In the early 1900’s cars still rode on carriage springs. Ride quality wasn’t nearly as important an issue back then as just keeping the car rolling over the rocky, rutted surfaces that often passed as roads. Early suspensions consisted of the front wheels being attached to the axle using steering spindles and kingpins. This permitted the wheels to pivot while the axle remained stationary. In addition, the up and down oscillation of the leaf spring was damped by a device called a shock absorber. These first shock absorbers were nothing more than two arms connected by a bolt with a friction disk between them. Resistance was adjusted by tightening or loosening the bolt. Needless to say, the shocks did not last very long and performance was distinctly lacking. Thankfully, shock absorbers have improved greatly since then!
Many people mistakenly think shocks support the vehicle’s weight. In fact, the main job of shock absorbers is to control spring and suspension movement. In technical terms, kinetic energy of suspension movement is turned into thermal energy (or heat energy), which is then dissipated through the hydraulic fluid. Shock absorbers are essentially oil pumps. A piston is attached to the end of the piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes, called orifices, inside the piston. Because these orifices let only a small amount of fluid through the piston, this slows down the piston and consequently slows down (or damps), spring and suspension movement. The amount of resistance a shock absorber develops is dependent upon the speed of the suspension, as well as the number and size of the orifices in the piston. All modern shock absorbers are velocity sensitive hydraulic damping devices; the faster the suspension moves, the more resistance the shock absorber provides. This is how shock absorbers adjust to road conditions and what allows them to reduce the rate of bounce, roll or sway, brake dive and acceleration squat.
The fluid displacement in shock absorbers happens on both the compression and extension cycle. The average car or light truck will have more resistance during its extension cycle than its compression cycle; the compression cycle controls the motion of a vehicle’s unsprung weight (the vehicle weight not supported by the springs, including the steering knuckle, brake assembly, tire and wheel), while extension controls the heavier sprung weight (the mass of the vehicle that is supported by the springs, including the body, engine and transmission).
There are two main types of shock absorbers available; twin tube and mono tube. Twin tube designs are also available as gas charged, PSD (Position Sensitive Damping), and ASD (Acceleration Sensitive Damping). Each has their own strengths and weaknesses having to do with ride control and comfort, as well as mounting options. Which shock is right for you will depend on your vehicle and how you use it, as well as how important certain ride characteristics are to you (ie, some people are willing to compromise on a comfortable ride for precision handling, while for others the comfortable ride is all-important).