2013 Baja Car Rear Suspension Redesign
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
Sahil Patel
September 2014
Thesis Advisor:
Dean Allen Arthur
ACKNOWLEDGMENTS
I would like to thank Dean Allen Arthur for his ongoing support, patience, and guidance throughout this project. I would also like to thank Mr. Ronald Hudepohl, Mr. Douglas Rife, Mr. David Conrad, and Mr. Bill Hansel for their assistance with the production and assembly. This project would not be able to have been completed without them.
ii TABLE OF CONTENTS
ACKNOWLEDGMENTS ...... II TABLE OF CONTENTS ...... III LIST OF FIGURES ...... IVV ABSTRACT ...... V INTRODUCTION ...... 1
PROBLEM STATEMENT...... 1 INTERVIEWS ...... 1 CUSTOMER REQUIREMENTS ...... 2 PROOF OF DESIGN ...... 3 PREVIOUS DESIGN ...... 4 RESEARCH ...... 5 DESIGN PROCESS...... 8 REFERENCES ...... 11 APPENDIX A - MANUFACTURING AND ASSEMBLY ...... 12 APPENDIX B - BUDGET...... 17 APPENDIX C - QUALITY CONTROL ...... 18
iii LIST OF FIGURES
Figure 1 – List of Customer Requirements……………………………………………………2 Figure 2 – Figure 2 – Proof of Design ….…………………………………………………….3 Figure 3 – Previous Linkage Mounting Points ……………………………………………….4 Figure 4 – Double A-Arm Suspension………………………………………………………...5 Figure 5 – MacPherson Strut Suspension……………………………………………………..6 Figure 6 – Multi-Link Suspension…………………………………………………………….7 Figure 7 – Motion Sketch of Rear Side Wheel ……………………………………………….8 Figure 8 – Motion Sketch of Camber Change……………………………………………...... 9 Figure 9 – Table with New Mounting Points………………………………………………….9 Figure 10 – 3D Final Model………………………………………………………………….10 Figure 11 – Dynamic Mass in Rear Axle…………………………………………………….10 Figure 12 – von Mises Stress Analysis………………………………………………………11 Figure 13 – Bottom Mounting Bracket………………………………………………………12 Figure 14 – Top Mounting Bracket………………………………………………………….13 Figure 15 – Bottom Link……………………………………………………………………..14 Figure 16 – Top Link………………………………………………………………………...15 Figure 17 – Bung Fitting……………………………………………………………………..16 Figure 18 – PVC Pipes……………………………………………………………………….18
iv ABSTRACT
The Society of Automotive Engineers (SAE) hosts a yearly intercollegiate Baja race at various locations throughout the nation. Every college that chooses to compete in this event assembles a team of undergraduate and / or graduate students to compete. This competition will include competitions such as acceleration, hill climb, land maneuverability, suspension and endurance. I will work on the redesign of the rear suspension that will be safe, reliable, cost effective, and easy to maintain. The suspension redesign will be my senior design project.
v 2013 Baja Car Rear Suspension Redesign Sahil Patel
INTRODUCTION
PROBLEM STATEMENT
Design an improved rear suspension in the 2013 UC Baja Car. The previous rear suspension design of the 2013 Baja Car has an understeer causing poor handling. The new design of the rear suspension must have a roll center of the rear suspension that is closer to the current center of gravity which is 24”.
The relationship between the roll center and the center of gravity creates a moment on the vehicle in turns. The greater the distance between the roll center and the center of gravity the greater the moment the vehicle must overcome (2).
INTERVIEWS
Ricardo Hinojosa was a former Bearcats Baja member, class of 2013. He is knowledgeable on the 2013 car due to his experience from building the car. He has expressed his desire to reduce the understeer on the car. According to him, the geometry of the linkages doesn’t match the suspension. This bit of information was important in developing my problem statement revolving around the roll center of the vehicle.
Joe Kobs was the former president of the Bearcats Baja team in the year 2014. He helped with the manufacturing of the 2013 as a 4th year Mechanical Engineering Technology student. He stated that the 2013 car also has a problem with understeering. According to him reliability of the suspension is imperative from his previous experience with the team. It was critical to him that I have a design that will be able to withstand the maximum forces during competition.
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CUSTOMER REQUIREMENTS
There are two primary customers for this project. The first is the Society of Automotive Engineers (SAE). Every year SAE host an intercollegiate Baja Competition at 3 various locations throughout America. These completions consist of 4 dynamic events; acceleration hill climb, maneuverability, and rock crawl/suspension. On a yearly basis they issue rules and regulations at which a vehicle must adhere to.
The second customer will be the University of Cincinnati Bearcats Baja team. They will be using this car for future competitions and also for practice purposes.
The following are the requirements that have been determined.
1. Safety The vehicle must be safe to operate. Vehicle must adhere to SAE Baja rules. 2. Reliable Must support person 200 lbs. weight in seat. The design must be able to last in a SAE Baja race. The camber angle must change less than 5° in the travel motion. The travel motion of the rear wheels must at least be within the shock limit of +4.00 inches and -0.50 inches vertically. 3. Easily maintained Quick disconnects for the assembly parts. Must be independent. Can be maintained by using standard tools. Minimal fabrication will done The left and right side must mirror each other 4. Cost Must stay within the allotted budget
Figure 1 – List of Customer Requirements
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PROOF OF DESIGN
Proof of design will be conducted by performing a test of the travel motion of the newly designed linkages. The travel motion must be willing to travel up from resting position a minimum distance of 4.25 inches. The distance of 4.25 inches is the distance determined by the range of movement in the shock shown in figure 7.
Figure 2 – Proof of Design
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PREVIOUS DESIGN The previous design of the rear suspension linkages did not form a roll center. The outer linkage mounting points (side mounted wheel side) were closer in the Y axis than the inner mounting points (side mounted to the chassis), as shown in the figure below. The outer side distance is 5.5 inches and the inner side is 5.75 inches.
Figure 3 – Previous Linkage Mounting Points
This results in a roll center that does not exist. When this happens the forces in a turn go in two different directions. The forces going in the two different directions assist in the issue of the understeer on the car (4).
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RESEARCH
Double A-Arm Suspension This type of suspension is also referred to as the double wishbone. It This system is heavier utilizes a dual “A” shaped arm. The two arms are on the same and takes up more horizontal plane and have a shock absorber running through the space than the arms. The top arm will be shorter than the bottom arm (4). Each arm counterparts. This is has two mounting points connected to the frame and one joint at suspension is the knuckle. This type of system is good for increasing negative complex and has camber angle. In turn there are great traction qualities for turns. The multiple parts camber of the tire is also easily adjustable in this type of suspension. associated with it. The costs associated with this particular suspension are higher than the counterparts. Any maintenance for the double A will be time consuming.
(4)
Figure 4 – Double A-Arm Suspension
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MacPherson Strut Suspension The MacPherson Strut system is very common for modern day cars. It is difficult to The shock absorber will be mounted vertically to the wheel hub. A maintain traction control arm will mounted from the frame to the wheel hub for some while cornering. This additional support (1). This is a simple design with low maintenance is not a good design required. There is less room taken up by the suspension than the for lowered vehicles. double A arm and in turn this is a lighter option. This will help with The camber is set and the car’s acceleration due to the fact that there will be less weight to not easily adjustable. carry. This is the most economical of the three mentioned This type of suspensions. suspension will have trouble moving horizontally.
(4)
Figure 5 – MacPherson Strut Suspension
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Multi-Link Suspension This type suspension is a cousin of the double A suspension. The This can get to be multi-link suspension uses three or more lateral arms and one or expensive to build. more longitudinal arms. There is a spherical joint at each end of the The design of this connecting arms. The shock absorbers can mount to the longitudinal suspension is the arms if desired or mounted elsewhere if that is a more desirable most complex of the option. This allows for more flex for the vehicle which makes it three types great for off road applications(3). This suspension is a happy mentioned. This type medium in terms of space in between the MacPherson and double A. of suspension absolutely must be finely tuned and if there are any deviations from the plan, it can cause damage to the vehicle and possibly driver. The car currently has a multi-link suspension, re- designing the linkages would be the most cost-effective plan moving forward. (4)
Figure 6 – Multi-Link Suspension
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DESIGN PROCESS
The very first thing to be determined in the designing of linkages is the appropriate travel motion of the rear wheels. Figure 7 is a motion sketch of the side view of the rear side of the vehicle.
Axis 4.00 inches
Shock
0.25 inches
Figure 7 – Motion Sketch of Rear Side Wheel
Adjusting the axis shown above shows the compression of the shock. When the car is motionless on the ground the shock is compressed at 0.25 inches. When the shock is in full compression it is 4.25 shorter meaning the travel needs to be +4.00 inches and -0.25 inches. The second thing to consider in the design process was the change in camber while the
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wheels move up and down. The optimum camber change rate I was looking for was ± 5°. This was done by creating another motion sketch (Figure 8).
CG
Roll Center
Figure 8 – Motion Sketch of Camber Change
By moving the wheels up and down the camber of the wheel can be measured as well as determining the optimal mounting points for the new linkages. The new changes in the mounting points and camber change were recorded in a table (Figure 9). This table was paramount in finding the best combination of roll center, new mounting points, and camber change.
Figure 9 – Table with New Mounting Points 9 2013 Baja Car Rear Suspension Redesign Sahil Patel
The only issue with this method of design was the length of the linkages was only considered in the X and Y direction. The total length of the linkage was determined via 3D modeling which is shown in figure 10.
Figure 10 – 3D Final Model
The final step in the design process was the Finite Element Analysis (FEA). The FEA was conducted using SolidWorks software. The benefit of FEA is that I will allow me to know where the possible failure would lie. The equation below in figure 11 shows the dynamic mass in the rear axle.
Figure 11 – Dynamic Mass in Rear Axle
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Figure 12 below is the von Mises stress analysis. The analysis shows how the assembly will deform in the worst case scenario. A force of 131.1 lbf is applied in the study. There are not any critical points that need to be worried about; they would be indicated by the bright red.
Figure 12 – von Mises Stress Analysis REFERENCES
1. "Mechanical Engineering." Comparison between MacPherson & Double Wishbone Suspension System ~. Web. 9 Feb. 2015. 2. Milliken, William F., and Douglas L. Milliken. Race Car Vehicle Dynamics. Warrendale, PA, U.S.A.: SAE International, 1995. Print. 3. Raiciu, Tudor. "How Multi-Link Suspension Works." Autoevolution. 17 June 2009. Web. 25 Oct. 2014.
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APPENDIX A- MANUFACTURING AND ASSEMBLY
BRACKETS
Figure 13 – Bottom Mounting Bracket
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Figure 14 – Top Mounting Bracket
Both brackets were made via profile milling. The brackets were the MIG welded to the frame tab.
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LINKAGE ASSEMBLY
Figure 15 – Bottom Link
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Figure 16 – Bottom Link
Bung fittings MIG welded to aluminum tubing cut to desired length for both top and bottom links. Each link has a left and right hand thread at opposite ends to allow for camber control. 3/8” bolts and nuts were used to mount the linkages to the car.
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Figure 17 – Bung Fitting
The bung fittings were cut to 1” pieces. Lathe turned to a diameter of 0.675” at 0.50” length.
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APPENDIX B- BUDGET
Build of Materials Item Size Quantity Price 4 Left Hand Heims Thread 8 $75.84 Joints 4 Right Hand Thread AISI 4130 12” X 12” 2 $45.12 Steel Plate .08” Thick 72” Aluminum 1" DIA x 6061 1 $38.67 0.125" THK Piping x 0.75" Aluminum 12” 6061 1 $9.29 1” Diameter Billet Total with Shipping ($32.09) $201.01
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APPENDIX C- QUALITY CONTROL
PVC replicas of the tubing lengths were created and fitted to ensure the tubing was modeled correctly in SolidWorks
Figure 18 – PVC Pipes
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