The two-wheeled front suspension system is composed of dual A-arms with coil-over shocks. The suspension system has been designed to have a mechanical advantage in the shock linkage using a scissor joint mechanism, which allows for a small frontal area. The upright and A-arm alignment is optimized for near-zero scrub for efficiency and minimum bump steer for stability.
Titanium 6/4 is utilized for all suspension components except for bronze bushing and steel fastening hardware. Because of the high strength-to-weight ratio of titanium, the suspension system is both lightweight and safe. Building on the experience from the 2010 car uprights, deep-penetration welds will be used to ensure that combined steering and braking forces will be withstood without incident.
Rod ends function as combined ball joints and kingpin. In accordance with the rod end stress calculation guide given on the ASC website (http://americansolarchallenge.org/wordpress/wp-content/uploads/2009/10/rod_end_calculations.pdf). The rod end shanks are made from 4130 Cr-Mo steel. High-strength Cr-Mo steel has a yield strength of approximately 161,000 psi. This, in conjunction with previous experience with these particular rod ends in the 2010 vehicle, leads us to believe that these rod ends are suitable for the worst-case suspension loads.
The weight of the vehicle was assumed to be 600 lbs for the loading analysis, which is appropriate based upon experience with the previous car weight estimations. The weight and center of mass of the vehicle is based upon 2010 scrutineering weighing results. We have found the anticipated center of mass of the vehicle to calculate the suspension forces on the uprights and A-arms under 2-1-1 bump, steer, and brake conditions.
The 2-1-1 loading scenario (or impact type) refers to simultaneous 2G bump, 1G steer, and 1G braking forces applied to the structural components being tested. All suspension and frame components are tested using the 2-1-1 combined loading.
Analysis is performed by placing the forces from the FBD calculations on the load bearing surface from the rod end coming off of the top of the upright. Appropriate pin joints and bearing surfaces were placed for this analysis and it is assumed that the shock has reached maximum upward travel for this test. Our upper suspension appears strong and rigid, with factors of safety of at least 2.75. The forces were calculated from FBD drawings made after conferring with lead mechanical inspector Dick Roberto in 2010. The rigidity and robustness of the suspension system has been verified in the 2010 car, and will be remade with little modification.
Lower Control ArmEdit
1G steer and 1G stop analysis is performed by placing a 1300 lbf turning force and 1600 lbf latitudinal force on the load-bearing surface from the rod end attached to the lower part of the upright in addition to a 200 lbf steering force. Both left turns and right turns were analyzed independently. The forces were calculated from the FBD drawings that we made after conferring with lead mechanical inspector Dick Roberto. The lower control arm 2G bump test is assumed to be negligible since the upper control arm reacts nearly all suspension forces.
Main Upright AssemblyEdit
Analysis is performed by placing a 2-1-1 force on the simulated tire patch. This analysis includes the upright, wheel, brake mounts, and hub. Table 3 shows the results from the front upright assembly analysis: the front upright assembly is shown to have reasonable displacements and factors of safety, due to the high tensile strength of the titanium members. Maximum displacement is taken from the tire contact patch.
NGM Compatible Custom Machined WheelEdit
The NGM compatible custom machined wheel was machined out of a solid billet of 6061-T6 aluminum. FEA tests were run on this wheel for the 2008 NASC and since then, the wheels have had more than 1500 miles driven on them with no visible wear or deformation; a full set of replacements will be kept available.
Rear Swing Arm AnalysisEdit
Analysis is performed by applying an inertial load, based upon 300 lb weight share to the rear wheel, to the projected tire contact location using a motor/wheel/tire placeholder for convenience. Sliding restraints were used for the suspension mount to take into account the low stiffness in the lateral direction. The rear swing arm analysis results are summarized in Table 4. It can be seen that this suspension section is fairly stiff and has sizable margins with factor of safety. Maximum displacement is taken from the tire contact patch.
There are three independent braking systems on the vehicle: (1) A dual redundant hydraulic disc brake system on the front wheels using two separate and independent calipers and master cylinders, (2) A bicycle brake with large pads on the rear wheel, and (3) Regenerative braking on the rear drive wheel. For the purposes of qualifications the regenerative brake is not counted as a braking system. The parking brake system operates via calipers on the front-right brake rotors actuated by a mounted master cylinder.
The brake calipers are mounted to the upright with 6061-T6 aluminum mounting plates with grade-8 steel hardware.
The rack and pinion assembly is made from aluminum billet, while the tie rods are made from 6061-T6 aluminum. The steering arms are made from Ti 6/4, welded to the uprights.
Wheels & TiresEdit
The wheels are designed with NGM compatible dimensions. They have been shown to operate safely in the previously OSU solar car and analysis is given above. They have also performed well in the current car’s previous races. The tires are tubeless, highway-rated, Dunlop D850.
The wheels are made from 6061-T6 aluminum while the hubs are made from grade 2 titanium.
Learning from our 2010 experience with battery pack construction we will be building a functionally identical nonconductive, chemically resistant, fire-resistant battery box. The battery box will be made again thick 2-core-2 fiberglass with a Nomex honeycomb core. It will be located in the most secure area of the chassis and designed for easy installation and removal. Ventilation ducts (not shown in figure) will route cooling air directly to the outside of the vehicle for safety in the event of battery out-gassing.
The battery box will be constructed with pre-impregnated fiberglass with a Nomex honeycomb core. It will be constructed using lay-up fiberglass seams and reinforced interior fillets. The construction technique will be functionally identical to that of the 2010 enclosure.