Vibration dampers or shock absorbers differ basically in the type of friction, which causes the transformation of oscillation energy into heat, see figure.
While shock absorbers of vehicles in the 20s and the 30s operated with mechanical friction, liquid dampers became generally accepted in the course of development, since the resulting exponential relationship between damping force and the spring rate allows for a better adaptation of the absorption characteristics to the vibratory vehicle system. It enables an asymmetrical configuration for rebound and compression with relatively ease. These dampers also predominantly show a better response (damper force application at very low compression velocities).
The first shock absorbers based on liquid friction were called the lever dampers. In these dampers, oil is forced through a valve by a piston which is displaced by a lever. The lever is often realised in the form of a transverse arm. This type of damper is rarely used nowadays. Telescopic shock absorbers are used almost exclusively today.
Apart from the absence of wearing parts such as levers, bearings and operating cams, these dampers are advantageous from the point of view of the possibility of more precise adjustments in damping in comparison to lever-type dampers. This is due to the fact that the spring travel and plunger lift mostly correspond. Due to the large plunger lift, the liquid recirculation is larger. The direct damper can hence operate with a much lower pressure (compared with the lever damper), which facilitates the design of the damper valves and has a favourable effect on its life span.