WHY CHECK GAS PRESSURE IN YOUR SHOCKS?
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Having the proper gas pressure in your shocks is essential to proper performance. In this Tech Tip we tell you what happens when the gas pressure is not maintained and how to properly set this pressure to a defined specification. Having too high of a pressure is just as negative on the performance of the shock as being too low.
Why is there gas pressure in shocks?
Gas pressure is used in shocks to prevent cavitation. What is cavitation? Cavitation occurs when the shock pressure drops very close to absolute zero and the oil vaporizes. It can happen in either the compression or rebound chamber, but it's the rebound chamber that is the major concern. If the pressure in the rebound chamber drops, it causes cavitation and that is what causes the crazy cavitation dyno curves. This troublesome cavitation in the rebound chamber starts during the compression stroke when you want more compression force then your damper’s gas charge will allow you to attain. So in order to fix cavitation for a given piston build you have to find a way to increase the compression chamber pressure so the given pressure drop caused by the main piston valving doesn't bring the rebound chamber pressure down to near zero. The two easiest ways to increase the compression chamber pressure is by adding more restriction in the base valve and/or increasing the gas charge.
If there is compression cavitation, it could be caused by a bottomed out or stuck separator gas piston, the loss of the shock’s gas charge, an oil replenishment restriction or simply a mechanical malfunction. If cavitation is present in the compression chamber it is short lived and quickly collapses back into a fluid because the compression chamber volume is variable and is directly acted upon by the gas charge. The rebound chamber cannot do this since it has to regain the pressure it lost via fluid flow across the piston.
Typically, the oil in a shock is clear, as seen in this see-through shock.
However, if the shock cavitates, it produces gas bubbles as seen here.
How does this cavitation affect the damping force of a shock? Here is a Force/Velocity graph showing the damping force for a properly performing shock.
When the gas pressure starts to fall below specifications, the forces start to deteriorate. The more gas pressure you lose the more extreme the force loss becomes. This extreme loss in forces due to cavitation is shown in this Force/Velocity graph.
Note the loss of damping force! Is this how you want your shock to perform? Losing gas pressure not only causes cavitation, but with cavitation comes “lag” in the shock. When this happens there is no damping at all for a portion of the stroke. Which chart shows the performance of your shocks?
Shocks, like engines, need proper maintenance to keep them at peak performance. Shocks work with the suspension to get the horsepower to the pavement. Keeping the tire patch on the pavement is critical in getting around the track fast.
You cannot check shock pressure like a tire!
There is no way to "just check" the gas pressure in your shocks. Unlike a tire, shocks have very little gas volume. DO NOT try to use a tire pressure gauge to check the gas pressure. It will not work. You will let too much gas escape, and your shocks will not work properly! Even when using an inflation tool, the pressure could drop as much as 50 psi because of the tool’s design. So, to properly gas charge your shocks, you will have to use an inflation tool.
We recommend using a Kaz Tech Inflation tool to properly pressurize your shocks. Our tool allows for an accurate re-charge without wondering if your pressure is correct. No more wondering about the final results.
How to gas pressurize your shocks:
The first step in pressurizing the shock is the installation of the Inflation tool. Screw the fitting onto the Shraeder valve as shown below. You can usually hand tighten the fitting, but if necessary use a wrench to just snug it up. Be careful not to over-tighten because you could ruin the brass sealing washer. It is best to just hand tighten.
Next, turn in the “T” to depress the Shraeder valve.
Now it is time to pressurize the shock. Inflate the shock with nitrogen gas 10-20 psi over the specified pressure. Why nitrogen? Because it is more temperature stable than compressed air and it is clean and free of moisture. Using compressed air can introduce contamination and moisture into your shock causing problems with the seals.
Next, bleed out the excess pressure to obtain the shock’s specified pressure.
Now turn out the “T”, and remove the fitting from the shock Shraeder valve. You will hear a slight hiss when you remove the fitting from the shock. This is OK! You are just releasing the gas in the Inflation Tool.
You’re done for today! However, race shocks, like race engines require maintenance. You check the oil in the engine everyday because the oil is the life blood of the engine and it keeps it at peak performance. Your shocks are no different. You don’t have to check the oil daily, but you do have to maintain the gas pressure. The gas pressure, like oil, keeps the shock at peak performance. Put it on your preparation checklist.
Every time you check the engine oil, brake fluid and tire pressures, re-charge the shocks gas pressure.