Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations that occur during driving. To absorb these unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and the pressure tube is normally attached to the unsprung portion of the vehicle. The piston is normally attached to the sprung portion of the vehicle through a piston rod that extends through the pressure tube. The piston divides the pressure tube into an upper working chamber and a lower working chamber, both of which are typically filled with a hydraulic liquid. Because the piston is able, through valving, to limit the flow of the hydraulic liquid between the upper and lower working chambers when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force that counteracts the vibration that would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle. In a dual tube shock absorber, a fluid reservoir or reserve chamber is defined between the pressure tube and a reserve tube. A base valve assembly is disposed between the lower working chamber and the reserve chamber to also produce a damping force that counteracts the vibrations that would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle.
Shock absorbers filled with hydraulic liquid have met with continuous success throughout the automotive industry. While meeting with success in the automotive industry, hydraulic liquid filled shock absorbers are not without their problems. One problem with these prior art shock absorbers is that they are not sensitive to the frequency of the vibrations. Complex systems have been developed to modify these liquid filled shock absorbers to provide a shock absorber that is relatively soft for high frequency vibrations while being relatively stiff for low frequency vibrations. Other problems associated with the prior art hydraulic liquid filled shock absorbers include the variability in their damping forces due to temperature changes of the hydraulic liquid. As the temperature of the hydraulic liquid changes, the viscosity of the liquid also changes, which significantly affects the damping force characteristics of the liquid and, thus, the shock absorber. In addition, any aeration of the hydraulic liquid during operation of the shock absorber adversely affects the operation of the shock absorber due to the introduction of a compressible gas into a non-compressible liquid. Finally, the hydraulic liquid adds to the weight of the shock absorber, as well as presenting environmental concerns regarding the use and disposal of a hydraulic liquid.
In an effort to overcome the problems associated with shock absorbers that utilize hydraulic liquid as the damping medium, shock absorbers that utilize a gas as the damping medium having been developed. The use of a gas, preferably air, as the damping medium produces a frequency dependent damper or shock absorber that is significantly less sensitive to temperature when compared to hydraulic liquid dampers, is not adversely affected by aeration over time, is lower in weight and, especially when the gas is air, is environmentally friendly due to the elimination of the hydraulic oil.
While gas shock absorbers have resolved some of the issues that relate to hydraulic liquid shock absorbers, they are not without their own problems. One problem associated with gas shock absorber is a relatively high static push-out force that reacts against the piston, tending to extend the shock absorber. This static load is caused by the high pressure gas within the shock absorber in conjunction with the fact that the piston rod is located on only one side of the piston.