- 1 E-Board and Design Leads
- 2 Design Report
- 2.1 FRAME AND BODY
- 2.2 DRIVETRAIN
- 2.3 VEHICLE DYNAMICS
- 2.4 BRAKES
- 2.5 THROTTLE CONTROL
- 2.6 ELECTRICAL
- 2.7 SAFETY
E-Board and Design Leads
- President: Phil Katz ('12)
- Chief Engineer: Jakob Maag-Tanchak ('12)
This year the University of Rochester (UR) is designing and building a vehicle to demonstrate ability to perform well in all of the tested areas. After several years of steady improvement in designs and competition results, the UR Baja SAE team has proven its capabilities. It is now looking to iterate on proven concepts to maintain its position among the best Baja teams in the country. Having achieved 19th place overall in the last race of the 2010-2011 season, the University of Rochester is looking to iterate on proven concepts and shore up any past weaknesses. The 2010-2011 roll cage is being completely reused, although it may be disguised by major redesign of the body paneling, rear bracing, drive train, and rear suspension of the vehicle. The basic assembly of the drivetrain will remain, but with major alterations to aid in the alignment of all of the components. Additionally, major electronic projects have been undertaken for the first time in team history, to set the new vehicle apart from the competition field. The University of Rochester has a history of creating effective, high value vehicles, routinely producing one of the fifteen least expensive cars in the field and performing in the top twenty five overall. The current design aims to continue in that tradition by improving performance while keeping costs down. Design
FRAME AND BODY
The frame was designed in accordance with 2011 Baja SAE Rules, and is also in accordance with article B8.2 of the 2012 Baja SAE Rules . Structural stability and driver safety were the main parameters governing the design. Clearance measurements were taken from all potential drivers while seated in the 2010 vehicle and wearing all required safety gear. The distances between the driver helmet and surrounding members, and between driver knees and SIM’s were measured in order to see how much length and height could be reduced compared to the previous year’s car.
According to Article B8.3.12 primary roll cage members must be “a steel shape with bending stiffness and bending strength exceeding that of circular steel tubing with an outside diameter of 1’’, a wall thickness of 0.120’ and a carbon content of .18%” Bending stiffness is given as EI, where E is the elastic modulus. This value is the same for all steels. I is the second moment of the area for the structural cross section, which is a function of diameter and wall thickness. I is given as
I=π/4 [(OD/2)^4-(OD/2-thickness)^4 ]. (1)
Bending strength is given as SyI/C, where Sy is the yield strength of the material, and C is the major radius of the tubing. Calculations show that 1.25’’ OD, .065’’ thick, 4130 tubing satisfies requirements. The larger diameter increases I, and 4130 has a higher yield strength than 1018 steel. The tube size reduction combined with overall height and length reductions significantly decreased the weight of the frame. To ensure the driver’s safety and structural integrity, the frame was analyzed under two different scenarios, front impact and rollover impact, using PATRAN/NASTRAN finite element analysis software. 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, and these results follow. Front Impact- The first analysis modeled a front collision of the car with a stationary object. A Technical paper written by KEVA Engineering, an established collision consulting organization, on Crash Pulse Modeling analyzed a mid-size four door sedan going at 35 mph and crashing into a wall. The result of the analysis wasa stopping time of 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 14.5 G’s was placed on all members of the car. Also, a force equivalent to the weight of the car, and the weight of the driver (assumed to weigh 175 lb), 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 the roll cage performs to a safety factor of 3.86 for the given collision.
The roll cage was also analyzed by utilizing a load, equivalent to the weight of the car at 2 G’s. This was deemed suitable because in observed impacts, vehicles almost always sustain a front impact that reduces speed before top impact. It would be impractical to design for a straight top impact since the front of the car always touches first. For analysis the 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 1.51. Figure  shows a high stress concentration around the point of impact, but the safety factor 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. For this scenario the 1.51 safety factor will suffice due to the low probability of the impact.
The car is built with rear bracing instead of front bracing because it allows the car to remain structurally sound while increasing aesthetic appeal while satisfying Article B18.104.22.168 of the rules. Using rear bracing allows for more freedom at the front end of the car by allowing us to run a “foot box” style that is unobstructed by front bracing members. Removing the front bracing members also enhances the driver’s vision by removing tubing from his or her line of sight. The rear bracing used has significant improvements over the previous design, mainly by allowing easy installation of the engine and CVT. Past cars required the engine to be partially disassembled in order to be installed or removed from the car. Bracing members have also gotten in the way during the removal of the CVT primary. Space for the engine and CVT has been increased to make maintenance easier. The improved serviceability 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.
In order to build a vehicle that would catch the eye of consumers, a unique “skeleton” body panel system was developed that would put the body panels in the plane defined by the SIM and LFS, while leaving the tubes exposed as shown in Figure [4 . ABS was chosen over fiberglass, aluminum, HDPE, and carbon fiber because its low melting temperature allows it to be easily shaped using heat. This removes the need for expensive ovens in a manufacturing environment because simple heat guns are enough to deform the plastic. Additionally, the use of ABS allows for the mass production of future vehicles using permanent molds and thermoforming. Once a wood mold is made, panels can be heated on to the mold, and correct sizing is guaranteed quickly and easily. In the past, we found aluminum difficult to shape flush with the corners of the frame, making it hard to close gaps as described in Article 9.7 of the rules. Fiberglass was not chosen because of the necessity for expensive epoxy and temperature controlled manufacturing environments. It also requires significant sanding in order to be painted well. Carbon fiber is impractical because of its high cost, and because it requires more stringent temperature control than fiberglass during manufacturing. The main benefit of carbon fiber is its strength to weight ratio, but strength is not a major factor in body panel design. Panels simply have to keep out debris, and even the extremely thin fiber glass panels used last year made it through the racing season without a single rupture. The panels that fit around the front suspension were made of two pieces instead of one. This allows the panels to come off for cleaning without removing the front suspension. In the past, the A-arms passed through these panels making this easy removal impossible. An added benefit of this feature is that it allows a horizontal tube to show throw, continuing the skeleton theme all the way to the front of the vehicle.
HDPE was used for the belly pan because it has been proven to sustain impacts without significant damage. Past HDPE belly pan plates are still in use on our older vehicles after many years and show little signs of wear, even after repeated impacts with boulders, logs, and other obstacles. Additionally, its smooth surface allows the vehicle to slide smoothly over rough obstacles. The front section of the belly plan is curved around the foot box to allow the vehicle to slide over obstacles it encounters head on. This feature also prevents the belly pan from getting caught and torn off, which has happened with past belly pans that ended at the front of the vehicle. A rear panel was added beneath the drive train to protect it from mud and intruding debris, even though it was not required by the rules. This panel was made of HDPE left over from the belly pan, maximizing material usage. Hitch- Last year, the front hitch acted as a front bumper that was intended to protect the front suspension and tie rods. It also served as a convenient spot from which to lift the front end of the vehicle. However, we decided that it would not have done much to protect the front suspension in the case of a significant front impact; it hampered the removal of the belly pan which it passed through, it was deemed visibly unappealing, introduced complexity, and created a requirement for extra fasteners. This year’s design incorporates a simpler hitch of half inch tubing. The new design is located above the belly pan so it does not get in the way, it does not require any hardware, and it still serves as a convenient lifting point for the front end.
This year, extra attention was placed on the efficiency of tab production. While simple and inexpensive, the sheer number of tabs on a Baja vehicle makes them worthy of such attention. In the past, each tab was cut individually, and a hole was drilled in it. For tabs requiring uniformity, they were milled at the same time ensure consistent width. This year, long strips of steel were cut from large pieces of thin plate using hydraulic shears. Holes were created using a turret punch instead of a drill, saving many drill bits and saving cost. Each tab was then cut off from the longer strip using a step shear. This type of methodology is necessary in production manufacturing.
The drivetrain was engineered to optimize efficiency, serviceability, weight, and size. The drivetrain consists of a standard Briggs and Stratton 10 hp OHV Intek engine with CVTech-IBC continuous variable transmission (CVT) and a series of speed reduction components. A Matex Products Inc. planetary gearbox coupled with a chain reduction compose the rest of the reduction. The final sprocket is fixed to a final drive shaft coupled to a pair of constant velocity (CV) axles compose the reduction. The CV axles are used to deliver power to the rear wheels. The reduction after the CVT allows for a total ratio of 10:1. At the maximum engine output of 3800 RPM, we will achieve a top speed of about 31 mph. The maximum acceleration for the car is 22.5 ft/s2 ¬at an engine RPM of 2600 RPM, which is the estimated engagement speed. Based on the analysis, it was found that rolling friction and air drag limit the vehicle from achieving speeds greater than 31 mph on a flat road. The drivetrain gear reduction was designed to utilize the full range of the CVT. The CVTech CVT has a low ratio of 3:1 and a high ratio of 0.43:1. With a total 10:1 reduction in the reduction train, the vehicle reaches its maximum speed when the CVT reaches a ratio of about 0.7:1. A margin was left so the car can reach higher speeds on downhill slopes. Last year’s design consisted of the same basic components but suffered due to misalignments. The major goal of this year’s iteration was to better align the components which was achieved using a custom aluminum case. The case consists of five parts, which also allows for good accessibility and ease of assembly. The full assembly of the drivetrain can be seen in Figure  Continuously Variable Transmission: A CVT allows shifting continuously through an infinite number of gear ratios in a fixed range. This enables the engine to run at its most efficient RPM for a range of vehicle speeds. The continuous shifting provides smooth acceleration without 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 clutch system. The design of CVTech’s CVT is very simple compared to CVT’s made by companies such as Polaris and Gaged. Fewer parts mean there are fewer points of failure during a race. Another advantage of the CVTech is its large range of ratios, which allow the vehicle to reach greater torque and acceleration.
Gear Ratio and Torque
There is an overall 10:1 RPM reduction after the CVT. With the CVT, the drivetrain gear ratio ranges from a total of 30:1 to 4.3:1. The CVT is followed by a 5:1 reduction through a Matex Products Inc. planetary gear set and a final 2:1 reduction through a sprockets-and-chain system. At the engine’s maximum torque of 13.8 ft-lbs at about 2600 RPM, the reduction outputs 357 ft-lbs of torque after rolling friction and drag are considered. The maximum torque was calculated for each component of the drivetrain to analyze the applied stresses. A simple equation characterizes the torque transmission after each gear ratio:
Where T = torque, P = power, n = reduction ratio, and ω = rotational velocity. Fatigue was analyzed using ASME standards. Planetary Gear Set: The planetary reduction is a feature that we employed for the first time last year and are reusing this year. 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 drivetrain, the drive shaft from the CVT secondary 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. Equation (3) is the formulation for the gear ratio: Ratio=(Ring gear diameter)/(Sun gear diameter)+1 (3) The planetary gear acts as an intermediate reduction and provides the largest reduction of 5:1 in our drivetrain. The advantage to the planetary gear is its ability to distribute the load among four planet gears, thus enabling it to achieve a large reduction in a very compact system. It is preferable to a regular gear train or a chain reduction, which would require significantly larger individual components to handle the stresses.
The chain drive provides the final reduction into the CV axle. As mentioned before, a 2:1 ratio is achieved using a 15-tooth sprocket and a 30-tooth sprocket. Chain drives provide great benefits over gear trains in spanning large distances, such as those between the CVT secondary shaft and final drive shaft. To span such a distance with a chain, extra links can be added until the distance is spanned. To make up this distance with gears, either very large gears or extra, smaller gears are needed. Neither option is appealing since large gears are expensive to manufacture and time consuming to lighten with speed holes. Extra gears add complexity to the system as well as machining time. Additionally, the 8620 steel used for gears can be prohibitively expensive when trying to keep vehicle costs down. In order to provide proper chain tension, a chain tensioner was placed inside of the gear case. A Delrin roller was used to apply tension to the bottom side of the chain which is naturally slack. The tension can be maintained by adding shims to the tensioner as the chain stretches with wear. Delrin was chosen because of its resistance to wear. The material was tested on an older vehicle and held up well after many weeks of testing. The sprockets are mated to the shafts via custom splines. Splines were used rather than/instead of keys because they are much stronger. The spline increases the area the force is applied to, effectively reducing stress as compared to a single toothed keyway. Splines also eliminate backlash in the shaft when braking and accelerating so they have a much higher fatigue life. The splined shafts were case-hardened in order to maximize the spline strengths. The chain being used is ANSI 520 O-ring motorcycle chain with a 5/8 pitch.
The drive shaft joining the CVT secondary and the planetary has a keyway to secure the CVT and a custom spline to fit he female input of the planetary. This input shaft is threaded on one end for a castle nut that retains the CVT secondary. This castle nut is spaced far out enough that it allows the secondary to “float” back and forth over an inch. This prevents misalignment between the primary and secondary since the secondary will slide to the position required instead of being constrained to a possibly incorrect position. The thread on the shaft is created using a die to aid in manufacturing. Threading on a lathe is time consuming and unnecessary for standard thread sizes. The output shaft from the planetary gearbox is designed to have a spline mating to the planetary gear carrier as well as a spline mating to the 15-tooth drive sprocket. The shafts are held fixed with a combination of x and y bearings. The final drive shaft is spline-mated with the 30-tooth sprocket and internally splined to conjoin with the CV axles. A mount is built into the shaft for the brake rotor to attach to; thus eliminating the use of a hub. All of the axles are retained using a combination of snap rings and shoulders to ensure they do not shift during operation. The CV axles utilize constant velocity spherical joints that allow for articulation in the shaft during rotation and suspension travel. The joint utilizes a ball and cage system. The cage system allows for the power to be transferred to the wheels while the CV joint articulates. The CV joints are used rather than universal joints because they can handle a greater misalignment in the shaft while maintaining a constant velocity from one end of the shaft to the other. Mounting and Guarding: The drivetrain is designed for optimal accessibility and serviceability. First the rear bracing members were placed above the CVT leaving space to move the engine in and out of the car with relative ease. Second, the engine mount configuration was changed to allow easier removal. The bottom mounting holes on an Intek engine are not easy to access with a wrench. To address this, the engine was mounted to a length of steel L bracket. The bottom bolts holding the engine to the brackets are not intended to be removed, although they can be. Instead, the entire engine/L bracket assembly is intended to go on and come off together. The L bracket is fixed to the frame of the vehicle by horizontal bolts which are much more accessible, greatly decreasing the time needed to remove the engine. The engine plate and the gearbox are both mounted on a frame, which keep the proper alignment of the drivetrain. The engine is separated from the L brackets using spacers machined so that the shaft holding the primary is exactly 242 mm from the shaft holding the secondary. Improper center to center distance is one of the greatest causes of inefficiency in a transmission, but the spacers solve this problem. The spacers are made from two blocks of aluminum instead of 4 sets of washers as was done in the past. This makes it easier to align the spacer holes with the holes in the L bracket and the holes in the engine. The height of the engine was moved down 7 inches from the previous year, considerably lowering the center of mass of the car and decreasing the likelihood of rollover. Last year the engine had been placed higher on the car to prevent flooding during water events. This year, due to the elimination of the water event, the drivetrain was able to be lowered to further lower the center of gravity.
Last year, the vehicle exhibited severe understeer which hampered the car’s ability to turn at speed. The main goal for this year concerning vehicle dynamics was to fix this problem. It was hypothesized that the lack of steering was largely to due to a center of gravity (CG) that was too far towards the rear of the car. To test the theory, we strapped a fifty pound weight to the foot box, and tested the car on a simulation track. With the additional weight in front, the turning radius was significantly improved, and the driver was able to induce oversteer in corners, which is a desired handling characteristic in a Baja vehicle. Oversteer allows the car to turn sharply around corners and is very helpful in the tightest turns. From testing, we determined that our first priority would be to shift the CG forward. In order to move the CG, one of this year’s drivetrain criteria was to iterate on the drivetrain to lighten it, reducing the rearward weight concentration. This was done by using smaller CV axles and smaller rear hubs. Last year’s car also had rear wheels very close to the firewall and a compact drivetrain, focusing most of the load on the rear tires. This year’s suspension was designed so that the rear wheels were 8 inches further back than last years, and the front wheels 2 inches further back, while leaving the engine (1/8th the car’s total mass) where it was in relation to the firewall. Moving the wheels, from which the CG is calculated, while maintaining last year’s basic weight distribution created an instant CG movement of 5in away from the rear axle. The movement of the CG 5 inches forward was calculated to change the weight percent distribution of the car from last year’s 37% front. With this CG movement, this year’s car has a distribution of 45wt% front. Even though the wheelbase increased by 6 in in the process, last year’s weight distribution caused the front tires to have such a low slip angle that the smallest possible turning radius at low speeds was still too high. Two years ago the static ride height of the car was too high (15”). This was caused by a high placement of the engine intended to keep the CVT out of high water. The car had a high tendency to roll over due to its high roll axis and high center of gravity (CG) when the flotation was not used. To increase stability, this year’s goal was a static ride height of 8” for increased stability in turns. This was mainly accomplished by dropping the height of the engine.
The team has had a tradition of simple double wishbone front suspension with air shocks. Simple A-arms incorporating a single bend were used to maintain ease of manufacture. The arms are constructed from 1” OD, .049 wall thickness 4130 steel tubing. Finite element analysis with NASTRAN/PATRAN was used to verify the structural integrity of the design. To build the A-arms A jig made of welded 1” square tubing was used. Jigs are used to constrain parts to the proper dimensions while they are welded and cool. The use of a jig allowed A-arms to be made consistently to the same dimensions without the warping. Jigs will provide quality control during mass production, because only properly fabricated tubes will fit into them.
The upper and lower inboard mount points were offset by 0.5” to induce a kingpin inclination of 7.5° from vertical. The mechanical trail, which is measured from the side view swing arm geometry (svsa), is approximately 1.35”. The kingpin inclination combined with the caster creates a camber change with turning for better handling. It was determined that the front suspension should have no camber change with vertical wheel travel to reduce scrub effects and to reduce steering instabilities and the force response the driver experiences. Since a positive camber produces camber thrust and is useful for stability in turns,  the camber change was put in place only with steering motion and not with shock compression and extension.
The A-arms are connected to the frame via Delrin bushing assemblies that limit friction as the suspension rotates. Stainless steel crush sleeves are used to maintain the integrity of the mount tabs and to stop the bushing from being crushed as the mounting bolt is tightened. The switch from mild steel to stainless steel crush sleeves is intended to limit corrosion that shortens the lifetime of the bushing suspension mount assembly.
The trailing link style suspension allowed for greater control of each degree of freedom than last year’s H-Arm design, although it requires more elaborate design solutions. It was decided that the links would allow for the appropriate amount of design control to induce a camber change of +7° with bounce, which would allow for better handling. This value comes from the approximated body roll, and is desired to be equal to the body roll angle for stability in turning at speed .
The suspension links include one toe link (also called a steering linkage or bump steer link) which can be modified to create bump steer. Bump steer is a toe change with vertical wheel travel. Default position is set to 0 degrees per inch of toe change, but can be modified for different handling characteristics, i.e. tighter turns in the land maneuverability event. The rear suspension steering characteristics are purely passive because body roll induces vertical travel of the rear tires. The front tie rods are exactly the same lengths as the rear suspension links, but the difference in lengths required for the rear suspension can be edited with the attached rod ends. 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 lbf 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. Additionally, in testing and actual racing, the performance of the Float shocks did not deteriorate noticeably due the heating of the air inside. It was decided that the Float shocks were optimal for a Baja SAE car, and so Fox EVOL shocks would not enhance the performance of the vehicle enough compared to the extra expense. The FLOAT shocks cost $580 per pair, while the EVOL shocks roughly $1500 per pair. That is a significant difference. In fact the choice of shocks is one of the largest factors that keeps the prototype cost low and makes the vehicle so appealing to consumers. The shocks were placed so that they did not interfere with removal of the CVT or CVT cover, which was a problem in past designs. Steering- Steering is achieved using an 11’’ rack and pinion connected to rod ends threaded into tie rods. The rod ends then turn the wheels by pushing on mount points attached to the front uprights. The rack and pinion was chosen for its low cost ($90) and weight (2lb).The tie rods have both left and right hand threads to allow for quick adjustments to toe as well as jam nuts to keep them from moving during operation. The design of the steering tie rods is the exact same as the rear suspension links. This is very useful because limiting variability makes mass production easier. It also helps in the area of spare parts, since a spare tie rod can also work as a spare suspension link. The steering column was placed so that the wheel would be within reach of all drivers. This year, the steering wheel does not incorporate a quick release in order to limit complexity and weight. During last years’ competition, the quick release was never used, proving that it is superfluous. Instead, the steering wheel is bolted to a flange which is welded directly to the steering shaft. Last year’s steering column suffered from lack of support close to the end where the wheel attaches. This allowed slight bending in the shaft. To fix this tubing was added to the frame so that the steering shaft could be supported closer to where force is applied by the driver. This support was provided by a light, inexpensive ($5) self-aligning nylon flanged bearing. To accommodate drivers with longer legs the angle of the steering wheel is adjustable, allowing the wheel to be moved above their knees. This is accomplished by bolting the flanged bearing into different locations along a vertical square tube, drilled with holes at different heights. The bearing has enough range of motion to accommodate the change in angle that accompanies the shift in height. A universal joint is necessary at the base of the rack and pinion to accommodate the changing angle of the steering shaft. The caster angle and king pin inclination were chosen to give some camber change while steering. The caster angle was chosen to be 7.5° and the king pin inclination was chosen to be 7.5° 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.
Custom uprights had to be machined in order to fit the new suspension geometry. Aluminum billet was machined to the proper dimensions and a custom machined, tapered spindle was press fit into the center. For purposes of mass production, the uprights would be cast and then finished using light machining. The hubs used were from a Honda TRX 250 because that model is a similar size and power to our vehicle. Using hubs from a more powerful ATV would add unnecessary weight since hubs generally increase in size with vehicle power. Delrin bump stops are bolted to the steering links of front uprights to prevent the wheels from turning too much. They are simple rectangles that interfere with the wheels at large angles. Understeer occurs at extreme wheel angles, and the bump stops prevent the wheels from reaching these angles. They also prevent the destruction of the front steering assembly when the wheels encounter hard obstacles like trees and boulders. A hard impact can snap tie rods and ball joints by forcing them too far back, but the stops prevent this.
CV shafts from a Honda TRX 300 were used because of their lighter weight compared to last year’s CV shafts, saving 6 lb per shaft. The hubs then had to be used from a TRX 300. These weighted .5 lb less per hub.. The rear uprights were machined from billet aluminum, which due to their simple design, does not need to be cast. The stress analysis for the front and rear uprights can be seen in Figure.
Maxxis RAZR2 22x7-10” tires will be used for the front and ITP Mud lite 22x7-10” tires will be used for the rear of the car. The angled, knobby tread pattern utilized by the RAZR2’s will improve turning and braking performance on the dirt terrain usually encountered during Baja SAE competitions. The tires are manufactured with 6-ply construction for higher durability and puncture resistance. The rear tires are also all terrain vehicle (ATV) “front” tires. We chose front tires for the rear axle, because rear ATV rear tires are too wide and would negatively affect the car’s handling. Although front tires are primarily responsible for handling and traction, they can be effectively used as drive wheels. Front wheel tread patterns are designed with most ATVs’ four wheel drive capabilities in mind. In the past, the strategy of using front tires for all four wheels has provided adequate driving traction and handling. The ITP Mud lite tires have a greater tread depth which will provide more traction in slippery portions of the course and during acceleration. They are also one of the lightest and most cost effective 6- ply mud tires on the market, and they perform very well in off-road environments. Another important factor in the decision to use the ITP Mud lite tires was that these tire treads are separated by larger gaps which allow the wheels to shed mud and clean themselves. This prevents mud from becoming clogged in the tire tread, thus maintaining traction and acceleration capabilities.
We chose to use ITP SS112 rims which are some of the strongest and lightest ATV style wheels on the market today. Based on the obstacles that our car may encounter, it is important that the wheels we choose are strong so that they will not dent. Damage to the wheels may cause the tires to leak air and go flat. Additionally these wheels have a good combination of low weight and high strength. Having used other wheels in the past, most notably Douglas rims, and having observed many other brands used at competition, the ITP’s have been proven to be the strongest and least likely to break on hard obstacles. Another aspect considered when ordering the wheels was the type of offset that would maximize handling and performance Negative offset is recommended for solid rear axle vehicles, but positive offset is recommended for independent rear suspension. Since our design has independent rear suspension we chose positive offset.
The main goals of this year for the brakes subsystem was to improve the bias system and choose braking components based on engineering analysis instead of copying them off from previous designs All equations used were taken from Stop Tech’s Technical White Pages. As described by Article 11.2 the braking system must be split up into two independent hydraulic circuits. The only logical way to do this is to have a separate circuit for the rear and front wheel sets. Because the drivetrain utilizes a spooled rear axle, it makes sense to use one caliper to stop both rear wheels at the same time. This cuts down the cost/weight of an extra caliper, brake pads, and lines/fittings. Braking onto the final drive shaft also decreases the unsprung mass by removing the calipers and rotors from inside the wheel hubs. Dynamic Weight Shift- The braking forces from the calipers must be able to generate a torque on the brake rotors that can overcome the torque imparted on the tires by the ground friction. The friction between the tire and the ground is dependent on the weight placed on each tire. However, it is not enough to simply measure the weight under each tire as it is standing still because during deceleration, weight is transferred from the rear axle to the front. The equation for weight transfer is " WT=" ("a" /"g" )"x" ("h" _"cg" /"WB" )"x " "V" _"t " (4) where a is the maximum deceleration of the vehicle, g is the acceleration of gravity, is the height of the cg, WB, is the wheel base, and is the weight of the vehicle. The maximum acceleration was assumed to be 1 g and the height of the cg was determined by the methods outlined in Race Car Vehicle Dynamics . Once the normal forces at maximum acceleration were known, the system used to generate the necessary stopping force could be developed. To make the decision process simpler, the diameter of the rotors was fixed from the beginning. A 7’’ rotor was chosen because larger sizes would not fit with their affixed calipers inside the wheel rim. A larger 8’’ rotor was chosen for the rear because it is responsible for stopping both rear wheels. Larger rotors are harder to find at reasonable prices so we chose the largest size caliper before the prices jumped. A 0.75’’ bore master cylinder was chosen for the front and rear because they are standard sizes for 400-500lb vehicles according to the Wildwood representative we spoke to. Once the master cylinders and rotors sizes were chosen, a compromise had to be found between pedal size and caliper piston area. A spread sheet was made that listed variations of pedal length, the force input needed from the driver, and available piston sizes. A pedal that creates a mechanical advantage of 4:1 was found to be satisfactory. Front Brakes- Out board braking is used in the front because the left and right A-Arms are independent. Each front wheel has 1.3’’ bore caliper. To simplify with the manufacturing of the vehicle, the rotors that come with the front hubs were used. The front calipers are floating style because the two pistons needed for fixed calipers would take up too much room inside the limited space of the rim. Also, floating calipers will self-align, which is helpful in maintaining good contact between the rotor and brake pads.
A Wilwood caliper with piston area of 2.20 in2 was used because of its reasonable price, bleed nipple location, and mounting hole locations. In the past, rear calipers were difficult to access, but the nipple location on the Wilwood Billet Caliper makes the brakes easier to bleed. Although it does not self-align like floating models, if mounted well, it is less likely to lock up because it does not rely on the same sliding mechanism.
The design for the bias bar allows for an easily adjustable braking bias. By turning a knob at an end of the bias bar a mechanism utilizing a spherical bearing alters the ratio of force being applied to each master cylinder. Ideally both sets of wheels should lock up at the same time and bias adjustments allow this to be controlled. Selection Process- The selection of the brake lines was based on price and reliability. Wherever the brake line is not required to move, hard lines were used because of their much lower cost. Flexible lines were used in the front A-arms because they must move up and down with the suspension. However, stainless steel braided line was used instead of cheaper rubber protected lines. This was done to avoid leakage and resist wear from the outside.
While generally satisfactory, several changes this year were made to improve the serviceability of the throttle controls. Accelerator Cable – For the accelerator cable, we used a bicycle shifter cable. The cable housing was constrained on either end by a small, machined aluminum block. The cable housing block allow for the actual cable to move freely no matter how the housing is routed from the pedal to the engine. Accelerator Pedal – The accelerator pedal is a purchased component featuring a pivot at the bottom of the pedal and a rough surface for better traction. There is also an adjustable stop so the pedal throw cannot exceed cable travel.
A major goal this year was to incorporate more advanced electronics into the vehicle to aid during racing and tuning. With relatively inexpensive equipment, this year’s design will have several technological advantages separating it from the competition.
This new equipment is comprised of a distress signal, tachometer, speedometer, and fuel level gauge. An important feature of this equipment is that, in addition to driver displays, all the data collected will be transferred wirelessly using an Arduino platform and XBee wireless module to a remote computer. This gives us the ability to log and analyze crucial performance data in real time, allowing advanced tuning ability and detailed monitoring during competition. The Arduino platform was chosen due to its easily customizable interfacing and its extensive online documentation. Also, many different peripherals for the Arduino are available and make future modifications and additions easily possible. In order to provide wireless data transfer, the XBee wireless module was chosen because it is one of the simplest and most reliable options for use with the Arduino platform. While other commercial products are able to transmit at faster speeds, they are much more expensive and fragile. Our goal was to develop equipment that could stand the test of time and be continually updated at little cost. The base platform developed is able to expand by adding modules, using a single module for each type of data being obtained. This will aid in mass production because the assembly can easily be split into multiple simple pieces. This will also pay off during the race season, as it will allow the team to make quick repairs and have inexpensive replacements available.
While a support crew is collecting data away from the vehicle, the driver will be able to see information displayed in a cockpit dash board. This will be the first time that the University of Rochester employs a full dashboard. Care was taken during design so that the driver’s view of the displays is not impeded by the steering wheel. Simple LED arrays will tell the driver which speed range, rpm range, and fuel range that they are operating in. Additionally, a multi-color LED on the dash will allow driver to receive messages from his crew. Making the LED turn red for example, could be an indication to pit, and turning the LED green could inform the driver to keep going. In the case of a consumer vehicle, the LED could be used to communicate messages such as “come back home.”
The usefulness of a distress signal has been demonstrated in countless previous races. Often during an endurance race, the pit crew is spread out over a large area in order to monitor different sections of the track. If the car must come out the race to be repaired, unless team member spots it as it comes out of the race, the team will not be notified until the car is towed to the pits. Precious minutes are then wasted as the team runs to regroup. The distress signal, which will be activated by a simple button placed in the cockpit, will give the team notice to make it to the pits in the time it takes the car to be towed. Because races often take place in areas with poor wi-fi and cell phone reception, the transmitters previously mentioned do not require either. The button for the signal can also be used to transmit extra data by using simple Morse-code-like patterns to describe the severity of the problem. In a production vehicle, the distress signal can be used to contact help in case of emergency. For example, if a customer went driving in the back woods around their homes, they could leave their computer open and anyone at home would know if they needed help.
The benefits of knowing how much fuel is left the gas tanks are obvious. One less obvious benefit is that a fuel gauge could be used to alert the team to possible fuel supply problems or leaking. Because we are not allowed to alter gas tank by Article 12.3.1, the fuel uses a method of detecting gas levels from outside the tank. If two strips of conducting material are placed on opposite sides of the tank, the fuel remaining acts as a dielectric and the capacitance across the tank can be measured. Simple calibration allows the capacitance readings to be converted into fuel level reading.
For the tachometer reading, sensors detect a signal from the ignition wire governed by the firing of the spark plug. The circuit generates different voltage levels in response to changes in the frequency of the signal. This voltage is then fed to microchips which illuminate a certain number of LED’s in the dash based on voltage level, and the exact value is sent to a computer via the on board transmitter. The speedometer acts on similar principles, but uses the magnetic disturbance generated by the brake rotor on the final drive shaft to establish a signal. Our first test circuit is shown in Fig  Testing was done by using a signal generator to simulate different engine speeds. Being able to transmit and record the speed-rpm curve for the vehicle is crucial for tuning the CVT.
Kill switches are needed to shut off the engine. A rear switch is required in case the driver is unable to actuate the cockpit kill switch. Only three styles of kill switches are allowed, as described in Article B3.2 of the rules. The WPS made model was used because it cost one dollar less than the next cheapest option. This might not seem like much but, each car must have two kill switches, and our prototype is intended to be mass produced. Two dollars multiplied by several thousand vehicles is a significant saving.
The brake light satisfies Article B3.4 of the rules. An LED model was chosen over an incandescent model because LED’s draw less current than incandescent bulbs. That gives the team and future customer the options of using a smaller battery and replacing the battery less often. The AutoSmart KL-25108RK , was chosen for its low cost. At $10, it is half as expensive as most other lights. For a run of several thousand vehicles, it is a considerable saving.
The major safety components in last year’s design were very effective and many elements were maintained in the current design. Seat- The seat concept, a unique but very practical part of this year’s design, was first used last year to great success. It was made to be light weight, durable and comfortable. The seat bottom and back are two separate pieces. The bottom was molded in foam to be a comfortable shape through trial and error, and the final shape was covered in fiberglass over to maintain form and add rigidity. A custom cover with built-in padding was then made to cover the seat bottom. The bottom of the seat has a radius profile cut into it that matches the USM. This helps lock the seat into place and also lowers the CG by lowering the driver height by half of an inch. The seat back is composed of a carved piece of insulation foam. The shape was slightly altered from last year’s design at the behest of the drivers to add lumbar support. The most unique feature of the seat is that it requires no fasteners. The driver’s weight and belt force keep the seat bottom from moving in addition to the profiled bottom which locks into the USM. The seat back is kept in place by the force of the shoulder harness pulling the driver against it. The lack of fasteners allows the seat to be easily removed for cleaning, which is extremely useful after muddy races. Easy removal is also useful because it allows easy access to the brake and electrical lines that run by the seat bottom. The design is several pounds lighter than our old JEG racing seat, but roughly the same price. Seatbelts- A Crow 5-pt harness, which was used for the first time last year, is being used again because of its lower cost compared to Simpson harnesses and greater reliability compared to G-Force harnesses. G-Force belts, while cheaper, tend to lock up when cached in mud. The Crow harness was exposed to extremely muddy conditions in SAE Kansas last year, but never locked up. It is constructed out of flexible material that prevents stiffening. This saves time during endurance race pit stops when the belts must be loosened for driver exchange. Fire Extinguisher- The design uses the minimum sized fire extinguisher as described by Rule B9.10 in order to save weight. The current extinguisher is 1lb less than the 10-BC extinguishers that we have used in the past. The First Alert FEGO wasl chosen because it is rechargeable which most other 5-BC models are not. Guarding-Although the addition of a thick aluminum gear case does add weight to the design, it does remove several steel plates that were needed last year to satisfy the guarding rule because it encapsulates the sprockets. In the past, guarding was difficult to assemble and remove, wasting valuable time during competition. For this car, packaging has been improved, increasing the serviceability of the car. The CVT guard is made of a combination of fiberglass and aluminum sheet. It is reinforced with a steel band about the axis of the belt to provide protection in the case of the belt flying apart. The CVT guard is made of two halves, and the outer half can be easily removed using steel latches so that the CVT can be serviced. The outer half can be removed in under 10 seconds which is significantly better than competitor designs using bolted fasteners around the perimeter of the guard. The guard is first made entirely of fiberglass to provide the necessary contours. After it is formed, the flat sections are cut out and replaced with aluminum sheet. This is done because aluminum has much better conduction and convection properties than fiberglass. Heat is one of the greatest causes of CVT inefficiency and wear. The aluminum allows the heat to leave the CVT, and extra cooling is provided by the forced convection created by the vehicle’s movement. Extra holes are drilled in the CVT guard to allow more heat to escape. These holes are drilled facing the rear of the vehicle to keep water, mud, and dirt from entering the CVT. The spill guard is made of fiberglass and formed so that all spilled fuel drains properly through a single outlet. It is mechanically fastened to the gas tank using aluminum strips that rivet to the guard and bolt to the gas tank mount holes. Past spill guards suffered from poor caulk-only connection, which is why the aluminum strip were employed this year. The guard is covered with a vinyl flap that prevents the guard from collecting mud and clogging up the gas cap. The splash guard consists of an aluminum sheet that entirely covers the engine and prevents splashed fuel from reaching sources of heat. The splash guard this year was made as small as possible to increase its stiffness and make mounting easier. SERVICEABLILTY- This year an aluminum quick disconnect, utilizing crimped connections, is being used instead of an e clip. This makes it easier to remove the engine for cleaning and oil changes. All electronic circuits will also use mechanical connection that can be easily removed so that they do not have to be cut and soldered every time the car is disassembled. In the way of hardware, all fasteners that are not predetermined (certain engine and brakes hardware) will use Standard English sizes to make repairs simple. Metric bolts will be avoided at all possible. In addition, the use of socket head cap screws will be limited due to their tendency to strip when covered in mud and Loctite. CONCLUSION The improvements of this year’s design include weight reduction, a more serviceable and more efficient drivetrain, the elimination of understeer, better suspension dynamics, and a more aesthetically pleasing appearance. The most unique features of the current design include a gear planetary-sprocket gear case, data collection and transmission systems, and a lightweight seat design that does not use any bolts. 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 for their funding and assistance. We would also like to thank all of the sponsors that make our Baja SAE build possible. References 2012 Baja SAE Series Competition Rules.. http://www.millerwelds.com/resources/articles/Best-Practices-for-GTA-Welding-of-4130-Chrome-Moly-Tubing.Miller. April 20,2012. Varat, Michael S., and Stein E. Husher. Crash Pulse Modeling for Vehicle Safety Research Tech. KEVA Engineering. Feb 1 2011. <http://www.nht sa.gov/DOT/NHTSA/NRD/Articles/ESV/PDF/18/Files/18ESV-000501.pdf>. Milliken, William F. Race Car Vehicle Dynamics. SAE International: Warrendale, PA; 1995. Pure Offroad. Feb 29,2012. http://www.pureoffroad. com/atv_wheel_offset_guide.htm. Stop Tech. Jan 28, 2012. http://www.stoptech.com /technical-support/technical-white-papers.