Shock absorbers have historically been the most mysterious component of a race car suspension system, and rightfully so. Although the basic function of a shock absorber is quite elementary, the technology of making it perform that function and satisfying racers' needs is still an evolving science-an art.
In a passenger car the shock absorber's main function is to damp wheel and chassis vibration associated with suspension movement. In racing, we use shocks to tune our chassis performance by controlling weight transfer to and from each of our tires.
The shock achieves this damping and control by providing forces that oppose the direction of movement and are dependent on the speed of movement. Unlike a spring, which stores energy and releases it mechanically, a shock absorbs mechanical energy and releases it in the form of heat.
Shock Absorber ConstructionThe two major types of shock absorber construction are twin-tube and mono-tube. Twin-tube shocks have historically predominated in most passenger car and circle track applications. However, recent trends marked with shock specialists on race teams have shifted toward mono-tube designs.
Twin-Tube Design And OperationA twin-tube shock is constructed with two tubes that form two chambers. The inner tube is the working chamber where the piston and rod move. The outer chamber is the oil supply chamber, which is filled with a column of oil beneath air. The air is at a pressure around 100 psi. Damping is achieved with a bottom valve and a piston valve. The construction of the valves varies with manufacturer. The most common is a stack of spring discs whose thickness and diameter determine the valving.
During compression, the piston is moving toward the bottom valve forcing oil to move through the piston valve that is nearly unrestricted in that direction. Because the piston rod is displacing oil in the working chamber, oil is forced through the bottom valve into the supply chamber. Compression damping is controlled by the restriction provided by the bottom valve.
During rebound, the damping is controlled entirely by the piston valve. Oil flows freely from the supply chamber to the working chamber to displace the volume lost by the shaft exiting the body.
With this understanding, it is worth noting that a twin-tube shock that does not have a separating bladder between oil and air in the supply chamber will not function upside-down. This is because air will always rise to the top and, if it is supplied to the working chamber instead of oil, the compression damping will be severely effected.
Mono-Tube Design And OperationMono-tube shock absorber construction consists of a single cylinder that contains a piston separating oil and compressed gas. The separating piston is free-floating. Consequently, the pressure will always be equal on both sides of the separating piston. The initial gas pressure varies but can be around 300 psi. Both rebound and compression valves are located on the working piston.
During rebound, oil is forced through the valve on the working piston facing the separating piston. The amount of restriction provided by this valve is solely responsible for rebound damping. The piston shaft exits the body and the separating piston moves toward the oil to compensate for this lost volume in the oil region.
During compression, oil flows through the valve on the shaft side of the piston. The amount of compression damping is determined by the flow restriction provided by this valve. The separating piston moves toward the compressed gas in order to equalize the pressure increase caused by the shaft displacing oil volume.
Heat FadeAs a shock absorber converts mechanical energy into heat energy, its temperature will rise. Heat in a shock can affect its damping characteristics in a number of ways. As the temperature rises, the oil viscosity decreases. Since the restriction to flow through the valves is dependent on viscosity, the damping forces will decrease as the oil viscosity decreases.