A reservoir on a shock absorber serves as a storage container for hydraulic fluid that is displaced during the compression and rebound cycles of the shock absorber. The reservoir helps to regulate the pressure of the shock oil within the shock absorber and prevent aeration ( Cavitation ) or foaming of the fluid, which can negatively affect the shock absorbers performance.
Oil cavitation, also known as aeration, is a phenomenon that can occur in shock absorbers. Simply, this is when air becomes mixed with the hydraulic fluid. This can happen when the fluid is subjected to high pressure or turbulence, causing bubbles or "cavities" to form in the fluid.
When cavitation occurs in a shock absorber, it can lead to a loss of damping performance and increased wear on the components. This is because the presence of air bubbles within the hydraulic fluid can cause a reduction in the fluid's effective viscosity, which in turn reduces its ability to effectively dampen the vehicle. As these bubbles move through the fluid, they create areas of low pressure that can cause the fluid to vaporize, which generates heat.
During operation, the reservoir allows for additional oil volume to be displaced during high-speed or high-temperature conditions, providing increased cooling and allowing for more consistent performance over extended periods. Oil is separated from the gas charge through the use of an IFP ( internal floating piston). This piston is equipped with an Oring and wear band to prevent contamination of the gas into the fluid. This helps to reduce the risk of shock fade and improves the overall damping performance of the shock absorber.
While it is a common misconception that a reservoir “cools” a shock absorber through increased volume and air flow over the reservoir ( in a standard configuration) , the primary cooling method is through the reduction of cavitation in turn allowing the oil to perform under pressure keeping it from vaporizing. Less vaporized fluid = less heat.
Overall, the use of a reservoir in a shock absorber can improve its cooling and damping performance, which in turn can help to extend the life of the shock absorber and improve the overall consistent performance of the vehicle.