In a vehicle's suspension system, to improve ride comfort, shock absorbers are typically installed in parallel to dampen vibrations:
Hydraulic shock absorbers are a common type, and their working principle is that when the chassis or body moves relative to the axle, the piston inside the shock absorber moves up and down, and the fluid repeatedly flows from one chamber to another through various orifices within the shock absorber chamber. At this point, the friction between the orifice walls and the fluid, as well as the internal friction between fluid molecules, creates a damping force against the vibration, converting the vehicle's vibration energy into fluid heat energy, which is then absorbed by the shock absorber and dissipated into the atmosphere. Shock absorbers and elastic elements jointly bear the task of mitigating impact and damping vibrations.
To ensure the normal operation of shock absorbers and elastic elements, their interplay needs to be regulated. During the compression stroke, the damping force of the shock absorber is relatively small to allow the elastic elements to fully function and mitigate impact. Conversely, during the rebound stroke of the suspension, the damping force of the shock absorber should be greater to quickly dampen vibrations. When the relative speed between the axle and the chassis is too high, the shock absorber needs to be able to automatically increase the fluid flow to ensure that the damping force remains within a certain range, preventing excessive impact loads.
In automotive suspension systems, telescopic shock absorbers are widely used, which provide damping in both compression and rebound strokes, and are thus called double-acting shock absorbers. Additionally, there are new types of shock absorbers such as gas-filled shock absorbers and adjustable-damping shock absorbers. The working principle of a double-acting telescopic shock absorber is as follows: During the compression stroke, as the vehicle wheel moves closer to the body, the shock absorber is compressed. At this point, the piston inside the shock absorber moves downwards, the volume of the lower piston chamber decreases, and oil pressure rises. The fluid flows through the bypass valve into the upper piston chamber, and then the fluid pushes open the compression valve and flows back into the reservoir.
The restriction of oil flow by these valves creates the damping force for the suspension's compression movement. During the rebound stroke, the wheel moves away from the body, and the shock absorber is extended.
At this point, the piston of the shock absorber moves upwards, the oil pressure in the upper piston chamber rises, and the bypass valve closes. The fluid in the upper chamber pushes open the rebound valve and flows into the lower chamber. Due to the presence of the piston rod, the fluid flowing from the upper chamber is insufficient to fill the increased volume of the lower chamber, thus creating a vacuum. At this point, the fluid from the reservoir pushes open the compensation valve and flows into the lower chamber to replenish it.
Because the stiffness and preload of the rebound valve spring are designed to be greater than those of the compression valve, under the same pressure, the total flow area of the rebound valve and its corresponding constant-flow orifices is smaller than the total flow area of the compression valve and its corresponding constant-flow orifices. This results in the damping force generated during the rebound stroke of the shock absorber being greater than the damping force during the compression stroke, thereby meeting the requirement for rapid damping.
