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 when the vibration between frame (or body) and vehicle bridge occurs relative motion, the piston in the shock absorber moves up and down. The oil in the cavity flows repeatedly from one cavity to another through different pores. 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 frame are close to each other), the damper has a smaller 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, in order to avoid bearing excessive impact load.

Cylindrical shock absorber is widely used in automobile suspension system, and it can play a role in compression and stretching stroke called bidirectional shock absorber. There is also a new type of shock absorber, which includes inflatable shock absorber and resistance adjustable shock absorber.

The working principle of two-way action cylinder shock absorber is explained. When compressing stroke, it means that the wheel of the car moves close to the body and the shock absorber is compressed. At this time, the piston 3 in the shock absorber moves downward. The volume of the lower chamber of the piston decreases, the oil pressure increases, and the oil flows through the flow valve 8 to the upper chamber (upper chamber) of the piston. The upper chamber is occupied by the piston rod 1, so the increased volume of the upper chamber is less than the reduced volume of the lower chamber. A part of the oil then pushes the compression valve 6 and flows back to the storage cylinder 5. The oil savings of these valves form the damping force of the suspension under compression. When the shock absorber stretches, the wheel is equivalent to far away from the body, and the shock absorber is stretched. Then the piston of the shock absorber moves upward. The oil pressure in the upper chamber of the piston rises, the flow valve 8 closes, and the oil in the upper chamber pushes the expansion valve 4 into the lower chamber. Because of the existence of piston rod, the oil flowing from the upper chamber is not enough to fill the increased volume of the lower chamber, which mainly causes a vacuum in the lower chamber. At this time, the oil in the storage cylinder pushes the compensation valve 7 to flow into the lower chamber to supplement. Because of the throttling effect of these valves, the suspension acts as a damper in the stretching motion.

Because the spring stiffness and pre-tightening force of the expansion valve are larger than that of the compression valve, under the same pressure, the total carrying area of the expansion valve and the corresponding constant-pass gap is smaller than that of the compression valve and the corresponding constant-pass gap channel. This makes the damping force produced by the extension stroke of the shock absorber larger than that produced by the compression stroke, and meets the requirement of rapid vibration reduction.

In order to accelerate the vibration attenuation of frame and body and improve ride comfort, shock absorbers are installed in the suspension system of most automobiles.