Bump, or compression, occurs when the shock shaft is being moved into the body. This occurs on the front of a bump, the back of a rut, the right side when turning left, the left side when exiting a left turn, the front under braking, and the rear under acceleration.
Rebound, or extension, occurs when the shaft is being pulled from the body. This occurs on the backside of a bump, the front of a rut, the left side shocks in a left turn, the right side shocks exiting a left turn, the front under acceleration, and the rear under braking.
How Shocks Affect Handling
The low- and medium-speed valving of the shock controls how fast weight is transferred. This affects the load on the tire and can change the handling balance while weight is being transferred. Once all weight has been transferred, the shock now longer influences handling. Since weight is almost always being transferred, the shocks are almost always affecting handling balance.
In general, rebound damping controls how fast weight leaves a tire while bump damping controls how fast weight goes onto a tire. Stiffer valving causes a shock to react more quickly; softer valving slows the reaction of the shock. Stiffer valving gets the load to change more quickly. Stiffer rebound valving gets the load off a tire more quickly and onto an opposite tire faster. Stiffer bump valving gets the load onto that tire faster. If all the valving, both bump and rebound at all four corners, changes equally, there is no effect on handling balance. If only bump or rebound is changed, then there is an effect. If only one end or one corner is changed, there is also an effect.
Analyzing Shocks
Shocks should be checked regularly for binds and pitting in the shafts. It is a good idea to check for dead spots by extending the shock fully and putting a sudden load on the shock by hand. Do the same with the shock fully compressed and pull out the shaft abruptly. A dead spot will be obvious, and that shock needs to be replaced or rebuilt.
