This year, the University of Rochester Yellowjackets focused on making design iterations and goal-based changes rather than fully redesigning most of the systems on the car. These changes include more compact mounting of the master cylinders, a significantly lighter gearbox, and a front suspension design with zero mechanical trail. The Baja SAE competition will evaluate the car’s ability to meet the demands of off-road driving while still being a marketable product. To achieve this goal, designs are focused on optimizing fabrication, cost, and overall performance. In addition, certain components of the vehicle are modified to cater specifically to the comfort of the driver. Each major section of the vehicle looks to embody one or more of these design elements, resulting in a marketable off-road vehicle. This year, we focused on design iterations and goal-based changes. Based on failures that occurred on our previous car, we also placed a greater emphasis on running finite element analysis. Other analysis software allowed us to optimize the steering geometry and front suspension, and for the first time ever we created a complete CAD model of the car. This meant we were able to catch errors during design rather than manufacture, and analysis led us more optimized designs.
As one of the most complicated systems on the car, even a single goal can call for an almost entirely new design. Last year’s vehicle tried to address the number and size of steering inputs by quickening the steering ratio to a more ergonomically feasible amount. Because insufficient changes were made to the steering geometry to address the increase in steering effort, the car was prohibitively difficult to steer. For the current year, it was deemed critical to decrease this effort as much as possible while still keeping the more ergonomic ratio. This was done through a redesign of the front uprights, which feature a mechanical trail of 0 in. and a scrub radius of 0.75 in. Front Geometry - The bottom a-arm failed in two different ways last year: (1) at the bump-stop in fracture at the end of the threaded insert and (2) at the shock mount in bending after impacting a rock. To solve these issues, this year the shock is mounted between two braces to spread the force, and the wall thickness of the bottom a-arms increased from 0.065 in to 0.095 in. Another issue from the previous car the ball joint connecting the top a-arm to the upright; due to an unanticipated rotation, it rubbed against the reinforcing ring on the inside edge of the aluminum pizza. This year, we switched to press-fit ball joints to provide greater clearances and more stable connections. Additionally last year it was found that whatever benefits were gained from the utilization of camber thrust were negated by the driver confidence sapping reduction in steering forces from negative-jacking. Thus castor was reduced such that it equaled the kingpin-inclination, as to avoid positive-jacking. It was also a goal to reduce the scrub radius to nearly zero, thus reducing ‘kickback’ while still giving some amount of ‘feel.’ However, the new design of A-arm was wider than previous year’s and required the mounting points be pushed inboard to keep the A-arm from colliding with the wheel through-out suspension and steering travel. Rear Geometry - The trailing link suspension design features a new, more integrated and structurally stable style of mounting the trailing link to the rear upright. The suspension links include one toe link which can be modified to create bump steer. The default position is set to 0 degrees per inch of toe change, but can be tuned to passively aid in tuning. For ease of manufacturing, the front tie rods can be used as the rear suspension links. The difference in lengths required for the rear suspension links can be edited with the attached rod ends. Shocks - Both the front and rear suspension use Fox Float R air shocks. The shocks weigh 2.1-2.25 lb each and provide weight savings over coil-over shocks. They feature a continuously adjustable air spring rate, and adjustable rebound damping for easy tuning. In testing and racing, the performance of the Float shocks did not deteriorate noticeably due to the heating of the air inside. Moreover, it was determined that Fox EVOL shocks would not enhance the performance of the vehicle enough to justify the additional expense. Front Uprights/Hubs – Aluminum uprights and tapered steel spindles were custom-machined in order to acquire the desired specifications. For mass production, the uprights would be cast and then finished with light machining. The block size required to produce the upright was minimized, which would allow more pieces to be cast per mold/pour. The hubs used are from a Honda TRX250, a vehicle of similar weight and power to our car, and are lightweight and inexpensive. Rear Uprights/Hubs – CV shafts, hubs, and bearings from a Polaris Sportsman 500 were used this year as they have very similar operating dimensions as the rear of this year’s car and thus could be used with limited modification. The rear uprights, like the front uprights, were made in-house but would be cast and finished with light machining for mass production. Tires and Wheels - Last year’s ITP Quadcross XC tires were applauded by drivers for their excellent grip in all terrains and conditions. With wheels, however, a decision was made to switch back to ITP SS112 wheels from two years ago, as the inner reinforcing ring of the ITP T-9 Pro Series Baja proved problematic to getting desired suspension geometry. The changed saves us .5 lbs and $60 per wheel, for 2lbs and $240 total.
To optimize our steering system with the front suspension geometry, we used Lotus Suspension Analysis software. This allowed us to create a practical design that minimizes unwanted steering effects. Our two main goals for the steering system were (1) for the driver to be able to steer the vehicle with ease and without fatigue for periods of up to four hours and (2) for less than two degrees of mechanical play to be felt in the steering system. To avoid internal stresses and bump steer during suspension travel, we ensured that the tie-rods were concentric with the a-arms; this was done by positioning the inboard tie-rod mounts on the line between the inboard mounts of the a-arms. The rack was placed behind the wheel spindle so that, during a frontal collision, the a-arms would collide before the (much weaker) steering links. To avoid hand-over-hand steering, which is hindered by the required wrist restraints, we chose a lock-to-lock input of the wheel to be 180 degrees. To avoid plowing of the tires, a maximum turning angle of 48 degrees on the inside wheel was chosen. Combining these parameters yields an overall mechanical advantage of the diver to the tires of 2:1. BRAKES - A major improvement of the current design over previous years is the in-line packaging of the master cylinders. Compared to the previous mounting configuration, there is much less mounting hardware and the design is vastly more compact, reducing the risk of interfering with the driver’s legs. Master Cylinders - We switched to MPC master cylinders with a 0.75 in. bore from Wilwood master cylinders. The through-bolt mounting design allows for the more efficient packaging, and the parts (both the cylinders and their mounting hardware) are significantly lighter. Calipers and Biasing - One of the most important requirements for any car is being able to successfully lock the brakes to stop the car as quickly as possible. Assuming that the front wheels will brake harder than the rear axle, a 60/40 braking ratio was selected as the design goal. To achieve this ratio with off-the-shelf parts that fit the other parameters of the car, we found that the front:rear torque ratio on the wheels is 3:2. These parameters drove the design and we chose Honda 380EX calipers for the front and MCP Martin 3050 calipers for the rear. Tie rods connecting from the brake pedal to the two master cylinders allow us to fine-tune the front-rear bias of the brakes during testing.
The drivetrain consists of the specified Briggs & Straton Engine (Model 105432), a CVTech-IBC Continuously Variable Transmission (CVT), and a gear reduction. Constant velocity (CV) axles transmit power from the gearbox to the rear wheels. The CVT continuously shifts between 3:1 (at engagement) to a 0.43:1 ratio when fully shifted. The gearbox reduction ratio of 12.93:1 allows the drivetrain ratio to vary from a low ratio of 38.79:1 to a maximum ratio 5.56:1 with the CVT shifted fully. The theoretical top speed of this year’s vehicle design at an engine speed of 3800 RPM is 44.7 mph, an obtainable goal as long as drivetrain efficiency stays above 50%. Gearbox - The reduction in the gearbox is achieved with two pairs of spur gears. The material selected is 8620 steel for both the gears and shafts. The properties of 8620 steel provide gears and shafts with high strength, durability, and shock resistance. The gearbox and reduction are the same from last year. To reduce weight we chose to web the spur gears, ensuring that the rim thickness remained greater than 0.8x the height of the gear and the inner thickness of the web remained greater than 0.25 in. These changes allowed for a weight reduction of 40% for the gear reduction. Splines mate the gears to their respective shafts for a very tough and accurate coupling. Snap rings align the gears on the shafts, and align the shafts in the case. The case is a two-piece design machined from 6061-T6 aluminum. The approximately symmetrical design of each half reduced material and manufacturing costs. Three bearings pressed into each half support their respective shafts. The double-sealed, single row ball bearings handle radial loads on the shafts due to torque and thrust loads from the CVT and drive-shafts. Gear oil provides lubrication, while shaft seals and a Buna-N O-ring gasket to seal the mating faces minimize case leakage. Stress and fatigue were calculated according to ASME standards.
For the current season, we chose to reuse the roll cage from the previous year, allowing us to phase in gradual changes to the frame. These changes include adjusting the front suspension mounting bars, a simplified pedal and brake mounting system, USM modifications, and the removal of SIM diagonal members. These modifications correspond to changes within the other vehicle systems. FEM analysis of both frontal impact and rollover simulations were performed to ensure design safety. Rear Bracing - Last year, the rear shock mount was positioned perpendicular to the extension of the SIM in the rear bracing. Its placement, in the middle of the member without cross-members to support the load, resulted in substantial bending throughout the middle of the rear bracing. To prevent this from reoccurring, two design goals were implemented: (1) to mount the shock in a location with more support and (2) to design a more rigorous shock mount tab to better distribute the force of the shock to the frame. To achieve the first goal, we chose to mount the rear shock at the joint formed by the upper rear bracing, lower rear bracing, and middle LC. This was accomplished by compacting the rear bracing, which also reduced the total size of the vehicle.