PURPOSE: To increase straightforward traveling, steering, and braking performances by connecting a control valve, switching over caster trail larger or smaller according to the operation of a power steering system and a brake system, to a caster variable actuator, in the above system for automobile.

CONSTITUTION: A caster trail control valve 10 is operated by a pressure difference generated in pipes 24 and 25 connected to an actuator 26 of a power steering system 2. On the other hand, the delivery pressure generated by a pump 20 of the power steering system 2 is applied to the chamber 5a of a caster variable actuator 4 through a pipe 30 and the caster trail control valve 10. Also, a pressure from the master cylinder 37 of a brake system 3 is applied to the chamber 5a. Accordingly, a caster trail L is so controlled that it becomes larger when a car travels in straightforward direction, smaller when it is steered, and larger when it is braked. Thus, braking, steering, and straightforward traveling performances can be increased.