Suspension system due to the impact of elastic elements to produce vibration, in order to improve the smoothness of the car driving, suspension and elastic elements in parallel installation of shock absorbers, in order to attenuate vibration, the car suspension system using shock absorbers are mostly hydraulic shock absorbers, its working principle is when the frame (or body) and the axle between the vibration of relative movement, the piston in the shock absorber moves up and down, the oil in the shock absorber cavity will repeatedly flow from one cavity through different pores into another cavity. At this time, the friction between the hole wall and the oil and the internal friction between the oil molecules form a damping force on the vibration, so that the vibration energy of the car is converted into oil heat energy, and then absorbed and dissipated into the atmosphere by the shock absorber. When the oil channel section and other factors remain unchanged, the damping force increases and decreases with the relative speed of movement between the frame and the axle (or wheel) and is related to the viscosity of the oil.

The shock absorber and the elastic element bear the task of cushioning and damping the vibration, and the damping force is too large, which will deteriorate the elasticity of the suspension and even damage the shock absorber connection. Due to the contradiction between the elastic element and the shock absorber, the surface has to be adjusted.

(1) In the compression stroke (the axle and frame are close to each other), the damping force of the shock absorber is small, so as to give full play to the elastic effect of the elastic element and mitigate the impact. At this point, the elastic element plays a major role.
(2) In the suspension extension stroke (the axle and frame are far away from each other), the damping force of the shock absorber should be large and the vibration should be rapidly dampened.

(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 kept within a certain limit to avoid excessive shock load.
In the automobile suspension system is widely used is a barrel shock absorber, and in the compression and extension stroke can play a role in vibration damping called bidirectional action shock absorber, and the use of new shock absorbers, which include gas inflation shock absorbers and resistance adjustable shock absorbers.

Description of the working principle of bidirectional acting cylinder shock absorber. When compressing the stroke, it means that the car wheels move closer to the body, the shock absorber is compressed, and 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 chamber above the piston (upper chamber). The upper cavity is occupied by the piston rod 1, so the increased volume of the upper cavity is less than the reduced volume of the lower cavity, and part of the oil is then pushed open the compression valve 6 and flows back to the oil storage cylinder 5. The oil savings of these valves form the damping force of the suspension under compression movement. When the shock absorber is extended, the wheel is equivalent to moving away from the body, and the shock absorber is stretched. At this point, the piston of the shock absorber moves upwards. The oil pressure in the upper chamber of the piston rises, the flow valve 8 is closed, and the oil in the upper cavity pushes the extension valve 4 into the lower cavity. Due to the presence of the piston rod, the oil flowing from the upper cavity is not enough to fill the increased volume of the lower cavity, and the main lower cavity produces a vacuum, at which time the oil in the oil reservoir pushes open the compensation valve 7 and flows into the lower cavity to replenish. Due to the throttling effect of these valves, the suspension acts as a damping effect when it is in extended motion.

Since the stiffness and preload design of the extension valve spring is greater than that of the compression valve, under the same pressure, the sum of the channel load area of the extension valve and the corresponding normally open gap is less than the sum of the cross-sectional area of the compression valve and the corresponding normally open gap channel. This makes the damping force generated by the extension stroke of the shock absorber greater than the damping force of the compression stroke, which meets the requirements of rapid vibration damping.

In order to accelerate the attenuation of frame and body vibrations to improve the driving smoothness (comfort) of the car, shock absorbers are installed inside the suspension system of most cars.