical axis of the vehicle when viewed “: Inches from the front or rear Bump Steer: Steering motion without driver input Lb: Pound resulting from the translation of the wheel and Lbf: Pound force suspension through its swept arc as a reaction to bump Lbm: Pound mass loading Slugs: English unit of mass Roll Steer: Steering motion without driver input resulting from the translation of the wheel and suspension through FPS: Feet per second its swept arc as a reaction to the rolling of the vehicle MPH: Miles per hour while corning psi: Pounds per square Inch Anti-Squat: This suspension characteristic uses acceleration-induced forces in the rear suspension to ksi: Kips per square inch reduce squat of the vehicle. A value of 100% means Kip: 1000 pounds that all of the weight transfer is being carried through the suspension linkage, not the dampers Shocks: Commonly referred to as a shock absorber, and performs the function of a damper between the road input and the response of the vehicle

APPENDIX B

APPENDIX C

COMPETITIVE RESEARCH

Rear inboard brake Integrated bearing carrier

Narrow chassis, long control arms

APPENDIX D

EXPENSES

Expense Tracking Spreadsheet (includes 2 full sets, plus spares)

Vehicle Budget Distribution

APPENDIX E

WEIGHT

Weight Tracking Spreadsheet

APPENDIX F

SCHEDULE

Overall Task Schedule

Example Detailed Task Schedule by Section

APPENDIX G

REAR SUSPENSION ASSEMBLY DRAWINGS

APPENDIX H

FIXTURE DRAWINGS