Four-wheel steering system for vehicle

A four-wheel steering system for a vehicle comprising a steering member, a front wheel turning mechanism for turning the front wheels in response to operation of the steering member, a rear wheel turning mechanism including means for sensing a vehicle condition and a rear wheel steering ratio determining means for determining the rear wheel steering ratio which is the ratio of the turning angle of the rear wheels to the turning angle of the front wheels where the steering ratio determining means has at least a predetermined first rear wheel steering ratio characteristic as a function of said vehicle condition and a second rear wheel steering ratio characteristic as a function of said vehicle condition which is shifted in an algebraically positive direction as compared with the first rear wheel steering ratio characteristic, and an actuator for turning the rear wheels according to the sensed vehicle condition and the rear wheel steering ratio determined by the rear wheel steering ratio determining means, characterized by a road condition detecting means for detecting conditions of the road surface which can affect the road gripping force of the wheels and a rear wheel steering ratio selecting means which receives an output of the road condition detecting means and outputs the first rear wheel steering ratio characteristic to the actuator when the detected road condition is such that the road gripping force of the wheels is high and the outputs the second rear steering ratio characteristic to the actuator when the detected road condition is such that the road gripping force of the wheels may be weakened.