Classification from the perspective of materials
Hydraulic shock absorbers: widely used in automobile suspension systems. The principle is that when the frame and axle reciprocate and the piston reciprocates in the cylinder of the shock absorber, the oil in the shock absorber housing repeatedly flows from one inner cavity to another through some narrow holes. At this time, the friction between the liquid and the inner wall and the internal friction of the liquid molecules form a damping force for vibration, so that the vibration energy of the car is converted into oil heat energy, which is then absorbed by the shock absorber and dissipated into the atmosphere.

Pneumatic shock absorber: a new type of shock absorber developed since the 1960s. Its structural feature is that a floating piston is installed at the bottom of the cylinder, and a closed air chamber formed by the floating piston and one end of the cylinder is filled with high-pressure nitrogen. A large-section O-ring is installed on the floating piston to completely separate the oil and gas. The working piston is equipped with a compression valve and an extension valve that change the cross-sectional area of ??the channel with the speed of its movement. When the wheel bounces up and down, the working piston of the shock absorber reciprocates in the oil, causing an oil pressure difference between the upper and lower chambers of the working piston. The pressure oil pushes the compression valve and the extension valve to flow back and forth. Since the valve generates a large damping force on the pressure oil, the vibration is attenuated.

Variable damping shock absorber: Recent cars use electronically controlled shock absorbers as standard equipment. It detects the driving state through sensors, and the computer calculates the optimal damping force, so that the damping force adjustment mechanism on the shock absorber works automatically.

From the structural perspective
Double-tube type: refers to the shock absorber with two inner and outer tubes. The piston moves in the inner tube. Due to the entry and withdrawal of the piston rod, the volume of the oil in the inner tube increases and shrinks accordingly. Therefore, it is necessary to maintain the balance of the oil in the inner tube by exchanging with the outer tube. Therefore, there must be four valves in the double-tube shock absorber, that is, in addition to the two throttle valves on the piston, there are also flow valves and compensation valves installed between the inner and outer tubes to complete the exchange function.

Single-tube type: Compared with the double-tube type, the single-tube shock absorber has a simple structure and reduces a set of valve systems. A floating piston is installed at the bottom of the cylinder (the so-called floating means that there is no piston rod to control its movement), and a closed air chamber is formed below the floating piston, which is filled with high-pressure nitrogen. The change in the liquid level caused by the piston rod entering and exiting the oil is automatically adapted by the floating of the floating piston, and it can also change the size of the throttle hole through external operation.

Working principle
In the suspension system, the elastic element is impacted and vibrates. In order to improve the smoothness of the car's driving, a shock absorber is installed in parallel with the elastic element in the suspension. Most of the hydraulic shock absorbers used in the automobile suspension system are hydraulic shock absorbers. The working principle is that when the frame (or body) and the axle vibrate and relative movement occurs, the piston in the shock absorber moves up and down, and the oil in the shock absorber cavity repeatedly flows 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 on the vibration, so that the vibration energy of the car is converted into oil heat energy, which is then absorbed by the shock absorber and dissipated into the atmosphere.

Compression stroke
During the compression stroke (the axle and the frame are close to each other), the damping force of the shock absorber is small, so that the elastic effect of the elastic element can be fully exerted to mitigate the impact. The volume of the lower chamber of the piston decreases, the oil pressure increases, and the oil flows through the flow valve to the chamber above the piston (upper chamber). The upper chamber is occupied by a part of the space of the piston rod, so the increased volume of the upper chamber is less than the reduced volume of the lower chamber, and part of the oil pushes the compression valve open and flows back to the oil storage cylinder. The throttling of the oil by these valves forms a damping force for the compression movement of the suspension.

Extension stroke
When the relative speed between the axle (or wheel) and the frame is too large, the shock absorber is required to automatically increase the flow rate of the liquid to keep the damping force within a certain limit at all times to avoid excessive impact loads. During the extension stroke, the wheel is equivalent to being away from the vehicle body, and the shock absorber is stretched. At this time, the shock absorber piston moves upward, the oil pressure in the upper chamber of the piston increases, the circulation valve is closed, and the oil in the upper chamber pushes the extension valve open and flows into the lower chamber. Due to the existence of the piston rod, the oil flowing from the upper chamber is not enough to fill the increased volume of the lower chamber, causing a vacuum in the lower chamber. At this time, the oil in the oil storage cylinder pushes the compensation valve open and flows into the lower chamber for replenishment. The throttling effect of these valves plays a damping role in the extension movement of the suspension.