In the suspension system, because elastic elements are impacted to cause vibration, the shock absorber and elastic elements are installed in parallel in the suspension in order to improve driving smoothness of automobiles; in order to reduce vibration, the shock absorbers widely used in the automotive suspension system are often hydraulic shock absorbers, its working principle is that: when the relative motion between the automobile frame (or automobile body) and the driving axle occurs due to vibration, the piston in the shock absorber moves up and down, the oil in the cavities of the shock absorber repeatedly flows from one cavity to another cavity via different holes, at this time, the friction between the hole walls and the oil and between oil molecules form the damping force, and the vibration energy of the automobile is converted into heat energy of the oil and then absorbed by the shock absorber and emitted to the atmosphere. Under the condition of the same total sectional area of oil channels, the damping force of the shock absorber increases or decreases along with the increase or decrease of the relative motion speed of the automobile frame and the driving axle(or wheels), and is related to viscosity of the oil. The shock absorber and the elastic elements take the task of buffering impact and reducing vibration. When the damping force is too big, the elasticity of the suspension will get bad, even the connecting piece of the shock absorber is damaged, so that the contradiction between the elastic elements and the shock absorber must be solved.

(1) In the compression stroke (the driving axle and the automobile frame are closing to each other), the damping force of the shock absorber is small so as to give a full play of the elastic action of elastic elements to buffer impact. At this time, the elastic elements play a leading role;

(2)In the stretching stroke (the driving axle and the automobile frame are getting away from each other), the damping force of the shock absorber should be big, so that the vibration can be reduced quickly;

(3)When the relative speed between the driving axle (or wheels) and driving axle is too high, the shock absorber should automatically increase the flow rate of the oil, so that the damping force is kept in a certain range so as to avoid too big impact load;

The telescopic shock absorber is widely used in the automotive suspension system, and the shock absorber which has the shock absorbing effect in both compression stroke and stretching stroke is the bi-directional action type shock absorber; in addition, some new shock absorbers including gas filling type shock absorbers and damping force adjusting shock absorbers are also used.

The working principle of the bi-directional action telescopic shock absorber is that: in the compression stroke, namely the automobile wheels move close to the automobile body, the shock absorber is compressed, the piston 3 of the shock absorber moves downward, the volume of the lower cavity of the piston is decreased, the oil pressure rises, the oil flows to the cavity (upper cavity) above the piston via the passing valve 8, and the partial space of the upper cavity is occupied by the piston rod 1, so that the increased volume of the upper cavity is less than the decreased volume of the lower cavity, some oil pushes the compression valve 6 and flows back to the oil cylinder 5, and the oil-saving action of these valves forms the damping force against the compressed motion of the suspension; in the stretching stroke of the shock absorber, the wheels are relatively away from the automobile body, the shock absorber is stretched, the piston of the shock absorber moves upward, the oil pressure of the lower cavity of the piston rises, the passing valve 8 is closed, the oil in the upper cavity pushes the stretching valve 4 and flows to the lower cavity; and because of the existence of the piston rod, the oil flowing from the upper cavity is not enough to fill up the increased space of the lower cavity, so that a certain vacuum degree is generated in the lower cavity, the oil in the oil storage cylinder pushes the compensation valve 7 and flows to the lower cavity for supplement, and the throttling action of these valves forms the damping force against the stretching motion of the suspension.

Because the designed rigidity and pre-tightening force of the stretching valve are greater than those of the compression valve, under the same pressure, the total sectional area of the stretching valve and the channels of the corresponding normal open gaps is less than that of the compression valve and the channels of the corresponding normal open gaps, so that the damping force generated in the stretching stroke of the shock absorber is more than that generated in the compression stroke of the shock absorber, and the aim of quickly reduce the vibration is achieved.

The following drawing shows the structural drawing of front and rear suspension shock absorbers of an automobile. Its working principle has been introduced above.