The rebound control side of the shock dynamics resists the following:1.Rear chassis rebound vertical movement on deceleration.2.Front chassis rebound movement on acceleration.3.And left side rebound (in some cases, but not all) vertical movement caused by load transfer as we negotiate the turns.

The amount of resistance that the shock provides with each movement, compression and rebound, does increase with the speed at which the shock is forced to move. Low speeds create low resistance and high speed movement creates higher resistance. So, we have two areas related to resistance, low speed and high speed.

Low Speed Control Low-speed shock movement is defined as shaft speeds that are between one to five inches of movement per second. Many racers use three (3) inches per second to evaluate the shock's resistance. These lower speeds are mostly associated with suspension movement caused by chassis roll and chassis dive at turn entry where the loss of speed is moderate. The low speed control dictates much of the handling side of the shock design and racetrack performance gains related solely to chassis balance and load redistribution.

Each shock has a piston mounted on the end of the shaft and one or more small holes in the piston allow fluid inside of the shock to flow freely from one side of the piston to the other. The size of the "bleed" holes regulate how quickly the fluid will flow back and forth and that is how the different levels of resistance are created for low speed control. All low speed adjustments on shocks that are built with that adjustment capability work by changing the size of the bleed opening to control the amount of flow. This adjustment can be for both rebound and compression or just one.

High speed control As we experience the greater velocities of shaft movement, we go into what is called high speed control with shaft velocities of from 5 to 10-plus inches of movement per second. Types of suspension movement that cause the higher shaft speeds in our shocks are:1.Bumps or holes in the racing surface (creating very high shaft speeds).2.The driver stabbing the brakes on entry and hard on the throttle on exit.3.Or a sudden change in banking angle, such as transitioning from banking onto the apron of the racetrack.

The piston mounted to the end of the shaft also contains a valving mechanism which allows the fluid to flow through slots that are 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 dampening rate associated with higher shaft speeds.

Shaft Displacement An important consideration in designing a racing shock is called shaft displacement. When the shock shaft is pushed into the shock body and into the fluid, it takes up space. Suppose we pull the shock shaft out as far as it would 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 into the volume of oil, it would not go because the shaft would be trying to compress and displace some of the oil. Oil will not compress, and in a sealed environment, 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 such as air and nitrogen will compress. 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 should not be air or any gas that would contain moisture (water) due to the heat generated by shock/fluid movement. The moisture would heat up and expand, causing 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.