The Design Of A Racing Shock

Keeping the Oil and Gas Separate The nitrogen gas we put into our shocks to compress and allow for the volume of the shock shaft must be separated from the fluids in all shocks. The gas can be contained inside a plastic bag or separated by another piston. Because the bags used in twin-tube designs 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 of gas created 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 be separated from the fluids at all times. That is because 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.

The Effect of Installation Ratios If the speed at which the shock moves determines the number of pounds of resistance, then how and where we install the shock is 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 that it is mounted to, the slower the shaft will move in relation to the wheel movement.

If shaft speed determines the shock resistance, then the slower the shock moves, the less rate of resistance it will have. Therefore, if I mount the same shock two inches from the ball joint verses mounting it six inches from the ball joint, I will have much more shaft speed and more pounds of resistance with the first mount. It makes sense that I would need a much stiffer shock when mounting it at six inches than I would need if mounting it at two inches. Many racers miss this important point.

Linear and Digressive DesignsThere are two basic piston designs used in most circle track racing. One is the Linear piston which has a high flow rate at low shaft speeds and hence little resistance, and increases resistance as the shaft speed increases. The rate of the shock continues to increase as long as the speed increases.

The other design, using a Digressive piston design, has a low flow rate at low shaft speeds which provides a lot of resistance and control, and then the resistance rate increases with increased shaft speeds to a pre-determined level and then tapers off. As the shaft speed continues to increase, the resistance stays uniform above a certain shaft speed. This "pop-off" characteristic works well for reducing the possibility of building excessively high amounts of resistance, usually associated with sharp increases in shaft speeds due to running over bumps and holes in some racetracks.

A New Trend Takes Shape Another development that has recently become popular involves using more rebound resistance than compression control. If we read the older automotive design books related to shock design for production automobiles, we see where the design criteria called for equal resistance in both directions, compression and rebound, in combination with the action of the springs. Because springs naturally resist compression and aid in rebound (unloading of load) we need less compression and more rebound control in our shocks to be truly equal in each direction of movement.