PROJECT MANAGEMENT & DESIGN II MAE 435. Formula SAE. Matthew Galles. Mr. Nathan Luetke. October 14th,

Transcription

1 PROJECT MANAGEMENT & DESIGN II MAE 435 Formula SAE Authors: Robert Costen James Fulcher Matthew Galles Christopher McHugh Xavier Thompson Ashley Wyatt Class Supervisors: Dr. Sebastian Bawab Mr. Michael Polanco Project Advisors: Dr. Sebastian Bawab Dr. Colin Britcher Mr. Nathan Luetke October 14th,

2 CONTENTS 1 ABSTRACT 1 2 INTRODUCTION 2 3 WHEELS INTRODUCTION COMPLETED METHODS PROPOSED METHODS 3 4 UPRIGHTS INTRODUCTION COMPLETED METHODS PROPOSED METHODS 4 5 BRAKES INTRODUCTION COMPLETED METHODS PROPOSED METHODS 7 6 SUSPENSION SUSPENSION INTRODUCTION COMPLETED METHODS PROPOSED METHODS 11 7 STEERING INTRODUCTION COMPLETED METHODS PROPOSED METHODS 13 8 FRAME INTRODUCTION COMPLETED METHODS PROPOSED METHODS 14 9 RESULTS MATERIAL SELECTION DESIGN DISCUSSION 16 i

3 LIST OF FIGURES Figure 1: Rear Upright Design Change... 4 Figure 2: Front & Rear Uprights... 4 Figure 3: Disk Braking System & Drum Braking System... 6 Figure 4: AHP Criteria Combined Results... 9 Figure 5: Camber Change with Vertical Load Calculator Figure 6: Camber Angle vs Degrees Vertical Load Figure 7: Push-Rod Suspension System APPENDICES Appendix 1: Gantt Chart Appendix 2: Budgeted Hours and Progression Appendix 3: Budget a b c ii

4 1 Abstract The Formula Society of Automotive Engineers (FSAE) competition requires students to design, fabricate, and test an open-wheeled race car. This paper presents completed research and development, resources, and processes used to complete the design and analysis process of a FSAE chassis working from an outside to inside approach; wheels to the frame. The team focused on stopping safely, which lead to changes in the braking system. The rear axle braking system was modified to include a rotor and caliper for each wheel. The suspension design is being revised to improve handling and reduce driver fatigue. The rear suspension will have a push rod system rather than a pull rod system. The front suspension geometry is being adjusted to provide optimal camber gain for vertical tire displacement and steering input. This is to ensure the largest contact patch is maintained between the tire and the ground. A new steering system will be designed with proper Ackermann angles to improve cornering. Frame modifications were implemented to reduce overall size and weight. The main hoop, mid hoop, and frame width were reduced. A different tire compound will be used to improve heat retention and slip angle. The front uprights will include proper caster and kingpin inclination angles (KPI) while maintaining a strong and light weight design. 1

5 2 Introduction In 2014, the ODU FSAE team finished with four hundred and thirteen points placing thirtyseventh of one hundred and nine teams. The team focused on a complete chassis design that would allow for a higher power to weight ratio. Suspension redesign will improve vehicle handling and steering to accommodate the increase in available power.. The ODU FSAE team integrated a new frame design with the suspension design. The combination of two designs created a car that placed well in competitions but did not have competitive handling because of excessive tire movement, bump steer, understeer, poor cornering and uneven steering caused by poor steering linkage placement. A competitive suspension design makes the car more predictable to maneuver through high speed turns and improves the size of the contact patch, allowing for better road holding and load distribution [1-5]. Because a competitive engine design was achieved on the car, the team is focusing on the frame design, suspension design, tire selection, upright design, brake component selection, and correcting the uneven steering by shifting the steering linkage placement. 3 Wheels 3.1 Introduction The wheel assembly consists of the tire, rim, and wheel center, which connects to the wheel axle. Different tire compounds affect traction of the vehicle and allow for controlled braking and maneuvering [4]. Different tire compounds have unique values for ground patch, heat retention, and 2

6 grip [1, 5]. The tire compound did not retain heat effectively which, in turn affects handling. Different size and material rims are available for purchase. Common choices for racing are carbon fiber and aluminum rims in either 13 or 10 sizes. 3.2 Completed Methods Carbon fiber rims were considered to reduce the overall weight of the car. Carbon fiber wheels have a 1.73 lbs to 7.1 lbs ratio with aluminum wheels for each rim. However, the increased cost is outside of the budget. 3.3 Proposed Methods Purchase the same wheels as the wheels or an equivalent. Complete research on tire compounds to select the tire. 4 Uprights 4.1 Introduction The upright assembly serves as the connection between the tires and the suspension, and consists of the upright, hub, wheel bearings, steering attachment, and braking components [4, 5]. The uprights allow for pre-designed caster and KPI which vary with ball joint location. 4.2 Completed Methods 3

7 The uprights were redesigned to reduce weight, and maintain reliability, and maintain proper suspension geometry. An upright design has been completed which allows for rear brake caliper mounts while reducing the size and weight of the assembly. The uprights will be constructed from 7075-T6 aluminum for weight reduction purposes. 4.3 Proposed Methods Figure 1: Rear Upright Design Change Finite Element Analysis (FEA) still needs to be performed on the proposed design. Figure 2: Front & Rear Uprights 4

8 To simulate corning on the uprights the center spindle or bearing support will be rigidly constrained while forces are applied to the upper ball joint, lower ball joint, and toe bar mounting point locations in opposite directions in various angles. To simulate braking on the uprights, a force will be applied in the same direction to the connection points, while the center spindles are rigidly fixed. Forces will be calculated based on 1.5 g-force. The overall weight of vehicle still needs to be calculated prior to analysis can be completed. 5 Brakes 5.1 Introduction There are three methods to reduce a vehicle s velocity: aerodynamic drag, a braking system, and drivetrain drag. Aerodynamic drag is not a critical braking factor at low velocity. The braking system is the most important component in safely reducing vehicle velocity when the magnitude of the drivetrain drag is less than the braking system output [6]. Disc and drum brakes are the two most common braking mechanisms used, while disc brakes are used exclusively in racing applications due to superior design parameters, including thermal dissipation, ease of adjustment, and simple mechanism design. A brake functions by converting the rotational kinetic energy of the wheel to heat by creating friction between the brake disc and the pads. The coefficient of friction between the disc and pad, the frictional force due to the actuator, and the actuator performance affect the performance of the braking system [7]. As a result, heat dissipation of the brake is of utmost importance in increasing braking efficiency [8]. 5

9 Figure 3: Disk Braking System & Drum Braking System The force of friction between the tire and the road is a limiting factor in braking force. Aerodynamic ground effects can be used to create high downward forces, increasing frictional force between the tire and road. This allows for increased braking without creating a wheel lock condition [9]. In order to achieve maximum deceleration, the front and rear brakes must operate at their respective traction limits [9]. The traction limit is dependent on the friction coefficient between the tire and road, and the dynamic weight of the front and rear wheels [8, 9]. The car had a braking distribution that was overly biased towards the front. 5.2 Completed Methods The car will move to a 2+2 caliper braking configuration. This will center the braking distribution bias. However, in order to achieve the proper braking distribution, a smaller rotor will be used in the rear to decrease the braking moment. This will still maintain a small desired bias towards the front. 6

10 Required Deceleration: a v t m / s 3s m / s (1.1) Required Braking Force: F ma kg 2 ( )( m/ s ) N (1.2) Front Braking Force (60%): 0.60 FF N N (1.3) 2 Rear Braking Force (40%): 0.40 FR N N (1.4) Proposed Methods In order to select the best brake components, the parts will be modeled in SolidWorks, and a thermal analysis will be performed using SolidWorks Simulation (Dassault Systèmes S.A., Waltham, MA). Newton s Laws of Motion will be used to calculate the dynamic weight of the front and rear wheels [8]. The fasteners to mount the disc to the wheel hub, and the brake caliper to the upright will be designed in accordance with well-known engineering practices [10]. Also, the moment calculations must be made from the contact point of the brake pad and rotor to the axle. A rear rotor must be designed using these calculations. A caliper mount will be designed for the rear upright. 7

11 6 Suspension 6.1 Suspension Introduction When designing an FSAE car, the suspension is a top consideration. All loads and accelerations must be transferred to the ground via the suspension. The car cannot properly perform if the widest section of tire is not grounded, the chief responsibility of suspension [11]. The camber angle is measured between the centerline of the tire and a line drawn perpendicular to the ground. It changes under different loadings and while maneuvering. Proper camber gain throughout suspension travel and steering input ensures the tire will remain in contact with the ground. Changing suspension geometry can move roll centers, causing tire hop or extreme roll [12]. Weight transfer must also be considered. If the roll axis is routed through the center of gravity, no roll will occur. However, rapid weight transfer will cause a loss of tire traction [13]. Therefore, camber and roll center changes must be analyzed when designing the suspension. 6.2 Completed Methods Two calculators have been created using Microsoft Excel. The first allows for dimensions of suspension components to be entered and provides camber change when vertical displacement is introduced. The second calculator will provide camber change for different steering inputs, caster angle and KPI. A level two analytic hierarchy process (AHP) was performed to reduce additional resources efforts in suspension design. Criteria, based on team member s feedback and observations of the performance, were selected for evaluation. Each member completed a pair wise comparison 8

12 ranking the criterions priority resulting in a six by six matrix for each member. This is shown in Figure 4. Each individual's matrices were combined using linear algebra into a single combined six by six matrix and eigenvectors were calculated. The criterions ranging was then utilized in creating additional AHP matrices to evaluate advantages and disadvantages of a push-rod and a pull-rod suspension system. Based on the results of the AHP, a push-rod suspension system was chosen for the design evaluation process. Figure 4: AHP Criteria Combined Results 9

13 Figure 5: Camber Change with Vertical Load Calculator Droop/unloading Jacking/Loading Figure 6: Camber Angle vs Degrees Vertical Load 10

14 6.3 Proposed Methods Design strong and durable control arms and uprights that will provide the car with the proper camber gain to maintain traction and prevent understeer. The control arm lengths and mounting points will be modified to ensure proper roll centers are maintained. After redesigning the suspension geometry and components, modeling and analysis will be performed using Optimum Kinematics (Optimum G, Denver, CO) so revisions can be performed. The FSAE car utilized a rack-and-pinion steering system. Over-steer and understeer were observed during harsh cornering. When the car was turned to the left side, the steering felt loose. Turning the wheel to the right proved to be stiff. Adequate adjustment to the steering system did not occur and caused a high steering ratio [14]. The misalignment and inaccurate placement of the steering system resulted in the front wheels following a different circular route than anticipated. The steering system requires the innermost wheel to navigate a much smaller radius than the outer wheel and therefore conducts a tighter turn [15]. The steering linkages geometry, based on Ackerman steering geometry, requires more turning of the inside wheel than that of the outside wheel [16]. There are two common types of steering gears: rack-and-pinion and recirculating-ball. Each end of the rack-and-pinion steering rack is enclosed in a metal tube with a tie rod connected [17]. The spindle and the steering arm are attached by the ends of the tie rod. The pinion gear, 11

15 connected to the steering shaft, is manipulated by the tie rod. When rotated, the pinion gear spins. Figure 7: Push-Rod Suspension System 7 Steering 7.1 Introduction The FSAE car utilized a rack-and-pinion steering system. Over-steer and understeer were observed during harsh cornering. When the car was turned to the left side, the steering felt loose. Turning the wheel to the right proved to be stiff. Adequate adjustment to the steering system did not occur and caused a high steering ratio [14]. The misalignment and inaccurate placement of the steering system resulted in the front wheels following a different circular route than anticipated. The steering system requires the innermost wheel to navigate a much smaller radius than the outer wheel and therefore conducts a tighter turn [15]. The steering linkages geometry, based on Ackerman steering geometry, requires more turning of the inside wheel than that of the outside wheel [16]. 12

16 7.2 Completed Methods With improved steering, the possibility of over-steer and under-steer will be eliminated. To eliminate the inconsistencies that can accompany steering, calculations will be performed to ensure the vehicle will not have a high steering ratio [13]. The accurate placement of the steering system will result in the front wheels following the correct circular route. The steering system requires the innermost wheel to navigate a much smaller radius than the outer wheel and therefore conducts a tighter turn [14]. The steering linkages geometry, based on Ackerman steering geometry, requires more turning of the inside wheel than that of the outside wheel [15]. 7.3 Proposed Methods The new Ackermann angle will be determined utilizing formula (1.1) 1 wheelbase Ackerman tan wheelbase track tan outside Ackerman inside Ackerman percent *100 front (1.5) The steering ratio will be calculated in order to keep the steering ratio as low as possible, thus, producing a quicker steering response. The new steering design will equalize the ratio in both directions. A rack-and-pinion steering system will be redesigned for the ODU FSAE race car. An improved steering system will help with the overall performance and suspension of the vehicle. 13

17 8 Frame 8.1 Introduction The frame of a race car protects the driver and components of the car. In the FSAE competition there are rules regulating the frame for safety reasons while still giving requirements found in real industry. 8.2 Completed Methods The frame size was reduced to lower weight and cost while maintaining safety requirements. The main hoop, that protects the driver s head, was brought down 3 while still allowing for the 95 th percentile male to fit in the car with a safety clearance over the driver s helmet. The driver s compartment was narrowed 1.5 on both sides for a total change of 3. The mid hoop was lowered by 1 while maintaining the straight plane from the hoop to the top of the main hoop without intersecting with the steering wheel or any part of the driver. 8.3 Proposed Methods Perform FEA on the frame. A frame will be fabricated utilizing the welding fixture designed by the team. All of the tubing of the frame structure will be one inch diameter chromoly tubing. 14

18 9 Results 9.1 Material Selection The material selected for the frame is chromoly steel. The selected tubing will allow for relatively easy fabrication and is readily available. It was determined that metal inert gas (MIG) welding with flux-cored wire provides an adequate weld joint but tungsten inert gas (TIG) welding provides a more aesthetically pleasing weld joint. Also, when fabricating the joints, it was determined that a bench grinder provides a cleaner joint the one cut with an angle grinder and a band saw is the ideal method for cutting the tubing. The braking components have been selected. A Wilwood Dual Master Cylinder will be used with Wilwood Dynapro calipers. Streamlined rotors have been selected for the front wheels. 9.2 Design A push rod suspension design was selected. With the push-rod suspension, the bending moment can be removed from the lower control arm in the previous design. SolidWorks was utilized to develop the upright and frame designs. It was determined this software can provide the proper stress analysis and accurate dimensions required for this project. 15

19 10 Discussion The purpose of our project is to design and construct a Formula SAE car that will outperform the teams car by twenty points. This will be accomplished by adjusting the steering, decreasing the tire size and improving the suspension. The results mostly came from online research and discussions with the team members. It has been determined that steering and suspension were primary issues with the vehicle. Research focused on the areas that would improve the current designs. It was determined that a change to steering creates a better angle between the steering column and the rack and pinion connection point, which in turn creates smoother turns requiring less effort to maneuver. The upright is being redesigned in SolidWorks. The rack-and-pinion steering system on the current vehicle is a good system; however it needs to be adjusted to improve the handling of the car. A rear brake rotor needs to be designed, fabricated, and tested to insure proper stopping distance and heat dissipation rates are maintained or improved from previous performance. Additional resources are required to modifying the current brake calipers and pads to work with the rear rotor. The future work to be performed is FEA and construction of the Formula SAE car. This includes constructing the frame out of steel piping and welding it together. A final upright design needs to be machined. A rear rotor needs to be designed and fabricated. References 16

20 [1] J. Axerio-Cilies and G. Iaccarino, "An Aerodynamic Investigation of an Isolated Rotating Formula 1 Wheel Assembly," Journal of Fluids Engineering, vol. 134, pp , [2] E. F. Gaffney Iii and A. R. Salinas, "Introduction to formula SAE&reg suspension and frame design," in 48th Earthmoving Industry Conference and Exposition, April 9, April 10, 1997, Peoria, IL, United states, [3] E. Uzunsoy and O. A. Olatunbosun, "A study of the effect of rear suspension auxiliary roll damping on vehicle-handling dynamics," vol. 219, pp , 01/ [4] D. Robertson and G. J. Delagrammatikas, "The suspension system of the 2009 cooper union fsae vehicle: A comprehensive design review," SAE International Journal of Passenger Cars - Mechanical Systems, vol. 3, pp , [5] N. D. Smith, "Understanding parameters influencing tire modeling," Colorado State University, Formula SAE Platform, [6] T. D. Gillespie, Fundamentals of Vehicle Dynamics: Society of Automotive Engineers, [7] S. M. Savaresi and M. Tanelli, Active Braking Control Systems Design for Vehicles: Springer-Verlag, [8] R. Stone and J. K. Ball, Automotive Engineering Fundamentals: SAE International, [9] M. Guiggiani, The Science of Vehicle Dynamics: Handling, Braking, and Ride of Road and Race Cars: Springer Netherlands, [10] V. D. Ingenieure and U. S. N. T. I. Service, VDI 2230: Systematic Calculation of High Duty Bolted Joints: U.S. Department of Commerce, National Technical Information Service, [11] B. A. Jawad and J. Baumann, "Design of Formula SAE Suspension," [12] A. Mihailidis, Z. Samaras, I. Nerantzis, G. Fontaras, and G. Karaoglanidis, "The design of a Formula Student race car: a case study," Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, vol. 223, pp , [13] D. E. Woods and B. A. Jawad, "Numerical design of racecar suspension parameters," SAE Technical Paper1999. [14] G. C. MARINESCU, N. DUMITRU, and V. OŢĂT, "ANALYSIS OF THE TORQUE APPLIED TO THE STEERING WHEEL IN STATIC CONDITIONS," Annals of the Faculty of Engineering Hunedoara-International Journal of Engineering, vol. 11, [15] S. Jianmin and Y. Qingmei, "Decreasing Vibration of Vehicle Using Combined Suspension System," in Robotics, Automation and Mechatronics, 2008 IEEE Conference on, 2008, pp [16] M. Thoresson, T. R. Botha, and P. S. Els, "The relationship between vehicle yaw acceleration response and steering velocity for steering control," International Journal of Vehicle Design, vol. 64, pp , [17] A. M. Odhams and D. J. Cole, "Identification of the steering control behaviour of five test subjects following a randomly curving path in a driving simulator," International Journal of Vehicle Autonomous Systems, vol. 12, pp ,

21 Appendicies Appendix A: Gantt Chart a

22 Hours Appendix B: Budgeted Hours and Progression Cumulative Expected Hours Cumulative Actual Hours Week b

23 Appendix C: Budget Budget Materials Fabrication Assembly Resources Costs Costs Costs Design Research $1,500 Drawings $1,500 Testing $1,500 Frame $2,000 tubing $960 Weld rod/wire $100 Suspension $750 $1,000 fasteners $150 dampers $250 heeve spring $150 A-arms $40 Uprights $200 $400 rims $300 tires $400 Brakes $100 calipers $150 Steering $625 $1,000 Rack and pinion $100 Column $50 c

24 Wheel $150 Quick dis-connect $100 Fasteners $50 Assembly Final Product $6,250 Competition Fees Entry fee/gas $500 Sub Totals $14,500 $3,775 $2,000 Total Costs $20,275 d