Patent ID: 7958981

Claim:
A shock absorber interposed in parallel with a suspension spring between a vehicle wheel and a vehicle body of a vehicle, comprising: a cylinder; a main piston partitioning the cylinder into a first operating chamber and a second operating chamber; a first passage connecting the first operating chamber and the second operating chamber under a first flow resistance; two pressure chambers partitioned by a free piston, the free piston having a predetermined pressure receiving area; a second passage connecting the first operating chamber with one of the two pressure chambers under a second flow resistance without passing through the other of the two pressure chambers; a third passage connecting the second operating chamber with the other of the two pressure chambers under a third flow resistance without passing through the one of the two pressure chambers; and a spring having a predetermined spring constant and resiliently supporting the free piston in a predetermined neutral position, wherein the first flow resistance, the second flow resistance, the third flow resistance, the spring constant and the pressure receiving area are set so as to produce a large damping force for damping vibration of the main piston when a vibration frequency of the main piston is lower than a first frequency, produce a small damping force when the vibration frequency is higher than a second frequency that is greater than the first frequency, and produce, when the vibration frequency is higher than the first frequency and lower than the second frequency, a damping force that gradually decreases as the vibration frequency increases.