Suspension damping is the resistance to movement by a piston travelling through hydraulic fluid. When a shock absorber or damper moves, the shock piston (connected to the shock shaft) moves through the fluid inside the shock tube. Pressure is created in the direction of movement and it is this pressure differential which creates the resistive force in the movement of the piston. This is known as the damping force. Control of the suspension damping force is the function of the oil, valves and orifices in the piston, which suspension engineers have determined based on the shock's intended application. How fast the piston is being pushed, called suspension velocity, also determines how the shock will react to a bump or dip in the road surface. As in the case with selecting springs, damping control is determined by application, and the optimal damping for comfort will be less than for fast road handling. Suspension damping modulates the velocity and resistance of a vehicle's suspension to unwanted movements. A good example of this is to look at a car with worn shocks. You will notice the vehicle wallowing and moving up and down like a boat in rough seas. A properly damped vehicle goes through a road with a minimum of body movement. The goal of a properly damped suspension is to have a vehicle settle down after a bump or dip in the shortest time possible.
The suspension component tasked to dampen the resonant motions of a vehicle's suspension is the shock absorber. This why it is technically called a damper. Aside from controlling the upward and downward movement of the vehicle body, a shock absorber must also dampen the unsprung components of the car, such as the hubs, wheels, axles and more often than not, the brakes. In a car will a solid axle, the shocks will also need to dampen the differential, which is a very heavy component. The movement of these components are also affected by the tires, which introduce some springing motions.
Suspension damping is velocity sensitive, which means that the damping force created by the shock depends on how fast the shock absorber is compressing or extending. While logic dictates that shaft velocity will double as you go through a bump at twice a given speed, it is also important to note that the shape of the obstacle is also as important. Given the same height, a square-edged bump will produce much more piston velocity than a gently rounded bump.
Compression damping is influenced by the size and shape of an obstacle and it will see a very wide range of velocities. Too much suspension damping on the compression stage cause unwanted tire deflection while too little causes brake dive and too much travel. Rebound damping, on the other hand, is mainly influenced by spring rate and sees less velocity changes. But rebound damping is equally important because too little damping will cause the suspension to oscillate uncontrollably.
It is generally preferable to have more low-speed damping for a good, firm controlled feeling, and less high-speed damping for a more comfortable ride on square-edged bumps. Once you recognize what adjustment you have and its effect, you can begin to fine-tune your suspension. But remember just because you have external adjusters doesn't mean you have much control of the damping curve. Remember also that with adjusters you can make an improvement in one area and actually make it worse in another with the same setting change. When playing with adjusters, record your initial settings make one change at a time and record all your results.
