Abstract: A hydraulic auxiliary drive system for the normally non-driven wheels of a vehicle to assist the main wheels. The auxiliary drive comprises at least one variable displacement motor having a swash plate for driving at least one of the non-driven wheels, a variable displacement pressure compensated pump supplying the motor with a constant pressure regardless of vehicle speed, a spring for biasing each motor swash plate toward zero displacement, a servo connected to the motor swash plate for varying the displacement of the motor, and a pressure source for supplying the servo. A manually operated pressure control includes a solenoid-controlled variable orifice for controlling the pressure level supplied to the servo to vary the torque output of the normally non-driven wheels. A control valve is positioned between the pump and motor for disengaging and reversing the auxiliary drive. Another embodiment includes a second servo connected to the motor swash plate for decreasing the motor displacement.

Abstract: A system providing auxiliary power to the non-driving steerable wheels of a four-wheel vehicle utilizing hydraulic power in a parallel circuit from the existing hydrostatic drive system. The steerable wheels are driven by hydraulic motors connected in parallel through a flow divider valve and supplied by the variable displacement pump of the existing drive system through a selector valve which has a free-wheeling position allowing the steering wheels to free-wheel with the return oil from each motor flowing in a closed loop back through the flow divider valve while providing charging pressure to that closed loop circuit.