Myth: Shock absorbers are installed in a car to make the ride soft and smooth.
Truth: Shocks are actually there to keep the wheels on the road, nullify suspension oscillation and maintain stability. Ride characteristics are more a function of spring rates and suspension bushings, although shocks have a secondary effect.
When a car travels along the road and the wheels strike any change in surface the springs compress rapidly in reaction to the car's mass acting upon them. The compressed springs will attempt to return to normal position (rebound) and that causes the body to be lifted. Since the spring has newly-stored energy, it wants to rebound past the normal position and this causes the body to react to that force. The combination of movements that result is called spring oscillation and, if uncontrolled, will result in an uncomfortable ride and poor handling.
Added to that is the wheels themselves, since they are known as unsprung weight. When they bounce up and down in reaction to road surfaces they further add to suspension oscillations.
To overcome spring and wheel oscillation manufacturers use a damping device, which we call the shock absorber. Over the decades many types of shock absorbers have been designed and used, but by far the most popular is the hydraulic double-direct-acting (or telescoping) shock.
The shock absorber is attached to the frame of the car at one end and the suspension component at the other. When the spring attempts to compress or rebound, its action is hindered by the shock. As the frame rises and falls in relation to the axle, the shock telescopes in and out. Its resistance to the telescopic movement is what hinders the motion of the suspension components and dampens oscillations. The purpose of this is to return the car to its normal level as quickly as possible to maintain stability.
Telescopic shocks consist of an inner cylinder (pressure), outer cylinder (reservoir), piston and piston rod. Valves are incorporated in the piston (piston valve) and at the bottom of the cylinder (base valve), and these act to meter the movement of the hydraulic fluid inside the shock. It is the resistance of the fluid as it moves from cylinder to cylinder that creates the shock absorbing effect.
In the resting position the pressure cylinder is full of fluid and the piston inside is in its midway location. This allows for full up and down extension as the car moves. The outer cylinder is only partially filled with fluid.
As the car moves and the spring compresses, the piston moves down in the cylinder, forcing the fluid to pass through the piston valve into the upper section of the pressure cylinder. Since the piston now occupies space that was formerly filled with fluid, some of the excess fluid must be forced through the base valve into the reservoir cylinder. The valve openings are calibrated to create resistance to that flow, thus damping the spring's compression.
During the spring's rebound action the shock's piston is forced upward (pulled by its top mount on the frame or body) and the fluid trapped above it is forced through the piston valve into the lower part of the pressure cylinder. To compensate for the reduced amount of piston in the pressure cylinder, additional fluid is drawn in from the reservoir through the base valve.
Most shocks are calibrated to provide more resistance on rebound than on compression, as this provides the best overall dampening of spring oscillation. That's why the shock is much harder to extend than to compress. Shocks can be internally calibrated to provide a soft, normal or firm action and that is why some ride "stiffer" than others.
Some aftermarket shocks are externally adjustable for firmness, especially in racing applications and most modern shocks are gas-filled to provide an even greater amount of oscillation dampening.