Checking Shock Absorbers

Checking of shock absorber


















Note: When testing and evaluating gas pressure shock absorbers, a fundamental difference between two designs must be made. Difference refers to expansion and to the separation of oil and gas chamber.

Gas pressure shock absorber with separating piston between oil and gas chamber (Bilstein, F & S starting November 1980).
Installation position of shock absorber with piston rod in upward direction.

Gas pressure shock absorber without separating piston between oil and gas chamber (F & S up to October 1980 and Boge).
Installation position of shock absorber with piston rod in downward direction.






1 Cylinder
2 Operating piston with spring washers
3 Piston rod
4 Closing package with piston rod seal and piston rod guide
5 Separating pistons
6 Baffle plate
a Oil chamber
b Gas chamber


Oil reserve in shock absorber

The oil reserve in the shock absorber is determined by the length of exposed piston rod, "a"
The temperature of the shock absorber should be approx. 20 °C when the oil reserve is measured.

In the event of an oil loss, the length of the exposed piston rod increases on shock absorber with separating piston; the length decreases on shock absorbers without separating piston.

If the permissible length are exceeded or not met, replace shock absorber because it has lost its effectiveness.

Shock absorber with separating piston






Push-in piston rod up to stop of operating piston on separating piston. Now measure exposed length "a"

Shock absorber without separating piston






Compress shock absorber - with piston rod in upward direction - until a clearly noticeable, additional resistance begins, that is until the piston makes contact with oil column. Now measure length of exposed piston "a".

Note: When checking oil reserve in shock absorbers without separating piston, any occurring intermediate noises are without significance.









Sight test

Check piston rod carefully for surface damage.

Check piston rod for bends. A bent piston rod is recognized by binding when inserted into guide bushing.

Note: For lubricating guide bushing outside piston rod seal, the piston rod is designed to provide a slight oil film.

The alignment of the suspension points is important for the correct function of the piston rod seal. In the event of leaks on piston rod seal, be sure to check whether alignment of suspension points is in order.

Rumbling and knocking noises

Check upper suspension for correct assembly, lower suspension for tight seat of fastening bracket and rubber mount in housing eye.

Determine oil reserve. It oil loss is very high, shock absorbers with separating piston show a tendency toward knocking, since during deflection the piston rod may knock against separating piston. On rear mounted shock absorbers, check alignment of upper suspension point on frame floor to lower suspension point on semitrailing arm.

A loose operating piston may be responsible for the knocking.

To check, push piston rod inwards in installation position of shock absorber, release and push-in again. If the operating piston is loose, a change between pushing and pulling will be noticed by a knocking nose.

Hissing noises

Shock absorbers with separating piston have a tendency toward hissing noises if the separating piston is leaking, since gas will enter oil system and will cause foaming. Such shock absorbers may actually still be fully operational, but should nevertheless be replaced.

Attention! Shock absorbers with separating piston without an absolutely solid separation of oil and gas chamber can be checked for noises etc. in installation position, that is, with piston rod in downward direction. If the noise test has been preceded by testing the oil reserve (with piston rod pointing in upward direction) or if the shock absorber has been in storage with piston rod pointing in upward direction, the oil is mixed with gas.

Noises can be evaluated only after pushing the piston rod several times inwards.