Designing a Rear Wheel / Four Wheel Clutch System for the 2025 Union SAE Baja Car

This work consists of the design process and engineering behind a rear wheel and four wheel clutch system for the Union SAE Baja Team 2024-2025 Car-codename Beachball Reinflated (RF). Beachball RF is the fourth generation four-wheel drive car developed by Union's Baja Team, continuing the chain drive/portal axle system developed for the second generation four-wheel drive car, Dustball. The development of this clutch system was motivated by experiences at previous Baja competitions since the transition of the Baja SAE rule book to mandate a four wheel drive system. The newly developed system allows the driver to select between rear wheel drive and four wheel drive based on terrain. Rear wheel drive systems feature significantly higher efficiencies and rear wheel force distribution. Based on efficiency estimation of Beachball RF's drive train, switching to rear wheel drive from four wheel drive could yield an approximate 25% increase in efficiency of the drivetrain. Introducing a clutch system of this sort involves a variety of difficulties stemming from Beachball RF's unique drivetrain. Beachball RF features a central chain drive with a rear and front chain and portal axles with sprag bearings (one way bearings) working as a pseudo differential. The sprag bearings in particular pose a great problem due to not being able to "speed match" dog clutch on engagement. This introduces significantly higher impact forces when the dog clutch is engaged at high speeds. In older manual vehicles, this is the "grinding gears" or "crunching" noises that occur on a bad shift. Additionally, the decision to make the clutch an electromechanical system was made. Functionally, this would allow the clutch to be engaged or disengaged by the press of a button, far more user friendly than requiring the driver to remove their hands from the steering wheel to engage using a lever. While there are advantages and disadvantages to both systems, the decision was heavily influenced by the addition of a data acquisition system to the car this year that would allow the clutch's performance to be influenced by data readings such as the rotational speed of the drivetrain as well the club's continued interest in an electronic CVT, which this could pave the way for. The design and engineering that went into the creation of this clutch system and the performance of the system at this year's SAE competition will be showcased alongside the rest of the Beachball RF.