A springing device must be a compromise between flexibility and stiffness. If it is more rigid, it will not absorb road shocks efficiently and if it is more flexible it will continue to vibrate even after the bump has passed. So we must have sufficient damping of the spring to prevent excessive flexing. The friction between the leaves of a leaf spring provides this damping, but because of the uncertainty of the lubrication conditions, the amount of friction also varies and hence damping characteristics to not remain constant. For this reason, the friction between the springs is reduced to minimum and additional damping is provided by means of device is called dampers or shock absorbers. In the case of coil springs, the whole of damping is provided by the shock absorbers. The shock absorbers thus control the excessive spring vibrations.

In fact the name shock absorber is rather misleading since it is the spring and not the shock absorber that initially absorbs the shocks. The shock absorber absorbs the energy of shock converted into vertical movement of the axle by providing damping and dissipating the same into heat. Thus it merely serves to control the amplitude and frequency of spring vibrations. It cannot support weight and has zero resilience. Therefore, damper is a better term technically to describe the shock absorber.

The shock absorbers are basically of two types the friction type and the hydraulic type. The friction type has almost become obsolete due to its non predictable damping characteristics. The principle of operation of a hydraulic shock absorber is that when a piston forces the fluid in a cylinder to pass through same hole, a high resistance to the movement of the piston is developed, which provides the damping effect. The hydraulic type has the additional advantage that the damping is proportional to the square of the speed. So for small vibrations damping is also small, while for larger ones the damping becomes automatically more.