University of Wisconsin-Platteville Society of Automotive Engineers Mini Baja Team

Transcription

1 University of Wisconsin-Platteville Society of Automotive Engineers Mini Baja Team 2009 Design Report SAE Mini Baja Wisconsin Competition, June 11-14, 2009

2 Copyright 2007 SAE International Vehicle Number 22 UW-Platteville 2009 Mini Baja Team Design Report Brendan Behrens Team Captain Kyle Droessler Co-Captain ABSTRACT The Society of Automotive Engineers sponsors competitions that challenge aspiring engineers to create a miniature off-road vehicle. The SAE Mini Baja competition objective is to design and fabricate a prototype vehicle that could be manufactured for consumer sale. The University of Wisconsin Platteville (UWP) has accepted the challenge to participate in this competition. An aspect of this competition is to compose a design documentation package that creates an overview of the vehicle s construction elements. The UWP Mini Baja team has created this report to describe their design. INTRODUCTION The purpose of designing and manufacturing a Mini Baja car was to create a prototype recreational off-road vehicle that could provide a fun, safe, and reliable experience for a weekend off-road vehicle enthusiast. In order to accomplish this task, different design aspects of a Mini Baja vehicle were analyzed, and certain elements of the car were chosen for specific focus. There are many facets to an off-road vehicle, such as the chassis, suspension, steering, drive-train, and braking, all of which require thorough design concentration. The points of the car that the University of Wisconsin Platteville decided to specifically focus on were the chassis, drive-train, and suspension. The most time and effort went into designing and implementing these components of the vehicle because it was felt that they most dramatically effect the off-road driving experience. During the entire design process, consumer interest through innovative, inexpensive, and effective methods was always the primary goal. FRAME DESIGN OBJECTIVE The objective of the chassis is to encapsulate all components of the car, including a driver, efficiently and safely. Principal aspects of the chassis focused on during the design and implementation included driver safety, suspension and drive-train integration, structural 1 rigidity, weight, and operator ergonomics. The number one priority in the chassis design was driver safety. With the help of the 2009 Baja SAE Competition Rules and Finite Element Analysis (FEA), design assurance was able to take place. DESIGN The main components of the frame are broken into two groups: the chassis and the roll cage. The roll cage is made up of the RRH, RHO, FBM, LC. The chassis is made up of LBD, LFS, SIM, FAB, and FLC (See page 11, the Acronym list, for member clarification). Material 1020 DOM was chosen for the roll cage because of its high toughness and ductility. A very tough material is important in a roll cage because in the event of impact, such as a rollover, the roll cage needs to absorb as much energy as possible to prevent the roll cage material from fracturing was chosen for the chassis because it has structural properties that provide a low weight to strength ratio is a chromium molybdenum alloy steel that has controllable properties. Attributes to 4130 include corrosion resistance and the ability to maintain a Bainite micro-structure after welding. This prevents the area around a weld from becoming overly brittle. 1 inch diameter tube with a thicker wall was used instead of 1.5 inch diameter tube with a thinner wall for manufacturability purposes. Although the thinner wall, 1.5 inch diameter tube would be slightly lighter than the thicker wall, 1 inch diameter tube, it would have been more material and more difficult to weld. Safety Roll cage safety features were first implemented in accordance with the 2009 Baja SAE Competition Rules, which served as a baseline. The first primary safety standard focused on during design was maintaining a minimum of 6 inches vertical distance from the driver s head to the bottom of the RHO and a 3 inch clearance between the rest of the body and the vehicle roll cage. These dimensions created a roll cage envelope that was safe for the driver. After the roll cage

3 envelope was created, the next aspect addressed during baseline design was roll cage structural integrity. Roll cage structural integrity guidelines can be found in the 2009 Baja SAE Competition Rules section 3, Roll Cage, Systems, and Driver s Equipment. All 2009 Baja SAE Competition Rules guidelines were implemented throughout the entire frame. Once the baseline requirements were met, other safety design points were implemented. The chassis was additionally designed to give the occupant extra space and protection with curved vertical supports and extra lateral bridge supports, which can be seen in Figure 1. These supports tie the right and left sides of the car together, increasing structural integrity and reducing the chance of driver ejection during roll-overs. To further improve the roll cage safety and verify its structural integrity, finite element analysis was completed on the roll cage. Roll Cage FEA Safety Analysis Simulated loads within a computer program were placed on a wire frame model of the roll cage at critical points to simulate the amount of force that the vehicle would undergo from its own weight and a driver in the event of a rollover. To conduct a finite element analysis of the chassis, an existing chassis design was uploaded from the computer program SolidWorks to a finite element analysis program known as Algor. The loading performed by the Algor FEA software modeled an end over end rollover. Different loads at various angles were applied at points on the top of the roll cage to simulate that scenario, as seen in Figures 2 through 7. The weight of the vehicle itself was assumed to be 450 pounds. Then 200 more pounds were added to the vehicle weight to simulate the weight of a driver. The combined values were 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 will not fail. The maximum stress during the simulation was found to be about one half the value of the roll cage s material yield strength. The maximum stresses and displacements are shown in Table 1. Location of Load Relative to Roof of the Chassis 135 Deg 90 Deg 45 Deg Max Stress (psi) Max Displacement (in) Table 1: FEA safely results In order to simulate a worst case scenario, the yield stresses of the two different materials on the Mini Baja car roll cage were found. Determining the yield strength of the roll cage is an important aspect, because once the materials begin to yield, the roll cage will lose much of its structural integrity. The 1020 steel had a yield strength of 47,863 psi, and the 4130 Chromoly had a yield strength of 170,000 psi. A loading configuration that would produce the highest stresses with the smallest load was then determined. Applying a distributed load 2 across the front cross member at 135 degrees with respect to the top of the roll cage seemed to achieve the highest stress with the smallest load, and would be similar to a forward flip landing on the front of the roll cage. After some trial and error, a maximum distributed load of 80 lbs/in ( lbs total) was determined to cause a stress of approximately 47,800 psi in one of the 1020 steel members. This means that it would take approximately 1215 lbs of load on the weakest member of the roll cage to cause a failure in the roll cage, as seen in Figure 8. The results from these simulations are accurate for the type and amount of loading that was applied to the known material and geometry. However, these loading scenarios generally do not exactly represent an actual rollover crash. To accurately depict a rollover incident, dynamic loading would have to be used to simulate the types of impact loading that would occur during an actual rollover. It would be very difficult to accurately model this event without known data gathered from an actual rollover. This data could be gathered using strain gauges attached to the frame of the vehicle. The results gathered from the FEA illustrate that the frame theoretically will not fail in a rollover until there is approximately 1215 lbs of force on the weakest member of the roll cage. The FEA results show a design that meets the expectations set for this chassis. With the data collected from the FEA simulations, the roll cage was found to have a theoretical factor of safety of approximately Safety Harness A five point racing harness attached to the most rigid members of the roll cage was utilized to ensure the maximum amount of driver safety restraint. Attaching the seat belts to the most rigid and structural chassis components guarantees reliability of the seat belt under the extreme forces possible in a collision. Using a quickrelease lever style seat belt clasp gives the driver the ability to get out of the vehicle in a safe amount of time in the event of an accident. SAE requires that a driver be able to evacuate a Mini Baja car in less than five seconds. The safety restraints provided in the car will be sufficient for keeping a driver safe in the event of a collision, while still allowing the driver to escape in the required 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 effective off-road vehicle. To complete the goal of integrating those components efficiently and effectively, all the components were solid modeled in the computer aided modeling program SolidWorks. After solid modeling was complete, all the components restrictions and requirements were considered. A few key drive-train requirements to be included in the chassis design consisted of the distance the primary and secondary clutches needed to be apart and keeping the center of gravity of the vehicle as low as possible. A few important suspension requirements considered during

4 the design of the chassis consisted of the angle at which the shocks needed to be mounted, the distance the A- arms needed to be mounted apart, and the anti-dive angle in which the front and rear A-arms needed to be mounted. Once all requirements were compiled, the suspension and drive-train were integrated into the chassis design. Another aspect of the chassis that was considered during the integration of the suspension was chassis deflection due to forces exerted through the suspension. To accurately minimize deflection in the chassis, FEA analysis was conducted and light weight Chromoly tubular members were added where the deflection was greatest. The simulated loads conducted through FEA helped determine where and how additional members should be added to the chassis. The method for simulating loads on the suspension points using FEA was similar to that of the rollover analysis, as previously described in the chassis safety section. Impact loading was simulated on the shock mounting points, at the angle in which the shocks were going to be mounted, until a member in the chassis reached its yield stress. Additional members were added to create the best combination of weight addition and structural rigidity. Each rear shock mount was able to withstand 800 lbf of loading before yielding, as seen in Figures 9 and 10. Each front shock mount was able to withstand 485 lbf of loading before yielding, as seen in Figures 11 and 12. Based on the information presented above, the strength of these mounting points will be enough to withstand the forces exerted on them in extreme off-road conditions. Withstanding forces that simulate extreme conditions ensures rigidity and reliability in normal offroad conditions. Also, as a result of steel having an infinite fatigue life, these tests were able to verify that under normal loading, the fatigue limit of the material will not be exceeded. It is well known that 90% of all material failures are due to fatigue, which is why it is so important that the stresses exerted on the chassis suspension mounts do not exceed their fatigue limit. Structural Rigidity Overall frame structural rigidity is important to enhance the capabilities of an off-road vehicle. To measure the overall frame rigidity, torsional rigidity analysis was conducted through FEA. The objective of the torsional rigidity analysis was to manipulate the chassis design within the FEA software to increase the amount of torque per degree of chassis deflection. By theoretically increasing this value, the actual vehicle could have the ability to be more torsionally rigid, making it able to withstand more intensive terrain without failure. To achieve this analysis, a simulated torque of 70 ft-lbf was placed on the back of the car, while the front of the car remained fixed, as seen in Figure 13. With the degree of rotation data collected from the FEA software, the torque was divided by the degree of rotation, creating a torsional rigidity value for the frame. The angle rotated under the 70 ft-lbf of torque was found to be degrees, as seen in Figure 14. The UWP Mini Baja car 3 frame has a 555 ft-lbf/degree theoretical torsional rigidity rating. It has been concluded that this meets expectation, and shows that the vehicle s frame is structurally 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 in vehicle performance. The frame is one of the largest and heaviest components of the car, and which is why special attention was placed on the vehicle s frame weight. The strategy utilized to minimize weight consisted of determining defined goals for the chassis and employing the correct material in the best places to accomplish those goals. Once baseline safety design requirements were met, FEA aided the material decision making process. FEA specifically helped determine whether a member was under high or low stresses, in the scenarios discussed previously, making the chassis design process efficient and effective. Low stress chassis members were made out of inch wall thinness 4130 Chromoly, and higher stress chassis members were made from inch wall thickness 4130 Chromoly. Chromoly was chosen because of its weight reduction capability and beneficial material properties, as was stated previously. Through accurately determining stresses on the chassis in different scenarios, weight reduction was able to be maximized through material selection and placement. The final weight of the chassis was measured to be 85 pounds. Operator Ergonomics The ergonomics of a cockpit can noticeably affect the quality of an off-road vehicle driving experience. This is why operator ergonomics was a factor that was considered in the design of the frame cockpit. The cockpit, consisting of the area in the roll cage where the driver sits to operate the vehicle, was designed for maximum comfort and ease of vehicle entrance and exit. The first aspect of the chassis that was designed around ergonomics was the firewall angle. The angle of the firewall, which inherently limits the amount an operator can lean back while driving, was set to 19 degrees, which is just less than the maximum angle required by the 2009 Mini Baja SAE Competition Rules. Letting the driver lean further back gives a more relaxed position to drive the car. As the rollover FEA analysis shows, there were no detrimental effects to structural integrity found in leaning the firewall back for ergonomic purposes. The next ergonomic improvement made to the chassis was side wall height. While still remaining within the 2009 Mini Baja SAE Competition Rules, the side wall height was set low enough to create easy entrance and exit, while still letting the driver remain safely encapsulated in the vehicle. The last specific ergonomic consideration made during chassis design was the decision to position the steering support in a way that makes it easy for people of all sizes to comfortably sit in the vehicle, while still being able to effectively support the steering column and house the

5 dashboard. The steering support remains out of the way of drivers knees and additionally makes it easier to enter and exit the vehicle. Manufacturability All design work for the UWP Mini Baja frame was done using SolidWorks. Using this program to produce a three dimensional model allowed easy revision of prebuild designs, and gave design team members a visual picture of what the frame would look like. After the design of the frame was finalized, a list of required support members was created and exported to Bend- Tech, allowing easy bending and fitting of various tubular frame components. Tube Bending To increase manufacturability, many bends were used as opposed to miters. By implementing bends into the design of the frame, the number of cuts and welds were decreased. Decreasing the number of cuts and welds lowers the production cost and increases overall chassis strength. For example, by using more bends, a CNC tubing bender could be used during manufacturing, in place of hand welded miter joints, reducing man-hours and production costs. All bends were designed to be made using a tube bender fitted with a 9-inch diameter die, which would eliminate costly tooling changes from the manufacturing process. Mounts All suspension mounts for the chassis were cut from inch cold rolled plate steel, using a CNC laser cutter. The inch cold rolled plate steel was chosen to give all mounts sufficient strength and durability 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 made using a gas tungsten arc welding (GTAW) process. All welds used 1/16 inch 2% thoriated tungsten ER, 70-S2 filler rod, and pure argon shielding gas. The GTAW process was selected because it provided the best control of heat affected zones while also reducing internal stress in the frame. ER 70-S2 filler rod was selected it order to allow the weld to flex slightly without cracking. Also ER 70-S2 has sufficient oxidizers to make welding easier. Pure argon was used to increase arc control. Before any joints were welded, all connected members were purged with pure argon to prevent scaling and oxidization on inner surfaces which would reduce the strength of the welds. All joints were ground and de-burred inside and outside of the joint prior to welding to ensure there would be no contamination during the process. 4 BODY AND COMPOSITES OBJECTIVES The purpose of the body is to prevent debris from entering the vehicle, with the intent of protecting the driver and the vehicle s components. The seat was designed to support the driver comfortably and safely while they are operating the vehicle. DESIGN The UWP Mini Baja car s body was kept very light through 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 a very light material that has desirable properties for a body panel. HDPE Plastic has a tensile strength of 4,600psi, shear strength of 3,380 psi, and it takes 4,570 psi to cause a 10% deflection in the material. These properties also make the body panels more highly puncture resistant. The HDPE panels provide the properties necessary to protect the driver and vehicle components from rocks and other debris. When the panels were integrated into the car, the panels were recessed into the chassis to provide visibility to the chassis members, making the car aesthetically pleasing. Dzus clips are utilized to affix the body panels to the vehicle. Dzus clips allow for the effortless removal of all body panels, providing access to all parts of the car. Hood and Dashboard The hood and dashboard of the car is made of E glassmat and polyester resin. E glass-mat is used because it is relatively inexpensive and provides the necessary properties to create an optimal hood and dashboard for the vehicle. E glass-mat has very good strength in all directions, compared to a uni or bi directional fabric. E glass mat has short and very strong fibers. Using the equation in Figure 15, the hood and dashboard was calculated to have 66,000 psi of tensile strength. This strength ensures the durability of the panels in all offroad conditions. The hood and dashboard, like the body panels, are held on by Dzus clips, which allow for easy access to all of the components in the front of the car. Seat The seat in this car was also designed to be very light weight. This was achieved by making a small seat out of fiberglass and having a detached headrest mounted on the fire wall. Many teams use a full size racing seat, made of aluminum. Aluminum racing seats give the driver very good support but, they are very heavy. The fiberglass seat was designed to provide the same, if not better, support than an aluminum racing seat, while being substantially lighter. This was done by creating lumbar support in the seat and shaping the seat to be generally ergonomic for people of all sizes. The seat, like the hood, was made using E glass-mat and polyester resin and has the same properties and strength as the hood. Mounting bolts on a plate were

6 fiber-glassed into the seat for easy mounting into the chassis. Ribs and creasing on the edges of the seat were utilized to make the seat mechanically strong enough to support all drivers able to operate the vehicle. The seat implemented in the UWP Mini Baja car provides a good combination of weight reduction and ergonomics. SUSPENSION DESIGN OBJECTIVE The objective of the suspension is to improve the stability and comfort of the vehicle through a variety of terrain. The main focus of the UWP Mini Baja car s suspension was to create an overall good performing suspension system that could perform in all terrains equally. DESIGN Overall Suspension The static ride of the vehicle was designed to be 13 inches high. Once a driver is positioned in the vehicle for operation, the suspension will sit at an optimal 12 inch ride height. This height was chosen for a combination of desirable ground clearance while maintaining a low center of gravity. This combination was necessary to keep this off-road vehicle versatile in all terrain. The ground clearance gives the vehicle the ability to overcome high rocks, hills, and bumps. The lower center of gravity will give it an ability to handle better in tight maneuvering situations at high rates of speed. Front Suspension The front A-arms were designed to be as long as possible to get a suspension ratio of 2:1, improve suspension response, and to have the greatest vehicle stability. These A-arms give the vehicle a front track width of 56 inches. The suspension ratio signifies the number of inches the wheel travels vertically compared to the number of inches the shock compresses. The 2:1 ratio was chosen because it gave the best combination of a soft and stiff ride. The ratio is able to do this through shock efficiency. As the suspension ratio gets closer to 1:1 the more effective the shock, creating a stiffer ride. As the suspension ratio surpasses 2:1, the shock effectiveness gets exponentially smaller, giving the A-arms the ability to move more freely, creating a softer ride. Another aspect of the front suspension that affects the shock effectiveness and ride comfort is the shock angle. The front shocks are mounted 30 degrees from vertical. As the shock angle becomes greater, the less effective the shock is and the softer the ride will be. The greater the shock angle, the greater the articulation capability, to a certain point, and the stiffer the ride will be. For the front suspension, a compromise between shock effectiveness, articulation height and ride comfort had to be made. Shock effectiveness was slightly compromised so articulation height could be greater and the ride comfort could be a combination of soft and stiff. The articulation height of the suspension was selected to 5 be 10 inches. To achieve this height and implement the shock as effectively as possible, the angle of the tangent line at the 10 inch articulation point determined the shock angle required. Greater articulation in the front was implemented to overcome aggressive terrain during approach. The 30 degrees of front shock angle also gave a good combination of a stiff and soft ride. It is important to have that characteristic, because it enables the vehicle to handle better in rigorous maneuverability situations, while still allowing the vehicle to be operated comfortably. Adjustability The front A-arms are very adjustable. An owner can adjust caster and camber very easily on the vehicle. The heim joints on the top A-arm, towards the inside of the car, make caster adjustments possible. A threaded joint on the wheel end of the top arms gives an operator the opportunity to adjust camber. The arms were designed to have zero camber gain throughout the motion of the suspension cycle. This was designed by setting the two A-arm planes from the frame joint to the knuckle joint parallel. Zero camber gain is a feature on the vehicle that allows the most tire surface area to be contacting the ground in any suspension position. A-arm Material and Structural Integrity The front A-arms are constructed of inch wall thickness, 1 inch diameter and 0.75 inch diameter 4130 round Chromoly tube. This material was chosen because of its strength to weight properties. Finite element analysis was conducted on the A-arms, simulating the maximum loads an A-arm would ever see. Under maximum loading, and with the addition of crosssectional braces, the A-arms performed with minimum deflection and no yielding, as seen in Figure 16. Ground Clearance The front lower arms were also designed for maximum ground clearance. A bend three quarters of the distance of the A-arm from the chassis creates extra ground clearance under the A-arms. The increased ground clearance in the front gives the vehicle an ability to travel on a wider variety of terrain that may be more intensive. Shock Mounting Shock towers were added to the chassis to achieve the correct shock angle, and increase the shock length for greater shock travel possibility. An adjustable shock tower brace was added to increase support of the shock tower under loading. FEA was conducted to verify minimal deflection under loading by the shock, as stated in the chassis section under drive-train and suspension integration. The adjustability of the shock tower brace aids in the tensioning of the shock towers and the accessibility to the front hood of the car, where various components are housed. Anti-dive An anti-dive angle of 10 degrees was set for the front suspension. This angle increases ease of handling and

7 improves comfort in aggressive terrain by making the shocks effective in all three axis. This angle can be seen in the chassis solid model pictured in Figure 1. Rear Suspension The rear suspension was more difficult to design, on the basis that the drive-train components needed to be integrated into it. The rear suspension was designed to accommodate a slightly lesser track width than that of the front suspension. The rear track width was designed to be 55 inches wide and create a slight over-steer in tight cornering situations, which allows for easier maneuverability at higher speeds. Many of the same principles were utilized in designing the rear suspension as the front suspension. The objective and reason for keeping the A-arms as long as possible was the same for both the front and rear suspension. The 2:1 ratio was also maintained in the rear suspension as it was in the front suspension, for the same reasons. A suspension geometric aspect that was different between the front and the rear suspension was the shock angle. Rear suspension articulation was not as much of a priority as it was with the front suspension. Having a high articulation limit for the front was seen to be more important than having it for the back. This was the case, because of drive-train CV joint limitations and the lesser necessity for articulation in the back. High front suspension articulation is more important than high rear suspension articulation because the front suspension needs that extra articulation during an approach into rugged terrain, where as the rear does not. Since this is the case, the shocks where mounted at their most effect angle, which is within 10 degrees of vertical. The majority of the forces are acting in the vertical direction, and this is why the shock is most effective when it is mounted as close to vertical as possible. Setting the rear shock vertical was also more beneficial in the rear because it allows the shock to dampen the extra weight in the rear to create a more comfortable ride. With that rear suspension configuration, the same balance of a stiff and soft ride was able to be created to match the front. Adjustability The top rear A-arms allow for camber adjustment of the rear tires. There is zero degree camber gain throughout the rear suspension travel, as was the case for the front suspension, for the same reasons. Caster adjustability was concluded to be unnecessary in the rear. A-arm Material and Structural Integrity The rear A-arms are constructed of inch wall thickness, 1 inch diameter 4130 round Chromoly tube. This was chosen for the same materials properties as described in the front suspension section. FEA was also performed on the rear arms, and proved them to be capable of handling the stresses exerted on them in extreme situations, as seen in Figure 17. As a result of the rear shock mounting location being offset, additional triangulation support was added to that section of the A- arms to minimize deflection and dramatically decrease 6 the possibility of yielding. Single link top A-arms were able to be utilized after analysis found that half of the forces acting on the bottom A-arms are acting on the top A-arms during normal suspension cycles. Once this discovery was made, drive-train integration became easier and material weight was able to be saved. Anti-dive An anti-dive angle of 5 degrees was set for the rear suspension. It is important to have a more aggressive angle in the front than it is to have in the rear. This is the case, because the front suspension needs to have more dampening along that axis to create a smoother ride and prevent diving after bumps and jumps. It was concluded that it would be effective to give the rear suspension the ability to react along that axis too, creating a more comfortable, but still aggressive ride. Shock Absorbers The Fox Podium X shocks are both externally and internally adjustable. The compression rate can be adjusted externally by a dial. The shocks can also be adjusted by changing the internal components. The Podiums have a simple shim stack design. Changing the diameter, thickness, and order of the shims will change how the shock will react with compression and rebound rates. The setup can also be drastically changed by replacing the piston. The diameter of the holes on the piston and the size of the bleed grooves on either the compression or rebound side of the piston can change how the shock reacts. The shocks can also be fine-tuned by adjusting the nitrogen pressure in the remote canister. With the help of a shock dynamometer, the shocks were set-up taking into consideration several factors, as seen in Figure 18. The front shock absorbers were setup differently than the rear shocks. Mini Baja cars seem to have the tendency to nose dive off of bump and jumps, so to further prevent that phenomenon, the front shocks were setup with a stiffer high speed compression rate than the rear shocks. The rebound rate was set so the shocks react fast enough that they are fully extended before the next bump, but not so fast that the car bounces after landing a hard jump. Low speed compression adjustment is also important for proper shock setup. Through the low speed compression adjustment, the shocks were setup to allow the vehicle to slightly roll while cornering, but not to the point where the vehicle rolls so much that it lifts the wheels off the ground using the majority of travel through the low speed compression adjustment. Also, by adjusting the low speed compression, the vehicle will be very controlled over small bumps, making for a smoother ride. Refer to Figure 29 for a solid modeled image of the suspension integrated into the vehicle.

8 STEERING SYSTEM DESIGN OBJECTIVE The steering system is designed to withstand the stress of safely maneuvering the vehicle through any type of terrain. DESIGN Simplicity and safety were the main design specifications for the vehicle s steering system. A small, lightweight rack with a 12:1 ratio was chosen as the main component of the assembly. The small size of this rack allows the geometry and joints of both the suspension arms and tie rods to align perfectly and completely eliminate bump steer. Custom stainless steel clevises provide a strong, corrosion resistant link between the rack and the custom aluminum tie rods featuring opposing threads for easy adjustability. Lightweight aluminum rod was chosen for the tie rod material for ease of manufacture, along with the fact that it will easily withstand the strictly axial forces applied to it. Steel tubing was used for the steering column due to the torsional loads it will need to withstand. A universal joint provided easy redirection of the steering column as it extends from the rack, along with a safety feature. If the vehicle sustained a severe head-on collision, the steering system would buckle instead of being driven into the driver. The forward design of the bearing mount loop allows for easy entering and exiting of the vehicle. A quick disconnect adapter for the steering wheel, which also allows for easy entrance and exit of the vehicle, completes the steering system. Tie Rods Intuitive analysis of a steering system shows that the forces exerted on tie rods produce almost strictly axial forces on them. Since the tensile strength of alloyed aluminum can approach that of steel and buckling is not an issue in such a short rod, the weight saving, low density of aluminum can be utilized in this case. The use of solid aluminum rod for the tie rods also introduces an ease of manufacture not available with steel tubing. Drilled and tapped holes on both ends of the tie rods allow for easy incorporation of a heim joint and clevis on either end. Furthermore, tapping one end of the tie rod with a left hand thread allows the toe adjustment to be completed by simply twisting the tie rod extending the tie rod with rotation in one direction and shortening it with the other. Steering Rack The front suspension design incorporates a narrow front end due to the long suspension arms. The long suspension arms allow for a better suspension ratio. By limiting the spacing between the inside suspension arm joints to 7 inches, the overall width of the car does not increase with the longer suspension arms. The size of the steering rack is directly limited by this spacing between the suspension joints in order to overcome bump steer. Bump steer is the phenomenon where suspension travel can move the tie rods in or out, 7 causing uncontrollable turning of the wheels. Properly aligning the suspension and tie rod joints into parallelogram geometry, shown in Figure 19, can completely eliminate bump steer. The small rack fulfills this design specification, along with reducing weight and freeing up space in the front end. The 12:1 ratio provides very responsive steering, but does not make the car too difficult to steer. Clevises The connection between the tie rods and rack is completed with custom, male clevises, shown in Figure 20. These small parts allow for full implementation of the heim joints included with the rack, without interfering with the rack mount. The use of stainless steel reduces the likelihood of failure due to corrosion. The differing metals in this area of the car (aluminum, steel, etc.) introduce differences in galvanic potential. This, along with the inevitable presence of electrolyte in the form of mud, produces the perfect conditions for corrosion. The complex geometry of these clevises also increases the probability of stress corrosion cracking. The passive film on 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 steering column just as it was for the tie rods; however, unlike the tie rods, the stress imposed upon the steering column is almost strictly torsional. The modulus of elasticity for aluminum is approximately a third of that of steel. This, combined with the equation for angular deflection, produces an angular deflection in aluminum approximately three times what it would be with steel. Since this sort of sloppiness is undesired in a steering system, steel is used. The calculations in Figure 21 confirm that the 0.75 inch steel shaft will withstand the stresses imposed upon it. The forward mounting loop for the steering column provides extra knee room for drivers of all sizes, along with providing room for easy exit of the vehicle in the event of an emergency. DRIVE-TRAIN DESIGN OBJECTIVE The drive-train is a very important part of the Mini Baja car, taking into consideration that all of the car s power is transferred through the drive-train system to the ground. The challenge is to harness the engine s 10 horsepower and distribute it to the ground in the most efficient way. The drive-train needs to be able to operate in the lowest and highest gear ratios while performing in all of the different aspects of the competition. DESIGN The drive-train design focuses on being highly variable while also staying very light and easily serviced. The drive-train allows the car to be vary between the gear ratios of 8.1:1 to 50.7:1. This gear ratio setup allows the car to have a start up speed of 2.4 MPH and a top speed

9 of 33 MPH. The system includes a continuously variable transmission (CVT), planetary gear box, and a chain drive to a Polaris Outlaw rear housing with custom built drive axles. Continuously Variable Transmission The Comet 790 Series CVT has gear ratios from 0.54:1 to 3.38:1. Utilizing this CVT gives the car many advantages that include a lightweight, simple, tunable transmission setup. The primary clutch consists of a centrifugal clutch that automatically shifts gears ratios under varying engine speeds and torque loads. The ability to tune the CVT comes from changing weights and springs in the primary clutch that will change engagement RPMs and time to maximum ratio. This allows the car to go fast and have a high gear ratio while traveling over flat terrain and have a low gear ratio while traveling over rough or steep terrain. Figure 22 illustrates the engine RPM to CVT ratio relationship. The CVT utilizes a 1 inch belt that requires a center distance of 9.41 inches, as seen in Figure 23. Planetary Gearbox The planetary gearbox that is used in the car is manufactured by Zenith and offers a great way to achieve the desired 5:1 gear reduction in a compact package, as seen in Figure 24. The planetary gearbox has many useful features, such as sealed bearings, lifetime lubrication, and a high energy density. The sealed bearings for lifetime lubrication are a very nice feature for the off-road enthusiast who wants to enjoy their vehicle with little maintenance. The high energy density allows the gearbox to withstand high torque loads while maintaining a small size. Its versatile mounting brackets allow it to be mounted very low on the vehicle, in turn lowering the vehicle s center of gravity. To make the planetary gear box possible for our application, a small amount of modification was required to the existing gearbox. The input required a custom shaft that would fit the driven clutch in the Comet 790 Series CVT and the output required a small modification to the existing output shaft. Both were designed and machined in-house at the University of Wisconsin- Platteville. Many different factors were taken into consideration during the design of the input and output shafts. The input shaft required a change in size from ¾ inch at the exit of the CVT to 1-½ inch diameter at the input of the planetary gearbox. The input shaft was designed with a 2:1 taper from 1-¼ inch to ¾ inch. A taper was selected over a straight transition in order to reduce stress risers. The existing output shaft was also modified with a 2:1 taper as the input shaft reduced for a 1.5 inch diameter shaft to a 1 diameter shaft. Chain Drive The output of the planetary gearbox consists of a 13 tooth sprocket and hub that drive a chain which leads to a 39 tooth sprocket mounted on the rear housing, as seen in Figure 25. The sprocket on the planetary gear box can easily be changed to accommodate different gear ratios for variety driving conditions. The chain 8 system uses a metric 520 O-ring chain that is lightweight, reliable, and able to withstand the applied forces. The maximum constant load on the chain during the highest torque situation was calculated to be 2,700 lbf. In addition to the forces generated during constant torque loading, forces due to torque spikes needed to be included, which increases the maximum chain load to approximately 3200 lbf. The manufacturer listed the yield strength of the chain to be 8,100 lbf which gives a factor of safety of 2.53 during maximum chain loading. Based on this, it was determined that metric 520 O-ring chain would be capable of withstanding the applied load. The design implements the use of a very short chain which minimizes the number of links and weight of the chain. Besides reducing weight, fewer links results in less stretch in the chain, because there are less links to be stretched. Utilizing an O-ring chain significantly increases the life of the chain, because of the reduced metal on metal wear that is common on a non O-ring chain. Using a chain in place of a belt also has the benefits of a longer drive life, no drive slippage, and the ability to withstand large torque loads. Rear Housing The rear housing consists of a Polaris Outlaw rear housing. This is a very good choice because it provides a drive to the rear axles in a small and light package. The housing provides excellent way to transfer power from the chain to the sprocket hub that drives the rear axles. The rear housing pivots on the bottom, so that chain tension can be adjusted by tilting the housing away from the planetary gearbox. A bolt that goes through a plate in the frame is threaded into a clevis which makes it very easy to adjust the chain tension. The rear housing is sealed with rubber boots which allows the rear housing to have lifetime lubrication, resulting in very low maintenance. Rear Axles To complete the drive-train, custom rear drive axles were machined to accommodate the unique drive-train setup. These drive axles have a 1 inch diameter, and are made from 4330 alloy steel. The required diameter of the shaft was calculated to be inch using the equation By using a 1 inch shaft, a factor of safety of 1.55 was utilized. Refer to Figure 29 for a solid modeled image of the drive-train integrated into the vehicle. BRAKING DESIGN OBJECTIVES The purpose of the brakes is to stop the car safely and effectively. In order to achieve maximum performance from the braking system, the brakes have been designed to lock up all four wheels, while minimizing the cost and weight. DESIGN

10 The breaking system is mainly composed of components from Polaris and Wilwood. Two front brake calipers and one rear caliper, mounted to the rear housing, have the ability to stop the vehicle effectively, and lock up all four wheels. The front brakes are composed of 7 inch diameter discs and dual 1.19 inch diameter piston calipers, made by Polaris. The rear brake consists of an 8 inch diameter disc and 1.18 inch diameter piston calipers, also made by Polaris. Both front and rear brake components are currently utilized on the 2008 Polaris Outlaw. Front brake disc with a dual piston design were chosen because of their superior braking ability, compared to single piston calipers. With the dual bore design, braking can be more effective due to the fact that two pistons increase the surface area acting on the brake pads. The Outlaw rear brake components were chosen because of their easy integration with the rear housing and their ability to easily meet the braking needs of the car. Remote master cylinders from Wilwood were chosen to increase the flexibility of mounting locations, while also creating more space for the driver. Two master cylinders were chosen instead of one to ensure that the braking system would still be able to perform, even if one were to fail. Therefore, if either the front or rear brake system fails unexpectedly, braking power will still be available to the driver. Steel brake lines run the length of the car, and flexible braided lines are used at the A-arms in the front for suspension travel and caliper pivoting. 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) 4 Coefficient of Friction disc/pad.45 Weight of Vehicle (lb) 500 Wheelbase (in) 56 Table 2- Brake system data used to calculated system specifications Through the use of a brake calculator and the input data listed in Table 2, it was determined that the selected components would perform to the expectations needed. Ideally, the braking bias should be 50-50, but there will always be a level of adjustment needed to optimize brake performance. Being able to adjust the braking bias is especially important for this car, because of having two calipers in the front and one caliper in the back. The bias needed to optimize braking performance was found to be almost exactly opposite of what would normally be expected of a brake bias, which is normally 60% in the front and 40% in the back. As a result of having only one rear caliper, 38% of the braking power needed to be biased towards the front, and 62% of braking power needed to be biased to the back. The pedal force needed to lock the brakes was calculated to be 110 lbs, which is within the capability of any driver. From the calculations performed, it was found that a torque of Nm and Nm were required in the 9 front and back axles respectively to lock up the wheels. The braking components provide Nm and Nm of braking effort in the front and in the back respectively, which is more than the required amount to stop the vehicle. This creates an acceptable factor of safety for the braking system on the car. Braking force to lock all four wheels verses the tire coefficient was also calculated and graphed for analysis, which can be seen in Figure 26. This graph is beneficial to illustrate the amount of force needed to lock up the tires on various terrains with different tire types. Figure 27 shows that with the car s braking components, it will be able to easily stop at any speed that it is capable of traveling. Table 3 below gives an overview of the braking system specifications 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 DESIGN OBJECTIVES The electronic system for the car was designed to fulfill two key purposes. First, the electronics system supports the mandatory safety equipment, specifically the brake light and the kill switch circuit. Second, the electronics provide useful instrumentation, in particular a tachometer. DESIGN The car s electrical system has been designed around three main power buses, each with an independently fused circuit. These buses are the safety lights bus, the unregulated power bus, and the regulated power bus. The safety lights bus is connected directly to the battery while the regulated and unregulated buses are connected through a 40A relay. Every powered electrical component on the car is connected to one of these three buses. These buses are managed from a centrally located sealed enclosure located in the dashboard. The safety lights bus powers the brake light. The brake light is activated by a brake pressure switch located in the rear brake line. The electronics are designed so that when the kill switch is depressed, power is disabled on both the regulated and unregulated buses, but the safety light bus remains connected so that the brake can still function. Because the kill switch closes the circuit when activated, the kill switch function is achieved by using a pair of diodes to simultaneously ground out the engine s primary coil and bypass the normally-open relay on the regulated and unregulated buses. One diode prevents the engine from grounding through the relay and the other diode prevents battery current from flowing back into the ignition coil. A 5Ω, 10W resistor is placed in series before the relay and kill switch pair to limit the current from the battery to the two branch circuits. A

11 second 5Ω, 10W resistor is placed in series with the kill switch. This is arranged so that when the kill switch is pressed, the relay, with an internal resistance of 78Ω, has a significant portion of its supply current shunted through the kill switch branch that has a resistance of only 5Ω. This reduced current flow through the relay branch and is enough to cause the relay to shut off, which then disconnects power to the regulated and unregulated buses, but still leaves the safety lights bus connected to the battery independent of the relay. The component that is connected to the unregulated bus is the tachometer. This bus was designed for expansion though, which may include off the shelf instrumentation, power for accessories, and other similar components. Anything connected to the unregulated bus is either not sensitive to electronic noise and voltage spikes in the power supply, or contains protection circuitry. The regulated bus contains the circuitry that conditions the signal for the tachometer. The original signal for the tachometer was intended to be generated by the manufacturer s signal sender. This signal sender is designed to connect to an external shaft from the engine. Because designing a shaft for this application would be expensive and difficult, a custom signal conditioning circuit has been devised. The ideal signal to the tachometer was found to oscillate at 8 khz and possess an amplitude of 60mV for the needle to read 4000 RPM. Any signal input with an amplitude of 25mV and below is read as zero RPM on the meter, which is a problem. This problem was solved through using an optoisolator along with a few other techniques. This electrical system provides a reliable and efficient way to manage all the electronic components on the UWP Mini Baja car. Refer to Figure 28 for the full electrical schematic of the UWP Mini Baja car. CONSUMER INTEREST The appeal of the finished product to a consumer is equally as important as all other aspects of the design process. A consumer must find the vehicle to be reasonably priced, aesthetically pleasing, exhilarating to drive, safe, and dependable in order to insure that the vehicle will be purchased. These consumer factors were continually considered throughout the design of the UWP Mini Baja vehicle. To create a vehicle that was cost effective, manufacturing processes were closely monitored throughout the design process. For example, the chassis was designed in SolidWorks and imported into Bend-Tech making the chassis easy to manufacture with computer aided machinery, lowering production cost. Utilizing similar processes throughout the rest of the vehicle s design and manufacturing lowered the overall price of the vehicle. The Mini Baja car body was design to be aesthetically pleasing. A fiber-glass molded hood and dashboard with recessed body panels that show off the chassis inner workings make the car look clean and fun to drive. 10 To make the Mini Baja car the best performing car at the track, special attention was given to the drive-train and suspension design. Those two components most significantly affect a consumer s attitude about the performance of an off-road vehicle. The drive-train gives the car a top speed of 33 mph, while still being able to provide 600 ft-lbf of torque at the wheels. The suspension was highly analyzed through the design process and performs just as well. The suspension will support handling at high rates of speed, while still being able to articulate aggressively for a wide variety of terrains. Paramount to all other requirements was the safety of the driver. Once baseline safety requirements were in accordance with the 2009 Mini Baja SAE Competition Rules, the design did not stop there. For example, Finite Element Analysis verified the robust nature of the frame, further ensuring consumer safety while operating this vehicle. 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 promotes reliability. Another aspect of reliability this vehicle provides is its capability to be easily maintained. Maintenance is able to be performed on all components of the vehicle easily, because the vehicle was fully solid modeled before manufacturing took place, allowing for the opportunity to eliminate component placement constraints which would make it difficult to perform scheduled maintenance like changing the oil or replacing a CVT belt. CONCLUSION Once all the design aspects have been combined into one complete vehicle, the result is profound. Safe, reliable, fast, aggressive, and just plain fun to drive is what the UWP Mini Baja car is all about. With a focus on the drive-train and suspension design, the vehicle will be able to handle any terrain that is put before it. The drive-train sports a respectable low end wheel torque of 600 ft-lbf, while still having an exhilarating top speed of 33 mph. The suspension creates a ride that is comfortable, yet aggressive when handling corners at high speeds. This vehicle only has a 10 HP engine, but as a result of effective design techniques it has the ability to conquer the most difficult terrain. Through weight reduction, increasing drive-train efficiency, and calculating and tuning the suspension accurately, the power restraint is a minimum factor in this vehicle s offroad ability. Not only will the performance catch a consumer s interest, but features such as a comfortable seat, a sleek body design, practical electronics, and cost effectiveness will impress even consumers that are not avid off-road vehicle enthusiasts. The careful design and the technology that went into this vehicle will prove itself during manufacturing, in the show room, and of course, at the track.

12 APPENDIX Fig. 4 Distributed load at 90 degrees relative to the roof of the chassis (Algor ) Fig. 5 Result of 90 degree loading (Algor ) Fig UWP Mini Baja Chassis (SolidWorks ) Fig. 2 Distributed load at 135 degrees relative to the roof of the chassis (Algor ) Fig. 6 Distributed load at 45 degrees relative to the roof of the chassis (Algor ) Fig. 7 Result of 45 degree loading (Algor ) Fig. 3 Result of 135 degree loading (Algor ) Fig. 8 Maximum loading until yield (Algor ) 11

13 Fig. 9 Loading on rear shock mount (Algor ) Fig. 13 Torsional rigidity loading (Algor ) Fig. 10 Result of loading 800 lbf (yield) on rear shock mount (Algor ) Fig. 14 Result of torsional rigidity loading (Algor ) Fig. 15 Body calculation Fig. 11 Loading on front shock mount (Algor ) Fig. 16 Maximum lower front A-arm loading (Algor ) Fig. 12 Result of loading 485 lbf (yield) on front shock mount (Algor ) 12

14 Fig. 17 Maximum lower rear A-arm loading (Algor ) Fig. 21 Steering equations Fig. 18 Force verses velocity plot produced by shock dynomometer Fig. 22 Speed verses RPM with CVT transmission Fig. 19 Steering diagram Fig. 23 CVT implemented on car Fig. 20 Steering Clevis (SolidWorks ) 13

15 Fig. 24 Planetary gear box assembly Fig. 28 Electrical schematic Fig. 25 Drive-train installed in the car Fig. 29 Solid model of UWP Mini Baja Car (SolidWorks ) Fig. 26 Brake force to lock verses tire coefficient Fig. 27 Brake distance verses speed 14