Although you have surely seen driving around how they cut off the tank of a rear shock absorber and it was of no use, it must be said that this is not the case with real shock absorbers. If you remove the tank from a good shock absorber, it will lose all its effectiveness, since it is a fundamental part of its operation. Likewise, if it is not properly pressurized it is of little use.
There is no more graphic way to understand how a reservoir shock works than this video. Specifically, it is a reproduction of the Öhlins system in which we can see the main shock absorber and the tank. This has a double function and is that, on the one hand, it allows a greater amount of oil to be in the shock absorber and, therefore, makes the system cool better. But the most important part is that it allows the shock absorber to be pressurized, as we mentioned before, and the operation is much more stable.
The shock absorber reservoir prevents cavitation (as long as it is well pressurized)
For this, the tank also has its own plunger that helps with stability, separating the liquid area from the gas area found inside. But there is still more and that is that as you can see in the video, the moment the system does not work correctly (in this case by removing the pressurization) the oil inside begins to cavitate.
The cavitation process is a hydrodynamic effect, in which bubbles begin to create in the liquid as if it were boiling due to the decrease in pressure. That’s why you see the oil becoming cloudy. Then, when they reintroduce the necessary pressure, as if by magic, those bubbles disappear.
What constant pressure allows is precisely to avoid the formation of bubbles and therefore a much more homogeneous performance. It’s also a fantastic reminder that suspensions need to be checked periodically by professionals every two years, and even more so in the case of pressurized shocks. By the way, there are also pressurized forks on some supersports and in competition, and the operation is basically the same.