From the material point of view: shock absorbers are mainly divided into hydraulic and pneumatic
1. Hydraulic shock absorber: Hydraulic shock absorber is widely used in automobile suspension system. 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 friction between the liquid molecules will form a damping force on the vibration.

2. Pneumatic shock absorber: Pneumatic shock absorber is 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, and the pressure oil pushes the compression valve and the extension valve to flow back and forth. Since the valve produces a large damping force on the pressure oil, the vibration is attenuated.

From the structural point of view: shock absorbers are divided into single-tube and double-tube types according to their structure.
1. Double-tube type:
The shock absorber has two inner and outer tubes, and 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 are four valves in the double-tube shock absorber, that is, in addition to the two throttle valves on the piston mentioned above, there are also flow valves and compensation valves installed between the inner and outer tubes to complete the exchange function.

2. Single-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). A closed air chamber is formed under the floating piston, which is filled with high-pressure nitrogen. The above-mentioned changes in the liquid level caused by the piston rod entering and exiting the oil are automatically adapted through the floating of the floating piston.