Undesirable longitudinal vibrations on road roughness can be caused by incorrectly selected spring rates and resistances to bump and rebound of shock absorbers. Often longitudinal rocking occurs on a car with weak shock absorbers and very strong springs: such a suspension does not swallow roughness, but pushes away from them, while vibrations of the body increase.
Bump resistance of suspension is created by springs and shock absorbers. If a compressive force is applied to a suspension, a suspension movement, by which it is compressed, depends on springs. Shock absorbers are necessary to dampen compression-stretching vibrations of springs and create additional resistance to the movement. How quickly bump or rebound occurs, mainly depends on shock absorbers.
When suspension springs are compressed, shock absorber rod moves inside shock absorber body, the shock absorber creates bump resistance. When shock absorber rod is pulled out of the body, the shock absorber creates resistance to so-called rebound.