Originally it had only little advantage over the single-tube shock absorber. Although the gas area between the tubes allowes a somewhat shorter construction, the disadvantage is less efficient cooling. The first examples also had an open compensation area and when unsuitably stored, could leak. In the meantime, this area is closed and the gas is held at a pressure of up to 8 bar.
Double-tube dampers are also operated with gas, indeed, with a little less pressure. The pre-pressure decreases foaming and the development of bubbles, the same as the single-tube shock absorber. The space between the two tubes, which is filled to approx. 2/3 with oil, serves as a compensation area. Two base check-valves regulate the influx and outflow respectively.
The typical characteristics of the hydraulic telescopic dampers can also be seen here. It is easier to compress them than to pull them apart. Thus the oscillations caused by the suspension can be efficiently damped, without limiting the comfort of the passengers.
Double-tube dampers are more suitable when it comes to tuning because they can be adjusted from the outside. If one compresses them completely, the nut on the damper-plunger is accessible using a suitable spanner. If this is then turned, the damping effect can be altered. If there is also a nut on the plunger-rod side, the compression and the rebound can even be seperately adjusted. In the two-wheeler field, through the plunger-rod in permanently installed dampers, this type of adjusting is indeed normal.
Because the regulating of a permanent base-valve is more easily possible than with a movable floating piston, the double-tube shock absorber has become the basis for further electronically controlled dampers.
Incorrect assembly or storage can allow gas to penetrate into the plunger area. 11/10