This invention relates generally to shock absorbers, and more particularly to shock absorbers of the fluid displacement type adapted especially for use on automotive vehicles, although usable in cushioning or braking shocks sustained by other devices and mechanisms.
Pistons for hydraulic shock absorbers of this type include valves which are caused to open more or less as a function of the difference in pressure on opposite sides of the piston head within the working chamber contained by the shock absorber casing. Oil passages controlled in such manner are normally complemented by the provision of additional passages which are not valved, such additional passages being typically referred to as "permanent passages."
These permanent passages function to effect a more comfortable ride when the shock absorber is used in connection with vehicle wheel spring suspensions. The cross-section of these permanent passages should, however, be comparable to the valved oil passages when open by the valves during reciprocation of the piston at a moderate speed.
The shape of these permanent passages is also important for effecting a more comfortable ride, and it is known that thin slits defining such passages produce a greater degree of comfort as compared to round holes. Permanent passages defined as round holes effect a sudden change from a laminar flow to a turbulent flow which therefore modifies the discharge rates and takes place much more rapidly and frequently than with the use of thin slits having the same cross-section.
However, resistance to the flow of hydraulic fluid through thin slits varies greatly in accordance with the viscosity of the hydraulic fluid used in the shock absorber. Therefore, those shock absorbers having permanent passages formed in the piston thereof in the shape of thin slits have an efficiency which diminishes substantially upon an increase in temperature of the piston. This, in practice, is an unacceptable drawback.
In order to diminish this loss of efficiency with rising temperatures, hydraulic fluids of low viscosity have been selected, although this has not been found a satisfactory solution because the low viscosity oils facilitate and increase the frequency of the sudden and uncontrolled changes from laminar flow to turbulent flow. This loss of efficiency from temperature factors is not, moreover, completely corrected by using a low viscosity oil, and this phenomenon, which affects many presently designed shock absorbers, has not yet been satisfactorily solved, even when the permanent passages are defined by round holes.