A sectional view of the twin tube damper is shown in Fig. 1.3-33. The working volume, in which the piston moves, is completely filled with oil while the reserve volume between the working cylinder and the container pipe is filled with up to 2/3rd its level by oil. This tube space serves as a compensating volume for the oil displaced by the immersing piston rod. The damping valves - valves at the base and piston valves - consist of a system of spring washers and valve bodies with throttle bores. The valves are held in position against the valve seat by a weak coil spring and also simultaneously function as non return valves.
The Gas Pressure Twin Tube Damper was developed specially for its application in suspension struts. With spring suspension and shock absorber strut wheel suspensions, the piston rod of the damper is not only used for the transfer of the damper forces, but also simultaneously for wheel guidance. In order to keep a deflection of the piston rod caused by the transverse forces within limits, large piston rod diameters (~ 25 mm) are necessary in comparison to dampers or spring carriers exclusively stressed by expansion and compression (~12 mm).
As a result of the larger rod diameters, during the inward and outward stroke of the piston rod, the oil volumes flowing from the working chamber into the compensating volume and vice versa are also correspondingly larger. This results in a larger risk of cavitation at the non return valves in the piston and base of the damper, which can lead to a brief failure of the damper. A significant improvement of the operational behavior of these dampers can be obtained if the oil reservoir is exposed to a relatively small gas pressure (usually 6-8 bar).
This way, a more precise operation of the damper can be achieved without having to accept the disadvantage of a mono tube damper, where the response of the damper is impaired by the clamping friction due to the piston rod seal which has to be dimensioned with high surface pressure resulting from the high internal pressure.