Design of Baja SAE Gearbox for Optimal Performance and Minimum Weight 2020-32-2311
Baja SAE is an intercollegiate competition where teams design and build a single-seat off-road vehicle that is powered by a 10 HP Briggs & Stratton engine. Due to this power constraint, it is crucial to optimize the vehicle’s weight and performance. The design process began by creating a vehicle dynamics simulation, which included engine performance, continuously variable transmission (CVT) shifting, tire slipping, vehicle mass, rotational inertia, air drag, rolling resistance, weight shift, and drivetrain efficiency. These calculations predicted the time to reach 100 ft and the top speed for various gear box ratios to aid in gear ratio selection. The rotational inertia of the drivetrain is 40% of the total effective mass of the vehicle when the CVT primary is engaged and 12% when the CVT has fully shifted into a numerically lower gear, with the largest contribution coming from the inertia of the CVT primary pulley. A sensitivity analysis showed that the vehicle mass and coefficient of friction between the tires and ground have the largest effect on gear ratio selection and times to reach 100 ft. A compound gear train was designed with gears having a minimum diameter and face width to reduce the inertia of the system and included calculation of static and fatigue failure from contact and bending stresses. The gearbox housing, shafts, and bearings were designed, manufactured, and integrated into the vehicle. Vehicle simulations were compared to testing results and showed good agreement in the cases tested. The final gearbox weighed 36% lighter than previous designs and had a 4% decrease in 100 ft times.