The Story of Shock Absorbers Discussed

  A suspension spring on a vehicle will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce until all of the energy originally put into it is used up. A shock absorber is a device that controls unwanted spring motion through a process known as dampening. A shock absorber will slow down and reduce the magnitude of motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid.

   A shock absorber is like an oil pump placed between the frame of the car and the wheels.  In a twin tube design shock absorber, one of the most common types of shock absorber, the upper mount is connected to a piston rod, which is connected to a piston, which sits in a tube filled with hydraulic fluid. The inner tube is known as the pressure tube, and the outer tube is the reserve tube. The reserve tube stores excess hydraulic fluid. When a car hits a bump in the road, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston. Holes in the piston allow fluid to leak through as the piston moves up and down in the pressure tube. Because the holes are tiny, only a small amount of fluid under a lot of pressure, gets through. This slows down the piston in the shock absorber, which in turn slows down the spring.

   The shock absorber works in two cycles, compression and extension. The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber of the shock absorber below the piston. The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the shock absorber piston. A typical vehicle will have more resistance during its extension cycle than its compression cycle.

Basically, the compression cycle controls the motion of the vehicle's unsprung weight, while extension controls the heavier, sprung weight. All modern shock absorbers are velocity sensitive. This means the faster the suspension moves, the more resistance the shock absorber provides. This lets the shock absorber adjust to road conditions and to control all of the unwanted motions that can occur in a moving vehicle, including bounce, sway, brake dive, and acceleration squat.