In order to improve vehicle ride comfort, shock absorbers are installed in parallel with elastic elements in suspension system. In order to attenuate vibration, hydraulic shock absorbers are mostly used in suspension system. Their working principle is that there is relative motion between frame (or body) and bridge when vibration occurs. When the piston in the shock absorber moves up and down, the oil in the shock absorber chamber flows repeatedly from one chamber to another through different holes.

At this time, the friction between the hole wall and the oil and the internal friction between the oil molecules form a damping force to the vibration, which transforms the vibration energy of the vehicle into the oil thermal energy, and then absorbed and emitted into the atmosphere by the shock absorber. When the cross section and other factors of the oil passage remain unchanged, the damping force increases and decreases with the relative velocity between the frame and the axle (or wheel), and is related to the oil viscosity.

Shock absorber and elastic components undertake the task of cushioning and vibration reduction. Excessive damping force will deteriorate the suspension elasticity and even damage the connecting parts of shock absorber. It is necessary to adjust the contradiction between elastic elements and shock absorbers.

(1) During the compression stroke (the bridge and the frame are close to each other), the damper has less damping force, so as to give full play to the elastic role of the elastic elements and mitigate the impact. At this time, elastic elements play a major role.

(2) During the suspension stretching journey (the bridge and frame are far away from each other), the damper force should be large and the vibration should be reduced rapidly.

(3) When the relative speed between the axle (or wheel) and the axle is too large, the shock absorber is required to automatically increase the liquid flow, so that the damping force is always within a certain limit, so as to avoid bearing excessive impact load.

Cylindrical shock absorber is widely used in automobile suspension system, and it can reduce vibration in both compression and stretching stroke. It is called bi-directional shock absorber. There is also a new type of shock absorber, which includes inflatable shock absorber and adjustable resistance shock absorber.