A suspension system, comprising:
- a hydraulic cylinder, which is provided between a wheel holding member and a vehicle body on each of front/rear left/right wheels of a vehicle, the hydraulic cylinder being configured to store working fluid and extend or contract depending on a change in a distance between the wheel holding member and the vehicle body;
- a first gas spring, which is provided so as to correspond to each of the hydraulic cylinders for the front/rear left/right wheels, the first gas spring comprising a first gas chamber and a first oil chamber communicating to the hydraulic cylinder in partitions, and being configured to function as a spring in a hydraulic system based on an amount of the working fluid stored in the first oil chamber changing depending on a hydraulic pressure in the hydraulic cylinder;
- a second gas spring, which is provided for each of the hydraulic cylinders, the second gas spring comprising a second gas chamber and a second oil chamber communicating to the hydraulic cylinder in partitions, and being configured to function as a spring in the hydraulic system based on an amount of the working fluid stored in the second oil chamber changing depending on the hydraulic pressure in the hydraulic cylinder;
- a spring switching valve, which is provided for each of the hydraulic cylinders, the spring switching valve being capable of switching between a state in which the communication between the hydraulic cylinder and the second gas spring is allowed and a state in which the communication is shut off;
- wheel rate switching means for controlling the spring switching valve to switch a wheel rate;
- and
- roll stiffness decrease means for setting, during rough road travel, one of the front left and right wheels and the rear left and right wheels as roll stiffness decrease subject wheels, and another one of the front left and right wheels and the rear left and right wheels as non-roll stiffness decrease subject wheels, to thereby cause the left and right hydraulic cylinders for the roll stiffness decrease subject wheels to communicate to each other while the spring switching valve is opened for each of the roll stiffness decrease subject wheels, and to thereby close the spring switching valve for at least one of the left and right wheels of the non-roll stiffness decrease subject wheels to cause the second gas spring shut off from the hydraulic cylinder for the wheel having the closed spring switching valve to communicate to the left and right hydraulic cylinders for the roll stiffness decrease subject wheels.