Shock absorber oils have to satisfy, jointly with the mechanical device of the shock absorber, a certain number of technical targets to ensure satisfactory behavior when in service on the car. This paper details the technical requirements for the oil with respect to the following properties:
damping diagram versus viscosity temperature dependance, shear stability, biphasic flow;
antiwear properties to avoid lateral seizure between piston and cylinder (critical in Mac Pherson-type suspensions);
low noise generation.
The latter characteristic has been thoroughly evaluated to identify the sources of noise and the means to control them. This paper distinguishes between two main noise sources: low frequency noise from road seal friction which is easily controlled through the use of friction modifiers and high frequency noise due to a biphasic flow which is more difficult to control. The influence of the nature of the gas and of the oil structure (OSWALD Coefficient) on noise generation as well as the mechanical means to control this noise are discussed.