Hornet Racing Baja S.A.E. Front Suspension (2018)
Baja SAE is a collegiate student design competition, in which students design, manufacture, compete, and promote their design considerations to a design judge panel. All competing vehicles are limited to a 10 hp. Briggs & Stratton engine, in which performance modifications to the engine are strongly prohibited. My responsibility with this club is the design of the front suspension. Also, the Baja SAE competition will be held May 30 through June 2, 2018. Since the competition is next year, this design is within the initial design stages, as constant design reiterations will be made.
The main objective of this front suspension design was to design for manufacturability. To promote the ease of manufacturability, not only are the front suspension components (uprights and control arms) mainly constructed from 0.(hidden)" (hidden) sheet metal, but also utilize symmetry where applicable. Additional consideration is placed into the sheet metal piece interaction in construction: edges of each interacting piece uses cut-outs along the mating edges, such that mating pieces can "jigsaw" in place (a benefit that will be experienced when welding the components together); additionally, intersecting faces utilize slots so that the joining pieces can easily link together (another benefit for construction). Also, since symmetry is a concept for design, the front suspension will use the same lower control arms for both left and right sides, and the same theory also applies to the upper control arms as well. Additionally, the front uprights also use the same sheet metal components for their construction, just their change in orientation is simply achieved by a respective change in orientation of the sheet metal pieces that comprise the constructed component. It should also be mentioned that the idea behind the use of a horizontal spherical bearing instead of a ball joint is due to space constraints, as well as suspension geometry requirements. The uprights maintain a spindle-to-ball joint relationship that causes the spindle to "mechanically trail" the kingpin inclination angle: the kingpin inclination angle is the geometrical line of action that acts through the center of the ball joints, and should theoretically intersect the ground in the middle of the tire's contact patch (the physical interacting surface area of tire tread and the terrain upon its operation); any deviation in distance from the kingpin inclination angle's intersection into the contact patch surface is known as the scrub radius, in which the longer the scrub radius, the greater the bump steer (bump steer is the effect that any surface imperfection will negatively be felt within the steering, which is experienced as "jolt" to the steering wheel). The mechanically trailing spindle is intended to counter bump steer effects, as the concept of a mechanically trailing spindle is similar in theory to a steering caster of a shopping cart; since the ball joint pivot of this upright design is not acting through the spindle, less bump steer effect is observed since the tire's path is lead in direction by the spherical bearings. Also, the spherical bearings are mounted horizontally, just so the mounting bolts have a double-shear connection. Another design consideration is the use of a "rotor bobbin" for mounting the brake rotor to the front hub, due to the fact that the rotor can be mounted closer to the upright, without the interference of the head of a cap screw, as well as a wire-tie system that is a competition rules safety requirement. The front spindle will use either 4140 or 4340 material. Further design considerations will be conducted based more so on cost and weight considerations. However, the diameter of the spindle was calculated utilizing an Excel spreadsheet, and material properties provided by (hidden)
To further promote manufacturability, the same jig will be used for the construction of both the upper and lower control arms. Design for this jig is similar to the design of the control arms, in which intersecting faces of the jig members also possess slots so that pieces will link together. This linking system is crucially important, since a situation in which a jig is not required to construct another jig. Bolts and spacers will be required to fix the control arms to the jig.
Front shocks: Fox Float X Evol shocks
Brake caliper: Wilwood PS1
Rims: Douglas Wheel A5, 4x156mm bolt pattern, 10" x 5" wheel dimensions, 4" backspacing
Tires: Maxxis Razr2, 22"x7"x10"
1) Smith, Carroll. "Engineer to Win". MBI Publishing, 1987
2) Smith, Carroll. "Tune to Win". Aero Publishers, 1978
3) Milliken, William F., and Douglas L. Milliken. "Race Car Vehicle Dynamics". Society of Automotive Engineers, 1995.
4) Budynas, Richard G., and J Keith Nisbett. "Shigley's Mechanical Engineering Design". 9th ed. McGraw Hill. 2008.
5) Dowling, Norman E. "Mechanical Behavior of Materials". 4th ed. Pearson, 2013.