2016 Baja SAE Series Frame Design

Transcription

1 2016 Baja SAE Series Frame Design 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 Jonathon Forrest April 2016 Thesis Advisor: Professor Allen Arthur

2 TABLE OF CONTENTS TABLE OF CONTENTS... 1 LIST OF FIGURES... 2 LIST OF TABLES... 2 INTRODUCTION... 3 ABSTRACT... 3 PROBLEM STATEMENT... 3 BACKGROUND... 3 RESEARCH... 4 PAST DESIGNS... 4 DRIVER ERGONOMICS... 4 MAINTENANCE ACCESS... 5 OVERALL FRAME WEIGHT... 5 DESIGN... 6 MATERIAL SELECTION... 6 DESIGN ANALYSIS... 8 FORCE CALCULATIONS... 9 FINITE ELEMENT ANALYSIS MANUFACTURING BENDING AND PROFILING WELDING OTHER COMPONENTS CONCLUSION ACKNOWLEDGEMENTS WORKS CITED APPENDIX A NAMED ROLL CAGE POINTS APPENDIX B VEHICLE BUDGET APPENDIX C - SCHEDULE APPENDIX D DRAWINGS

3 LIST OF FIGURES Figure Frame... 4 Figure Frame... 4 Figure Frame... 4 Figure 4 - UMW Baja Car... 5 Figure 5 - Revision 1 Wireframe... 8 Figure to 2016 Objective Comparison Figure to 2016 Objective Comparison Figure 8 - Front Impact FEA Results Figure 9 - Side Impact FEA Results Figure 10 - Rear Impact FEA Results Figure 11 - Top Impact FEA Results Figure 12 - Manual Tube Bending Figure 13 - TIG Welding Frame Figure 14 - Ergonomic Comparison 2014 vs LIST OF TABLES Table 1 - Material Selection 7 2

4 INTRODUCTION ABSTRACT Each year the Society of Automotive Engineers (SAE) hosts three intercollegiate competitions in the United States for the Baja SAE Series. The cars that compete in this design challenge utilize a common 10 horsepower Briggs and Stratton horizontal shaft motor and must abide by all design rules as published by the SAE. Competitions include design, cost and sales judging as well as several vehicle function tests. These tests include but are not limited to brake and acceleration tests, hill climbs, sled pulls, suspension and traction and finally a four-hour endurance race. PROBLEM STATEMENT Jon Forrest (MET) and Devon Dobie (ME) propose to build a new frame which is focused on resolving three main problems: improving driver ergonomics, improving maintenance access and maintaining or reducing weight while maintaining proper structural strength. Since 2012 the club has made significant strides in design and manufacturability. These three problems however seem to be reoccurring issues that 2016 frame team wishes to resolve which will overall make a more comfortable ride, faster repair times and a more competitive car. BACKGROUND The University of Cincinnati s Baja SAE team has been competing continuously for the last four years in which three of those years we have built new cars (2012, -13, -14). The 2013 and 2014 cars were the teams most competitive and innovative cars but each of them had their own shortcomings. The 2013 car made a huge step in weight reduction, ergonomics and manufacturing while the 2014 car mostly focused on ergonomics and integrating our custom designed and built gearbox. However, the 2013 and 2014 cars both failed to meet some basic ergonomic requirements that made customer ride uncomfortable and maintenance very difficult. 3

5 RESEARCH The research for the construction of a new Bearcats Baja frame is stemmed off of three main sources. These sources include and are not limited to past Bearcats Baja frame builds, past competing teams concepts and the SAE Baja Rules [4]. The problem statement spells out that the frame team would like to focus on three major aspects which include driver ergonomics, maintenance access, and frame weight maintenance or reduction. All of these are stemmed from issues that have risen from previous frame design or have been previous frame design goals. PAST DESIGNS The research from our past teams comes from the three senior design reports [1][2][3] for Baja from 2012, 2013 and The 2012 car frame (Figure 1) was oversized, heavy and very poorly put together. This was essentially the first car the Baja team put together and it was manufactured using manual tube benders, manual pipe notchers and MIG welding which caused a lot of inconsistencies and excessive weight when assembled. This was improved upon in 2013 (Figure 2) with basic goals of dropping weight and improving the manufacturing processes. The weight goals were met by slimming the frame up, removing unnecessary tubing, outsourcing tubing bending and profiling to a company with CNC capabilities and moving to a TIG welding process. The 2014 car (Figure 3) sought to improve upon this car by lowering weight again and improving driver ergonomics. The 2014 team however had a larger hurdle to overcome which arose from having larger drivers for competition that year. This led to a wider frame to meet SAE rules and increased weight overall. Poor attention to driver ergonomics caused discomfort to driver s legs as well as poor egress times per SAE rules. Figure Frame Figure Frame Figure Frame DRIVER ERGONOMICS Most of the ergonomics are governed by pre-established Baja SAE rules [4] based purely on the safety of the driver. Additional ergonomics such as length and width have maximum values but are largely determined by the team s research. In the 2014 car, due to limited leg room and short cockpits, the driver s shins were pressed against the DF members causing bruising under normal operating conditions as well as interference with the steering wheel and drivers knees. When observing several members of the team sitting in the past cars, it 4

6 was noted that everyone s knees were bent and pointed toward the side impact member while their legs enter the nose of the frame at an angle as if doing a butterfly stretch. A primary goal is to make the cockpit area slightly longer which would bring the drivers knees down and out of the way of the steering wheel while reducing the angle of your legs into the nose area thus relieving shin pressure. MAINTENANCE ACCESS The 2012 car was mostly easy to maintain due to its large size, but fell short in the drivetrain area where some parts could only be removed by completely disassembling the rear end of the car. The 2013 and 2014 cars were more compact with the focus on driver ergonomics but also fell just barely short on maintenance access due to small hand clearances and difficult tool clearances in the drivetrain and cockpit areas. The 2016 team is looking into a front end design similar to UMW s (Figure 4) to improve master cylinder and steering access. The focus here is to design a vehicle that can be easily maintained by allowing proper hand and tool clearances for fast repairs. Figure 4 - UMW Baja Car OVERALL FRAME WEIGHT The 2013 frame was the first year that the frame weight was lowered significantly. The 2014 car improved several parts of the frame too but decided to have a goal to maintain or lower weight if possible, which they were able to maintain. For 2016, the goal is to maintain and possibly even lower the weight of the vehicle by removing unneeded bracing in the nose and moving to a smaller outside diameter tubing for the fore aft bracing since it does not have to be made of primary tubing. 5

7 DESIGN MATERIAL SELECTION The Baja SAE rules set two basic requirements for material selection. The first requirement is that primary tubing must conform to rule B which states: The material used for the Primary Roll Cage Members must be: (A) Circular steel tubing with an outside diameter of 25mm (1 in) and a wall thickness of 3 mm (0.120 in) and a carbon content of at least 0.18%. OR (B) A steel shape with bending stiffness and bending strength exceeding that of circular steel tubing with an outside diameter of 25mm (1 in.) and a wall thickness of 3 mm (0.120 in.). The wall thickness must be at least 1.57 mm (0.062in.) and the carbon content must be at least 0.18%, regardless of material or section size. The bending stiffness and bending strength must be calculated about a neutral axis that gives the minimum values. Bending stiffness is considered to be proportional to: E Modulus of elasticity (205 GPa for all steels) I Second moment of area for the structural cross section Bending strength is given by: EI S y I where: Sy Yield strength (365 MPa for 1018 steel) c Distance from neutral axis to extreme fiber[4] c The second requirement is from rule B8.3.1 stating that all secondary tubing must be steel tubes having a minimum wall thickness of 0.89 mm (.035 in) and a minimum outside diameter of 25.4 mm (1.0 in)[4]. In order to properly select primary and secondary members, an Excel chart (Table 1) was made to compare tubing properties of SAE minimum requirements compared to several other tubing selections. 6

8 Table 1 - Material Selection 4130 chromoly steel tubing was chosen for the primary and secondary members due to past frame success, favorably higher ultimate strength and lower weight per unit length. When compared to the rule material of 1018 cold drawn steel with 1 OD x wall thickness, the primary members met and exceeded rule B B by using 1.25 OD x wall thickness. The selected tubing has a calculated bending strength of in-lb (837.8 in-lb greater than rule material) and a calculated bending stiffness of 1,267 kip-in 2 (294 kip-in 2 greater than rule material). Bending Strength = S yi c = (63,100 psi)(0.043 in4 ) = 4,301.1 in lb in Bending Stiffness = EI = (29,732.7 ksi)(0.043 in 4 ) = 1,267 kip in 2 The secondary members were chosen to be 1.00 OD x wall thickness, which also exceeded rule B

9 DESIGN ANALYSIS The design of the vehicle was completed using Solidworks 3D sketch and weldment features. We began by setting up a series of construction lines and planes to get a general vehicle layout. Each plane was carefully set to ensure that revisions to the frame s angular and linear dimensions could be made easily. We began by drawing out a wireframe (Figure 5) that met all of the basic roll cage requirements spelled out in article 8 of the rules. Per the problem statement the frame team wanted to focus on three main goals: driver ergonomics, maintenance access, and weight maintenance or reduction. The 2013 and 2014 car frames were great improvements upon each other but suffered to meet basic driver ergonomic requirements, maintenance accessibility and even a few roll cage requirements. Starting with driver ergonomics, the first goal was to improve legroom and shin to frame clearance. In both the 2013 and 2014 frames, the DF L and DF R members are vertical and are 14 Figure 5 - Revision 1 Wireframe and 13 inches apart. The short cockpit area and the small width between the DF L and DF R was the major reason why legroom and shin clearance was so poor. The final design included angling the DF L and DF R out slightly for better shin clearance. This change left the D L to D R distance to be 17 inches and F L to F R distance to be 12 inches. This would leave the driver s shins 14 to 15 inches of shin clearance once the raised floor is added for brake and steering components. The cockpit area was also extended roughly 2.5 inches to allow the drivers knees to be lower and out of the way of the steering wheel (seen objectively in Figure 6 and 7, where the red frame is 2014 and grey frame is 2016). The shape of the nose was also changed to help improve vehicle egress times. A driver must be able to egress a vehicle in less than five seconds per point B9.2 of the 2016 BAJA SAE Technical Inspection sheet [5]. It was observed at the 2015 Baja SAE competition in Auburn that our driver s feet were getting caught on the top bar of the nose during egress that connected points D L and D R. According to rule B8.3.2, the frame is not required to have a lateral cross member between points D L and D R because our car is classified as a Nose car. Also, along with members DF L and DF R being angled outwards, they were also angled forward. This did Figure to 2016 Objective two things; shortened up the top of the nose Comparison 1 8

10 allowing more room for the driver to egress the vehicle between points D and S while leaving adequate room for steering and front suspension mounting (seen objectively in Figure 7, where the red frame is 2014 and the grey frame is 2016). At this point in the design the seat and steering components were modeled and loosely placed within the frame to ensure adequate room. The final positioning of these parts would be determined when the frame was completely assembled so steering angle and seat height could be tuned to a more comfortable position based off of an actual setting rather than predicted in a model. The idea behind designing Figure to 2016 Objective Comparison 2 maintenance access into the frame was to improve maintenance time. The frame was designed so that minimal support tabs were used to secure the firewall and skid plate as well as left adequate room under the seat to access the seat bolts if need be. There also was considerable discussion with extending this initiative onto other systems such as drivetrain, steering and braking where maintenance has been problematic to impossible to complete in a timely manner. Unfortunately due to time constraints for completing manufacturing, final brake components and final front suspension components could not be given maintenance access thought due to unforeseen subsystem design delays. However, adequate room was left for both systems with the idea that the frame would be blank for future Bearcats Baja teams to could easily adapt new designs. Lastly, the improvements to weight were based on removing unneeded structural support members around the nose area and changing the OD of the fore aft bracing tubing from 1.25 OD x wall thickness to 1 OD x wall thickness. It was observed that the majority of the front end tubing support was not needed based off of past finite element analysis worst case scenario data and they were excluded from this design as seen in Figures 6 and 7. FORCE CALCULATIONS As with past designs, the force calculations were based on an assumed overall vehicle weight of 600 lbs. (18.63 slug), which would include a 350 lb. vehicle and a 250 lb. driver. The biggest difference for 2016 focused on calculating an impact force based off of published crash data impulse times rather than derive an impulse time from a previously accepted deceleration force of 9 g. The drivetrain team predicted a maximum theoretical top speed of 30 mph (44 ft/s) when the vehicle was complete. The forces were calculated based on worst case scenarios which were decided to be a front impact at 30 mph into an immovable object, side and rear impacts by 9

11 another 600 lb. vehicle moving at 30 mph and top impact if the car were to drop six feet onto one corner. After reading through Matthew Huang s book on Vehicle Crash Mechanics [6], graphical results of crash test data for a truck in a 31 mph barrier crash was found to have an impulse time of seconds. The following calculations were made to calculate the impact forces: Stopping Distance V = d d = V t t d = (44 ft 0 ft ) s s s d = 3.3 ft Deceleration d = V2 V2 a = 2a 2d 44 ft/s2 a = 2(3.3 ft) a = 293 ft s 2 = 9.1 g s Front, Side and Rear Impact Force f = ma f = (18.63 slug) (32.2 ft s 2 ) (9.1 g) f = 5470 lbf Six Foot Drop Velocity mgh = 1 2 mv2 v = 2gh v = 2 (32.2 ft s 2 ) (6 ft) v = ft s Top Impact Force f = m V t f = (18.63 slug) f = 4884 lbf (19.66 ft s) (0.075 s) 10

12 FINITE ELEMENT ANALYSIS Force analysis was completed on the frame using Solidworks built in finite element analysis (FEA) capabilities. The four analysis (front, side, rear and top impacts) were all completed using static load studies and joint fixtures were chosen case by case to yield the most realistic force distribution through the frame. It should be noted that finite element analysis is subjective and prior to testing is largely theoretical with respect to design. The following is the results of the FEA: Front Impact (Figure 8) Impact Force: 9.1 g s or 5470 lbf Max Stress: 75.9 ksi Factor of Safety: 1.28 Figure 8 - Front Impact FEA Results Side Impact (Figure 9) Impact Force: 9.1 g s or 5470 lbf Max Stress: 91.9 ksi Factor of Safety:

13 Figure 9 - Side Impact FEA Results Rear Impact (Figure 10) Impact Force: 9.1 g s or 5470 lbf Max Stress: 95.1 ksi Factor of Safety: 1.02 Figure 10 - Rear Impact FEA Results 12

14 Top Impact (Figure 11) Impact Force: 4884 lbf Max Stress: 94.3 ksi Factor of Safety: 1.03 Figure 11 - Top Impact FEA Results Questioning of the driver safety did arise from the results of the worst case scenarios low factors of safety. The reason why the design was deemed acceptable by the author could be perceived as a grey area in safety versus performance that can be seen in any real world design application of reliability versus performance. If the frame was designed to withstand a 50 mph front impact, the car would be overdesigned to the point that its weight would negatively impact overall vehicle performance. Sport side-by-sides currently on the market have the ability to do upwards of 80 mph which would undoubtedly total the vehicle in a collision at that speed. There is an inherent danger that should be understood and accepted with off-road racing and the 2016 frame FEA proves that it will be safe all the way up to its top speed. MANUFACTURING BENDING AND PROFILING The 2016 frame subsystem team made the decision from the beginning of design to manufacture this prototype frame at minimal cost while still delivering comparable quality to previous years frames. The most successful decision made towards this goal was deciding to not outsource CNC bending and laser profiling at VR3 (located in Ontario, Canada), which is a savings of nearly $2,000. Instead the frame tubing was cut to appropriate lengths and bent 13

15 at Ohio Hydraulics located in Sharonville, Ohio and was profiled using a simple logical technique that involved a Sharpie, precision angle grinding and calibrated eye balls. There was an accepted risk with this approach largely due to the high probability of making a bad cut, which only turned into scrapping two tubes out all 39 tubes that made up the frame. In this case, the in house manual tube bender was used to replace the scrapped tubing (Figure 12). WELDING Figure 12 - Manual Tube Bending The welding process chosen for the frame was Tungsten Inert Gas (TIG) welding (Figure 13). The decision was made based off of research for welding 4130 Chromoly tubing, availability of proper filler wire and cleanliness of the process tubing can be MIG or TIG welded and both are commonly used in the aerospace industry; however the author chose TIG welding due to the higher ability to control the weld puddle and penetration of the weld to prevent weld burn through from excessive heat. The filler material selected for the fabrication of the frame was ER70S-2 for it high strength and favorable fatigue properties. Any non-structural tubing members were MIG welded to cut down on fabrication time. It was also deemed unnecessary to have the frame heat treated due to the thin wall thicknesses that had been used but typically any 4130 over wall thickness would require weld stress relief to prevent cracking. Due to this manual method Figure 13 - TIG Welding Frame of construction, the vehicle was assembled using a variety of tools to ensure proper construction such as levels, digital angle finders, a square and measuring tape. OTHER COMPONENTS Firewall Skid plate Fire extinguisher Seat Restraint system 14

16 CONCLUSION In review, the frame team was able to achieve all of the three main problems addressed in the problem statement which were improving ergonomics, maintenance access and maintaining or reducing weight. Compared to the 2014 frame, it can be seen in Figure 14 that ergonomic improvements were made when one of the team s drivers sat in the car. It can be noted that there are improvements of forearm to leg space, steering wheel to knee space, leg angle and shin to DF L and DF R member relief. The overall weight of the 2016 frame came out to 76 lbs, which is +1 lb compared to the 2014 frame which came in at 75 lbs. Overall cost savings from 2014 to 2016 frame were approximately $1,650. On top of these successes, it can be concluded that the 2016 frame is a competitive platform for future University of Cincinnati Baja teams to use and compete in for years to come. ACKNOWLEDGEMENTS Figure 14 - Ergonomic Comparison 2014 vs I would like to thank Dean Allen Arthur for his continuous support of this team over the years. I would like to thank Devon Dobie (ME) for being a fantastic partner through the design and manufacturing of the 2016 Frame. A big thanks to TW Metals for the very generous donation of material. Finally I would like to thank my parents for their continuous support and to the alumni that I ve met through the Bearcats Baja program for mentoring me throughout the years and encouraging me to pursue this project for Senior Design. 15

17 WORKS CITED [1] Biteman, Brooks. Baja SAE Frame Design. Thesis. Cincinnati: University of Cincinnati, [2] Kobs, Joe University of Cincinnati Baja SAE Chassis. Thesis. Cincinnati: University of Cincinnati, [3] Ratliff, Michael Baja Frame and Chassis. Thesis. Cincinnati: University of Cincinnati, [4]. "2016 Collegiate Design Series Baja SAE Rules." SAE International. < [5] SAE. "2016 BAJA SAE Technical Inspection Sheet." Baja SAE < [6] Huang, Matthew. Vehicle Crash Mechanics. Boca Raton, FL: CRC, Print. 16

18 APPENDIX A NAMED ROLL CAGE POINTS All named points are implied to have a Left and Right hand side, denoted by subscript L or R (e.g. A L and A R ) [4] 17

19 APPENDIX B VEHICLE BUDGET 18

20 Frame APPENDIX C - SCHEDULE Key Planned Actual Holiday Graduation Year Month Week Milestones and Important Events R&D Design (Fit w/ Primary & Secondary Ideas) Alpha (Complete CAD) Revisions to Final Design FEA Order & Acquire Parts Fabricate & Assemble Physical Testing P A P A P A P A P A P A P A P A Bearcats Baja - Frame Schedule August September October November December January February March April Midnight Mayhem FREEZE Tenn Tech 19

21 APPENDIX D DRAWINGS 20

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