E-Board and Design Leads
University of Rochester Baja SAE Team Members:
- Susana Acosta
- Max Greenberg
- Kimchua Heng
- Phil Katz
- Bryan Knouse
- Jakob Maag-Tanchak
- Joel Ramirez
- Robert Towle
Advisor: Professor Sheryl Gracewski
# 039 University of Rochester Baja SAE Design Report
2011 Birmingham Competition
Brent Gordon, Kohei Hamano, Philip Katz, Brandon Martindale, David No, Sam Sadtler
This year the University of Rochester is designing and building a vehicle to demonstrate ability to perform well in all of the tested areas. The car is designed to be light weight, durable, fast and efficient. This is accomplished by carefully designing components of the car to meet these goals. INTRODUCTION The Baja SAE collegiate design competition challenges teams to design vehicles that perform at a high level, while strictly following a set of rules to ensure the safety of the competitors involved. The car is designed as a prototype for an off road vehicle to be marketed to the public. This means that the car should be durable, serviceable, competitively priced, able to perform at a high level, and also aesthetically pleasing to the customer. The car must also be able to navigate through water, making it truly an all terrain vehicle. The competition held in Birmingham, Alabama, will test the car on the quality of its design, the cost of the vehicle, and its performance in both dynamic events and an endurance race. The dynamic events will include suspension and traction, land maneuverability, water maneuverability, acceleration and a load pulling event. The endurance race is a four hour long race incorporating various aspects from all of the above events, testing the durability of the vehicle and the abilities of drivers and teams to perform well under pressure to complete as many laps as possible. The University of Rochester team has built an entirely new car for this year’s competition series and has worked to improve in the areas in which the cars of previous years have fallen short. Some of these areas of improvement include vehicle dimensions, tuning of several components using better testing methods, as well as improving the aesthetic appeal of the car. DESIGN FRAME AND BODY
The frame was designed in accordance with the rules to improve on last year’s design, while maintaining structural stability to ensure driver safety. Clearance measurements from all potential drivers were taken seated in normal driving position in the old car while wearing all required equipment. The distances between the driver helmet and side surfaces, and between driver knees and SIM’s were measured in order to see how much length and height could be reduced compared to last year’s car, while still conforming to section B8.2.and ensuring that the 95th percentile male driver is comfortably accommodated . Figure  depicts the finalized frame design.
Optimization- To reduce weight compared to last year, the minimum required tube sizes were used in place of the 1.25’’ diameter tubes used predominantly the year before. This tube size reduction combined with overall height and length reductions significantly decreased the weight of the frame, which will improve acceleration. To ensure the driver’s safety and structural stability, the frame was analyzed under two different scenarios in Patran; Front and Rollover Impact. The maximum stress was obtained for each of the two analyses and compared to the yield stress for 4130 Seamless Steel (63,100 psi) . A safety factor was obtained for each case.
Front Impact- The first analysis modeled a front collision of the car with a stationary object. KEVA Engineering’s research paper on Crash Pulse Modeling analyzed a crash on a mid size four door sedan going at 35 mph and crashing into a wall, finding that it stopped in 0.11 seconds  . This is equivalent to 14.5 G’s of acceleration. In order to model this, the front of the car was constrained in all axial directions, and a forward inertial load of 14.5G’s was placed on all members of the car. Also, a force equivalent to the weight of the car, not including the weight of the tube members, was placed in the impact direction. Figure  shows finite element analysis on the frame, having the elements with highest stresses highlighted in black and red colors. The analysis shows that we have a safety factor of 4.32 under the given conditions.
Rollover Impact- The roll cage was also analyzed by utilizing a load, equivalent to the weight of the car, at 2 G’s. This load was placed on the top bend of the vehicle between the FBM and RHO, pointing inward and downwards of the car. The maximum stress was obtained and compared to the yield stress of the tubing, yielding a safety factor of 1.5. Figure  shows a high stress concentration around the point of impact, but the margin of safety accounts for the worst case scenario in which the curve analyzed hits the ground at a perpendicular direction. This is an unlikely scenario because when the car rolls, the frame does not come into full contact with the ground but is impacted at an angle with only a portion of the full load being applied.
Rear bracing- We chose to use rear bracing on the car because it allows the car to remain structurally sound while increasing the aesthetic appeal of the car and meeting section B220.127.116.11 of the rules . Using rear bracing allows for more freedom at the front end of the car, making it more attractive. Removing the front bracing members also enhances the driver’s vision by removing the bars from his or her line of sight. In the past, problems were encountered with the rear bracing interfering with the drive train assembly, specifically the CVT. To fix this problem, point S (shown in figure RC 5 in the Baja SAE rules) was moved upward by extending the SIM further up the RRH. This makes it so the rear bracing members do not get in the way of the CVT or the guarding placed on the CVT. It also creates more clearance for the engine, improving accessibility and allowing easier maintenance. It was a problem in older cars that the engine needed to be partially disassembled in order to be installed or removed from the car. The new clearances in the rear bracing make the entire back end of the car more serviceable. This decreases the amount of time it takes to assemble the car and perform repairs, and it will help keep the car out of the pits and on the course longer during the endurance race.
Body Panels- Body panels were made out of fiberglass to increase the aesthetic appeal of the vehicle. Previous attempts with aluminum panels demonstrated the difficulty in shaping aluminum to closely adhere to the frame, yielding gaps. Fiberglass is easy to form and cut so that all gaps are eliminated, including those under the .25 inches as allowed by the rules. Fiberglass can be formed into contoured shapes, such as those utilized on our front hood, allowing for much more interesting body shapes. This greatly enhances the aesthetic appeal of the vehicle which in turn makes it more marketable to consumers.
Panels were mounted mostly using tube straps fastened with rivets, although a few welded tabs were also needed. The reduction of tabs decreased the cost associated with fabricating and welding tabs, and the use of rivets helped to keep down the weight and cost of mounting hardware.
Skid Plate- HDPE will be used for the skid plate because it has proven to sustain significant impacts without significant damage. 3/32’’ sheet will be used instead of 1/8’’ to save weight. DRIVETRAIN The drive train was designed with considerations taken in accessibility, achieving high acceleration while taking up minimal space and weight. The drive train consists of a standard Briggs and Stratton 10 hp engine with a CVTech-IBC continuous variable transmission (CVT) and final speed reduction components. The reduction components include a Matex Products Inc. planetary gearbox coupled with a single sprockets and chain system going into a pair of CV joints. Honda CV axles are used to deliver power into the rear wheels. At the maximum engine output of 3800 RPM at overdrive, we achieve a top speed of about 32 mph with a capability of a 9.25 ft/s2. The main motivation for this alternation from last year’s vehicle came from difficulties accelerating after braking and making it up hills. Additionally, due to the nature of the course, it makes little sense to design a car for a top speed that will never be reached. Gear Ratio and Torque- The CVT is followed by a 5:1 reduction through the planetary gear set and a final 2:1 reduction through the chain sprockets, giving us an overall reduction of 10:1 at overdrive. At the engine’s maximum torque of 13.8 ft-lbs, the reduction outputs 112 ft-lbs on the wheels. The maximum torque was calculated for each component of the drive train to analyze the stresses applied. The following equation was used in our calculations:
where T = torque, P = power, n = reduction ratio, and ω = rotational velocity . Continuously Variable Transmission- A CVT allows shifting between an infinite number of gear ratios, which enables the engine to run at its most efficient RPM for a range of vehicle speeds. The continuous shifting provides smooth acceleration without any manual driver control. Another advantage of the CVT is that it is easier to install compared to geared transmissions. It also weighs much less than a geared transmission containing multiple gear ratios. Planetary Gear Set- A new feature in our drive train compared to last year’s vehicle is the addition of a planetary gear set. A planetary gear system consists of a core sun gear; planet gears that revolve around the sun gear; a carrier holding the planetary gears; and an annulus that encases the aforementioned gears. In our drive train, the drive shaft is linked with the sun gear and an output shaft is attached to the carrier, while the annulus is held fixed. Reduction is achieved between the input RPM of the sun gear and the output RPM of the carrier. This combination of gears allows for maximum strength in the system because the planetary gears distribute the load over three gears thus reducing the risk of failure due to large breaking loads. It also allowed for the elimination of one stage of a two stage chain drive system, which in past years has been the primary culprit of complications. The following equation defines the gear ratio for a sun-input/carrier-output planetary system: Ratio = (Ring gear Diameter / Sun Gear Diameter) + 1
Our gear box acts as an intermediate reduction, providing the largest reduction in our drive train (5:1) . The planetary gear set is a compact system which allows the drive train to be tightly packed. The Matex gear set used in our car is only 146 mm in diameter. Chain Sprockets- The chain drive provides the final reduction into the CV axle. As mentioned, a 2:1 ratio is achieved using a 15-tooth sprocket and a 30-tooth sprocket purchased from McMaster-Carr. A spline joint is applied between the sprockets and the shafts to maximize power transmission. The splines are machined by Rochester Gear, Inc. on the sprockets and the shafts. The hub attached to the smaller 15-tooth sprocket will be cut down to reduce rotational momentum. A stress analysis allowed us to determine the optimal hub thickness for the maximum torque applied. The rear 30-tooth sprocket will be converted into a “sprotor” with appropriate machining. A sprotor is when a sprocket in the final drive is also converted into the break rotor. This design ultimately saves space and weight. The chain is an ANSI 520 O-ring motorcycle chain. This chain does not need to be greased, so the sprotor will not be lubricated and lose braking force. Last year’s car utilized an ANSI 50 chain, which snapped multiple times during competitions. Using links designed for dynamic loading will address this problem for our car this year. An alternative, which we considered, to the chain reduction was applying another gear system after the planetary set. However, the optimal dimensions of the gears calculated through AGMA stress analyses could add significant weight to our car and we opted to use a final chain reduction, which is more effective at taking larger loads . Shafts- The drive shaft adjoining the CVT secondary and the planetary set has a keyway to secure the CVT and a custom spline for the planetary set. The output shaft from the planetary has another spline with the gearbox and a second spline connection at the other end with the smaller sprocket. The output shaft was purchased as an auxiliary to the Matex gearbox and was machined to reduce the weight. The shafts are held fixed with bearings to our custom mounting system, which is discussed further in the next section. The final CV axle has another custom spline connection to the rear sprotor, and held fixed to a set of bearings attached to the mounting. The CV axles utilize constant velocity (CV) joints that allow for misalignment in the shaft during rotation. The joint utilizes a ball and cage system. The balls rotate in the cage in bending so that the axles are free to bend. However, the cage prevents them from rotating under torsional loading, allowing the axles to rotate and bend at the same time. Mounting and Guarding- The entire drive train is fixed to a removable mounting in the back end, which allows for ensured alignment of crucial drive train parts. A major aspect of our mounting system is the serviceability for securing the engine to the frame. By having separate bolt connections on the side of the engine, it addresses the issue in our last year’s vehicle which made it difficult to access the engine bolts because space was limited by the firewall. Another benefit to having an engine plate is that it can be isolated from the frame which will reduce noise and vibrations cause from the engine. This should provide a better driving experience. In past years making guarding for the back end has traditionally been labor intensive, poorly constructed and inaccessible. By reducing the size and weight of the drive train and containing it in a box of the frame it allows for effective guarding to be attached and removed for easy access to vital components. The height of the engine was moved down 7 inches, considerably lowering the center of mass and thus decreasing the likelihood of rolling over. Last year the engine had been placed high on the car to prevent flooding during water events; however this meant that the car could easily be rolled. This year, to deal with the water course, we will have a water tight yet vented CVT as well as installing a snorkel kit on the engine air intake. VEHICLE DYNAMICS Table  shows a table with the important characteristics of the car such as track width, wheel base and weight. Front Suspension Geometry- Some of the key considerations were the ability to handle cornering at higher speeds, maneuvering off camber turns easily, and traversing obstacles and jumps smoothly. The suspension elements also need to be able to withstand the rigors of competition. The control arms for the front suspension are an iteration of the arms used in previous years. In order to meet the design goals, the suspension geometry was designed using front view swing arm (FVSA) and side view swing arm (SVSA) views to determine how wheels would move as the suspension elements move . After having issues cornering at high speeds in the past, it was determined that having wheels that cambered with suspension movement would be ideal for increasing the stability of the car. When the tires camber into the turn, it creates camber thrust. This helps the car navigate turns by counteracting the centripetal acceleration due to the motion of the vehicle. The FVSA view was modeled to determine the geometry that would allow the wheels to camber in the correct directions for cornering applications. This meant that an unequal length, non-parallel set up was chosen for the front view geometry of the a-arms . The FVSA was also used to determine the roll center of the car for the front suspension. The roll center was lifted off the ground to bring it as close as possible to the center of gravity. Since moments causing the car to roll are created by forces generated at the center of gravity by centripetal acceleration, bringing the roll center closer to the center of gravity decreases the length of the moment arm. A smaller moment arm with an equal amount of force lowers the total moment that forces the car to “roll,” making the car more stable. This geometry is shown as Figure . The side view geometry was used to determine the “anti” characteristics of the suspension. It was determined that the suspension would have zero anti-dive built in to the geometry. Anti-dive is good to keep the car from diving under braking forces, opposite to the direction of suspension travel. However, similar forces are seen when attempting to traverse large obstacles. It is not ideal to have the suspension bind up in these situations due to an anti dive set up. In order to achieve this goal, the suspension was set up so that, in the side view, the arms are parallel to each other as well as the ground. Percent anti dive is proportional to angle between the line drawn from the suspension to its instant center and the horizontal. Since the arms are parallel to each other as well as the horizontal, this angle becomes zero and the percent anti dive is therefore zero as well. The front a-arms used are unique in the sense that they push the tires forward of the front end of the car so that the wheels hit any obstacles before the chassis crashes in to them. This is particularly helpful on obstacles such as “whoops” where there is a high probability of crashing the nose of the car into the obstacle. This feature was useful on past cars and allowed the car to retain more speed through obstacles. 4130 steel tubing was used to create both the upper and lower a-arms. The top arms are made from .049” wall thickness tubing and the bottom arms are made from .065” wall thickness tubing. The .049” tubing was the minimum tube thickness available due to the difficulty of bending anything thinner. Both top and bottom had finite element analysis performed on them to ensure that they could withstand the forces seen during competition events. Analysis was performed using the built in SolidWorks finite element solver and loads of 1 lb were used so that the results were easily scalable. These results are shown in Figures [5,6]. Rear Suspension Geometry- The design of the rear suspension was based on many of the same considerations as the front suspension. The team, historically using a swing arm style rear suspension, decided to make the switch to an independent rear suspension. Making the switch to independent rear suspension allows for easier tuning so that the overall handling capabilities of the car are increased, allowing for a better overall performance. After deciding to make the switch to independent rear suspension, a decision matrix was used to determine which style of rear suspension would work the best for us. The types of suspension that were discussed were trailing arms, semi-trailing arms and H-arms. H-arms were chosen to be used and the decision matrix is attached as Table . The design goals for the rear suspension were the same as the front suspension. The FVSA and SVSA views were used to determine the characteristics of the suspension. The FVSA view was used to set up the correct camber gain for the rear wheels. This means that the inside wheel cambers positively (away from the car) and the outside wheel cambers negatively (towards the car) while navigating a turn . Also, similarly to the front suspension, the roll center of the car was raised above ground in order to limit the amount of body roll felt by the car during cornering. The FVSA view for the rear suspension is attached as Figure The SVSA view was used to determine the anti-squat characteristics. It was decided that the percent anti-squat, which is calculated in the same manner as anti-dive, would be set to zero. It was set to zero for the same reasoning as setting the anti-dive to zero, “binding” of the suspension is not desired due to forces perpendicular to the shock mounting  4130 steel tubing was used to build the upper and lower H-arms. Both the bottom and top arms are made from 1” tubing with a thickness of .065”. This thicker tubing was chosen for the bottom arms so that it will be better equipped for impact with rocks, logs and other obstacles. The top arms were made of this thicker tubing because they will experience the loading from the shocks at the cross members. Analysis was performed for load cases most likely to make the arms fail using the SolidWorks built in finite element solver. Loads of 1lb were used to make the results easily scalable. These results are shown as Figure[8,9] Shocks- Fox Float air shocks are used for both the front and rear suspension. The shocks are 19.8” long fully extended and have 6.2” of travel. The shocks are light weight at only 2.1 lb and provide weight savings over heavier coil-over shocks utilizing steel springs. The shocks feature infinitely adjustable spring rates as well as adjustable velocity sensitive damping rates so that they can be easily tuned to meet the specific needs of the car. It was decided that to save on cost during the Cost Event, that the EVOL, extra volume edition of the shocks, would not enhance the performance of the vehicle enough compared to the extra expenses. Steering- The goal for the steering design was to be able to adjust steering to meet the needs of the event. For low speed turning, a true Ackerman steering angle is used, but for higher speed applications, a more neutral steering angle is used. This is accomplished by utilizing a bolt on steering attachment at the front uprights. A rack and pinion unit was purchased with custom extensions and nylon sliders mounted to each side of the rack. The rack is placed foreword of the shock mount leaving the tie rods exposed. This was necessary due to the unique shape and placement of the a-arms, so a bumper was added to protect them. The tie rods have both left and right hand threads to allow for quick adjustments as well as jam nuts to keep them from moving during operation. The castor and king pin angles were chosen to give some camber change while steering. The castor angle was chosen to be 10¬° and the king pin inclination was chosen to be 10° as well. These numbers were chosen so that the mechanical trail created will keep the steering stable at high speeds but will not make steering too difficult at low speeds. The steering column has been comfortably placed so that a range of drivers can comfortably operate the car without having to make difficult changes to the set up. Front Uprights/Hubs Our team has decided to machine our own uprights this year. We have chosen to use a piece of billet 6061-T6 aluminum and will use a manual mill rather than CNC. The uprights will be machined to accommodate a spherical ball joint with a straight, 5/8-18 stud. On the lathe, we turned a 4340 steel spindle that will be press fit into the upright. The spindle was made to accept a set of Honda OEM, cast aluminum wheel hubs which proved to be more cost effective to buy than make ourselves. Figure  shows a picture of the upright design along with a one pound load simulation, which allowed the stresses to be easily scaled. Rear Uprights/Hubs- The rear uprights are designed specifically to fit the rear h-arm suspension. They are also designed to mate with custom made hubs for the rear drive train. The uprights are made of 6061 aluminum so that they are strong enough to withstand the forces acting on them from the suspension as well as the hubs but lighter than steel uprights. They were fabricated by machining a plate to the correct specifications before welding in a cylinder to mate with the hub. Since the aluminum was welded, it was then heat treated to return it to its original temper and strength. The hubs are designed to mesh with the uprights and have the proper bolt pattern for the rear wheels. Figure  shows the design of the rear uprights as well as finite element analysis of the loading most likely to cause the member to fail. The analysis was performed using a 1lb load so that the results can be easily scaled to any loading magnitude. Tires- Maxxis RAZR2 22x7-10” tires are used for the front tires. The angled, knobby tread pattern utilized by the RAZR2’s gives them good traction which increases turning, acceleration, and braking performance on the packed dirt terrain usually encountered during Baja SAE competition. They utilize 6-ply construction for better durability and puncture resistance. ITP Mud Lite 22x8-10” tires are used as the rear, driving tires. While mud tires are not necessarily ideal for packed dirt settings, the chevron pattern works well for water propulsion. Since the competition has a significant water portion, these tires will give us an advantage in the water. They are also 6-ply and will not have durability issues. Both the RAZR2 and Mud Lite tires have been used in the past with success. BRAKES Front Brakes-The front brakes each utilize a standard Honda caliper with aftermarket drilled rotors. The rotors have holes drilled in to them so that the hot gases generated when braking can be vented. Rear Brakes- The brakes will be utilizing a sprotor as discussed in the drive train section. We chose to use this design because it will reduce rotating mass by eliminating the need to have both a sprocket and a brake rotor. Master Cylinders- MCP master cylinders were chosen due to their compact size and proven reliability. They allow for the necessary pressure a 95th percentile driver can provide to the pedal. These two master cylinders were mounted in such a way that both are actuated simultaneously, one delivering pressure to the front brakes, and one delivering pressure to the rear brake. An adjustable bias bar is used to ensure that the desired amount of braking force reaches the front and rear wheels. ENGINE The engine used is the required Briggs & Stratton 10 horsepower OHV Intek Model 205432 – Type 0036-el engine. The only modification to the engine was to remove the gas tank and place it above the engine compartment to better protect the engine, allow space for a spill guard, and to improve accessibility to the engine itself. This modification also allows for the engine to be mounted lower on the car and for the gas tank to be more easily refilled.
Gas Tank- A remote mounting system was fabricated such that the gas tank sits slightly above the engine. This placement of the gas tank allows the spill guard and splash guard to be one unit saving weight, material cost, and fabrication time for the unit.
ELECTRICAL Brake Light- A surface mount LED brake light was chosen over an incandescent bulb. Its simplified mounting requires only four clearance holes for the hardware. We are switching from incandescent lights to reduce the draw on the battery. Kill Switches- One driver accessible and one bystander accessible kill switch is wired to the vehicle’s engine for instant shut off in case of emergency. The switches are a push style kill switch grounded to the frame. The 40mm diameter red exterior can be easily seen. They are wired in parallel to the engine so that either will short the sparkplug and stall the engine. The driver accessible switch is mounted on the dash so that the driver can easily push it when fully belted. SAFETY Seat- A custom fiberglass seat has been made to be light weight, durable and comfortable. The seat was modeled after seats purchased in previous years that were deemed comfortable. The old seat was then used as a mold to make a fiberglass shell. A memory foam pad is attached to the seat and the whole seat is wrapped in custom fit nylon cover. This seatback is angled to the same degree as the rear roll hoop so that there is no wasted space. Seatbelts- We chose a Crow 5-pt harness because of its lower cost compared to Simpson harnesses. The Crow harness is constructed out of flexible material that prevents this stiffening. This will save us time during endurance race pit stops when the belts must be loosened for driver exchange. Guarding- In order to ensure the safety of spectators, as well as team members, moving parts of the drive train are guarded. In the past, guarding was difficult to assemble and remove, wasting valuable time during competition. For this car, packaging has been improved so that guarding can be simplified, increasing the serviceability of the car. Fire extinguisher- We have decreased the size and weight of our fire extinguisher by choosing a 5BC extinguisher over the more common 10BC extinguisher. This change saved us 1 lb in fire extinguisher weight. Also, the REC-5 is one of the few rechargeable 5-BC extinguishers on the market, meaning that it does not have to be replaced after use. WATER Flotation- The primary goal for our floatation design, besides being buoyant enough to float the car, was that it be robust enough to repeatedly strike hazards on the track. We began by estimating the weight of the car plus the heaviest driver at 600 lb with an extra allowance of 50 lb for mud accumulation during the event. Next, the amount of water needing to be displaced was calculated by dividing the weight of the car by the net buoyancy of one cubic foot of submerged floatation. An initial design in SolidWorks was then modified in several iterations to achieve a center of buoyancy under the car’s center of mass and a waterline reaching from between one half to two-thirds up the rear wheels. The float was constructed from a combination of liquid expanding polyurethane foam, and laminated sheets of polystyrene insulation foam with a lightweight steel armature. The main body of the float was first made using a plywood form and the pour foam. The upper layers of the float were then created by gluing blue extruded polystyrene foam to the top of the main float. Then, after adding contours and shaping the float to fit the bottom of the car, we encased it in fiberglass. See Figure  for our model of the completed float. We chose to add extra layers of fiberglass to the leading edge of the float to address past failures in that area. To attach the float to the bottom of the frame, ratchet straps were placed between layers of fiberglass and tied in to the frame.
Propulsion- One of the main challenges of the water portion of competition is propelling the car at a reasonable speed. In order to do this, fenders are mounted to the rear tires. The fenders are made out of fiberglass and designed to wrap around the tire, allowing a vane of water to be pushed out the back and propel the car. The fenders have a sliding plate built in to them so that the vane producing portion of the fender can push as much water backwards as possible. This maximizes the propulsion forces, thus maximizing the forward velocity of the car. The fenders are to be mounted to the rear uprights so that the fenders move and change camber with the tires, avoiding any interference issues when the suspension moves. TUNING CVT Tuning- To maximize the efficiency of the CVT, the proper combinations of CVT weights and springs must be used. To determine which primary weights, primary springs and secondary springs must be used, a full factorial array of the combinations of springs and weights will be tested. The data will be analyzed using analysis of means and analysis of variances. The contributions of the interactions of the different weights and springs will be used to determine the best combination to use. The best combination is determined by the weights and springs that cause the CVT to shift with the power band of the engine. If the CVT shifts to follow the power band of the engine, the RPM output of the engine is ideal and the greatest power is being delivered to the tires. This ensures that the car is performing at its peak. In the past, the CVT was not tuned and the car shifted incorrectly, slowing the car during competition. Table  shows the full factorial experiment to be performed to tune the CVT. A full factorial experiment has advantages over traditional “one at a time” tests that will allow us to better perform the testing. The biggest factor affecting our tuning is the fact that full factorial experiments allow the analysis of how different factors affect each other which is important because the different variables work together to achieve optimum performance. Another advantage of the full factorial test is that it accounts for error in results without the need for repetition. FASTENERS Fasteners are an important part of the Baja car that is often overlooked. Using the proper fasteners ensures that failure will not occur at a fastening point on the car. While it is crucial that fasteners do not fail, it is also important that they are not stronger than they have to be. Improper choice of fasteners adds unnecessary weight and cost to the car. Article 14 of the Baja SAE rules dictates the requirement for fasteners, and all of these rules were followed when choosing fasteners for the car . Specific care was taken to choose fasteners based on the load cases that are being applied. For instance when loading a fastener in shear, half height lock nuts were used because axial forces are limited. In addition to this, load cases resulting in shear forces are kept to double shear so that the loaded area is increased and stresses are decreased. While the weight savings per fastener are small, over the entire car, choosing the correct fasteners leads to significant weight savings while retaining the structural integrity of connection points. CONCLUSION This year’s design has made improvements including weight reduction, decreased size from unnecessarily large models of previous years, a more serviceable and more efficient drive train, better suspension dynamics, and a more aesthetically pleasing appearance. Most importantly, members have taken away engineering experience and have been able to apply what they learned in the class room in such a way that cannot be quantified. ACKNOWLEDGMENTS The University of Rochester Baja SAE team would like to thank the Mechanical Engineering Department, the Student Association, and the River Campus Machine Shop at the University of Rochester for their assistance with funding and organization. REFERENCES 1. Online Materials Information Resource. http://www.matweb.com. 2011 2. Varat, Michael S., and Stein E. Husher. Crash Pulse Modeling for Vehicle Safety Research. Tech. KEVA Engineering. Web. 1 Feb. 2011. <http://www.nhtsa.gov/DOT/NHTSA/NRD/Articles/ESV/PDF/18/Files/18ESV-000501.pdf>. 3. Shigley, Joseph, Charles Mischke, and Richard Budynas. Mechanica Engineering Design. 7. New York, NY: McGraw-Hill, 2003. 1056. Print. 4. Marks, L.S., and T. Baumeister. Marks' Standard Handbook for Mechanical Engineers. 11. New York, NY: McGraw-Hill, 2007. 2304. Print. 5. Milliken, William F. Race Car Vehicle Dynamics. SAE International: Warrendale, PA; 1995 6. 2011 Baja SAE Series Competition Rules 7. "Planetary Gear Ratios and Rating." NEUGART. Web. 17 Feb. 2011. <http://www.neugartusa.com/Service/faq/Plratio.pdf>.
Table 1 – Vehicle Characteristics
Approximated Weight 460 lb Wheel Base 57.25” Front Track Width 55.25” Rear Track Width 54” Ground clearance at ride height 10” Top Speed 32 MPH
Table 2 – Pugh Decision Matrix Trailing Arm Semi Trailing Arm H-Arm Adjustability - + + Ease of Design + + + Ease of Construction + + + Performance - + ++ Durability - - + Total - +++ ++++++
Table 3 – Full Factorial CVT Tuning Experiment Design
Control Factor Level
A. Primary Spring Strong Weak B. Secondary Spring Strong Weak C. Masses High Low TC Columns
A B C AxB AxC BxC AxBxC
1 -1 -1 -1 1 1 1 -1 2 -1 -1 1 1 -1 -1 1 3 -1 1 -1 -1 1 -1 1 4 -1 1 1 -1 -1 1 -1 5 1 -1 -1 -1 -1 1 1 6 1 -1 1 -1 1 -1 -1 7 1 1 -1 1 -1 -1 -1 8 1 1 1 1 1 1 1
Figure 1 shows the bare frame design
Figure 2 shows the car in a front impact load scenario
Figure 3 shows the car in a rollover load scenario
Figure 4 shows the FVSA front suspension geometry
Figure 5 shows the front top a-arms in a side loading scenario
Figure 6 shows the front bottom a arms in a shock loading scenario
Figure 7 shows the FVSA view for the rear suspension
Figure 8 shows the bottom H-arm in a side loading scenario
Figure 9 shows a top H-arm in a shock loading scenario
Figure 10 shows a front upright and a worse case load scenario
Figure 11 shows a rear upright and a worse case load scenario
Figure 12 shows a model of the floatation