When the frame and the axle reciprocating relative movement, and the piston in the changing rear shock absorbers in the cylinder reciprocating movement, then the oil in the shock absorber shell will repeatedly from a cavity through some narrow small pores into another cavity. At this point, the friction between the hole wall and the oil and the friction within the liquid molecules will form the damping force of vibration, so that the vibration energy of the body and the frame is transformed into heat energy, which is absorbed by the oil and the shock absorber shell, and then scattered to the atmosphere. The size of the damping force of the double shock absorber increases and decreases with the increase and decrease of the relative speed of the frame and the axle (or the wheel), and is related to the viscosity of the oil. The viscosity of oil used in shock absorber is required to be affected as little as possible by temperature changes; It has the properties of resistance to vaporization, oxidation and corrosion of various metal and nonmetal parts.