The piston mounted to the end of the shaft also contains a valving mechanism that allows the fluid to flow through slots designed into the piston. These valves consist of disks that open as the pressure increases due to more rapid shaft movement in either compression or rebound. These disks are used to control the damping rate associated with higher shaft speeds.
Shaft Displacement
An important consideration when designing a racing shock is called shaft displacement. When the shock shaft is pushed into the shock body and the fluid, it takes up space. Suppose we pull the shock shaft out as far as it will go, fill the shock body with oil, and then reseal the shock body. If we tried to push the shaft into the shock body and the volume of oil, it would not go. The shaft would be trying to displace some of the oil and oil cannot compress, so none of the oil can escape.
We need to create a space inside the shock and fill it with a substance that will compress. Gases will compress, so every shock needs to have a certain volume of gas along with the fluid, in order to allow for the displacement of the fluid as the shaft moves into the shock body and takes up space.
The gas cannot be air or any gas that contains moisture (water) due to the heat generated by shock/fluid movement. The moisture will heat up and expand and cause high-pressure buildup inside the shock. Nitrogen is a dry gas that suits our purpose and is widely used as a gas filler in racing shocks.
As the piston moves inside...
As the piston moves inside the shock body, the fluid flows through thepiston first using the slow-speed bleed holes drilled into the pistonbody. As the speed and pressure increases, the discs are forced open andthe fluid is free to flow through slots cast into the piston and theflow is increased. The thickness of the disc, as well as the number anddiameter, all help determine the exact amount of resistance tocompression and rebound.
Keeping the Oil and Gas Separate
The nitrogen gas we put into our shocks to allow for the volume of the shock shaft must be separated from the fluids in twin-tube shocks. The gas can be contained inside a plastic bag, and the plastic provides the seal between the gas and the fluids. Because the bags are not pressurized, these shocks are referred to as "non-pressurized" shocks. In truth, as the shock shaft is pushed into the shock body, some amount of pressurization must take place due to the displacement of the shaft and smaller volume inside the body. The gas bag must contract, which creates a small amount of pressure.
In mono-tube shocks, separation is accomplished by installing a second separator piston, which provides a seal that separates the fluid from the gases. A valve that is installed in the shock body at the gas chamber end of the shock allows us to pressurize the gas inside the shock. This pressure ensures that the gas will be forced to remain separate from the fluids at all times. The seal on the piston will usually allow air to seep past the piston from the fluid side to the gas chamber, but seals the heavier fluid from escaping into the gas chamber.
Installation Ratios Effect
If the speed at which the shock moves determines the number of pounds of resistance, then how and where we install the shock is also a consideration. If the shock were to be mounted at the center of the front wheel or on top of the ball joint, then the vertical wheel speed would equal the shock shaft speed. This is never the case in a stock car because of installation issues. In the front suspension, there is always an installation ratio and shock angle that affects the relationship between the vertical wheel movement and the shock shaft velocity. The farther the shock is mounted from the ball joint and the greater the shock angle from 90 degrees off the control arm it is mounted to, the slower the shaft will move in relation to the wheel movement.