The gas reservoir and its function in a pressurized shock is the topic of much discussion. To demonstrate it’s function, I decided to create a quick post on the topic. Gas pressure inside your reservoir type shock is separated from the shock oil either via a bladder or a separator piston. This is true whether you have an RFY, Penske, or Ohlins brand damper. Assuming that the damper is properly filled with fluid, pressurization improves the performance of pretty much all dampers. The introduction of gas pressure to the reservoir applies pressure to the oil, thusly reducing its compressibility. The reduction of compressibility leads to a decrease in shock oil cavitation. Cavitation is the formation of bubbles in the oil as a result of the piston forcing the oil through it’s orifices. Those bubbles reduce the viscosity of oil and ultimately mean you end up with less damping force.
On high end shocks where a compression piston exists, you can get away with lower gas pressure. However, budget shocks like the RFY, TEC Alloy, etc, that have no compression piston, you must maintain a relatively high gas bladder pressure. With the absence of the compression piston, gas pressure must be high enough to force oil through the main piston during the compression stroke. Without this, cavitation will almost certainly form. To summarize, you should never run your RFY or TEC shocks with low canister pressure, it is vital to the proper operation of the damper.
When rebuilding shocks, I usually apply a minimum of 75 psi to the reservoir. In my experience this is enough pressure to overcome the friction inherent in the system and return the shock rod to full extension. This should also be enough pressure to assure that during the compression stroke oil will be forced to open the valving on the main piston, rather than simply displacing into the reservoir. There’s a video below that illustrates this point.
It is important to note that reducing or increasing reservoir pressure will not effect spring rate in a meaningful way. If you bottom too often or feel that your ride is too stiff, adjusting spring rate is a more appropriate adjustment. Reservoir pressure is more closely related to damping.
The virtues of gas pressurization are many, as are the way in which various shocks utilize that system. If you have questions or comments feel free to comment.