In the compression stroke, it means that the car wheel moves close to the body, the shock absorber is compressed, and the piston 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 to the upper chamber (upper chamber) of the piston. Part of the space of the upper chamber is occupied by the piston rod, so the increased volume of the upper chamber is less than the reduced volume of the lower chamber, and a part of the oil is pushed open the compression valve and flows back to the storage cylinder. The oil saving of these valves forms the damping force of the compressed motion of the suspension.

When the shock absorber stretches, the wheel is far away from the body, and the shock absorber is stretched. At this time, the piston of the shock absorber moves upward. When the oil pressure in the upper chamber of the piston increases, the flow valve closes, and the oil in the upper chamber pushes open the extension valve and flows into the lower chamber. Due to 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 degree in the lower chamber. At this time, the oil in the oil storage cylinder pushes the compensation valve 7 to flow into the lower chamber for replenishment. Because of the throttling effect of these valves, the suspension plays a damping role in stretching motion.

Because the stiffness and pretension of the spring of the expansion valve are larger than those of the compression valve, the total load area of the expansion valve and the corresponding normal gap is smaller than that of the compression valve and the corresponding normal gap under the same pressure. In order to accelerate the vibration attenuation of the frame and the body, and improve the ride comfort of the car, the shock absorber is installed in the suspension system of most cars.

The shock absorption system of automobile is composed of spring and shock absorber. The shock absorber is not used to support the weight of the car body, but to suppress the shock of spring rebound after shock absorption and absorb the energy of road impact. The spring plays a role in easing the impact, changing "one impact with large energy" into "multiple impacts with small energy", while the shock absorber gradually reduces "multiple impacts with small energy".