In a radical departure from convention, MSOE’s mini-Baja vehicle, top, uses a hydrostatic drive system, above, in lieu of a conventional geared drive train. Photo at right shows close-up of bent-axis hydraulic wheel drive motor.
A 2002 design project by senior undergraduates at the Milwaukee School of Engineering (MSOE) makes extensive use of sophisticated hydraulics technology for an offroad vehicle in the Society of Automotive Engineers’ (SAE) annual Mini Baja Competition.
The contest simulates real-world engineering projects and related challenges for students to design and build an off-road vehicle that would survive the severe punishment of rough terrain. Students had to work as a team, not only to design, build, test, promote, and race a vehicle within the limits of the rules, but also to generate financial support for their project and manage their class loads as well. The competition includes rigorous tests of maneuverability, acceleration, top speed, braking, hill climbing, power pulling, and a four-hour endurance race.
A departure from tradition
MSOE has been involved in the competition for the last several years. However, this particular design represents a radical departure from the mechanically dominated power trains traditionally used. Instead of the conventional geared transmission, drive shafts, differential gearing, and transfer box for four-wheel drive, MSOE’s concept vehicle embraces hydrostatic drive technology with sophisticated electronics for performance and capabilities that clearly would be impractical for mechanical drives.
Key features of MSOE’s mini Baja include:
• hydrostatic transmission with energy management (load sensing, power limiting, and pressure compensation) at the pump,
• energy storage (via hydropneumatic accumulators) for speed boost on demand,
• selectable two- or four-wheel drive with maximum power utilization under all operating conditions — hill climb, acceleration, and power pull,
• front engine placement for nearly 50/50 weight distribution,
• articulated steering (via a hydraulic rotary actuator) for tight turning radius,
• CANbus control of electrohydraulic components, which allows vehicle performance to be adjusted based on track conditions, and
• electrohydraulic valve control of differential wheel speed, which permits full power while turning by eliminating skip and bounce.
The Baja car uses a wheel motor and front suspension setup often found mobile equipment. Tom Labus, P. E., professor of mechanical engineering, reveals, “This is one area where substantial improvement can be made in future designs by incorporating a small planetary gearbox with a high-speed motor to reduce the overall size and unsprung weight.”
