Steering system for vehicle with four drive wheels

Steering system for vehicles comprises a frame (20) with a front axle (4) and a rear axle (1), at least one of these carrying two steered wheels (14,15), the wheels of each axle being driven by a hydraulic motor (12,42) via a differential unit (11,41), the steered wheels (14,15) being connected, for steering control, to a cylinder-piston unit (160) fed by a usual power steering device, in which at least the hydraulic motor (12,42) associated with one axle is of variable piston displacement, and is associated with piston displacement adjustment means governed by the position of the cylinder of the cylinder-piston unit (160) which controls the steering.