A shock absorber is a suspension component that controls the up-and-down motion of the vehicle’s wheels.
Though the devices are called shock absorbers, the job of absorbing the jolts that result when the wheels pass over bumps or dips is handled mostly by the springs. Shock absorbers would more accurately be named dampers, as they are called in the United Kingdom. Their main job is to damp the motion mentioned above, keeping the vehicle’s body from bouncing down the road on its springs. In fact, the simplest shock absorber test is to push down on the vehicle’s bumper; a healthy shock absorber or MacPherson strut will allow the body to rebound but quickly come to rest. If the vehicle goes into a prolonged bounce cycle, the shock needs to be replaced. Bad shocks or struts aren’t safe because they fail to maximize tire contact with the road, which degrades handling and braking.
Each shock absorber contains a piston that slides through a cylinder. Hydraulic fluid on either side of the piston head provides the resistance against which the up-and-down motion of the wheels must work as it compresses and expands the shock absorber. A valve in the piston head, and typically another at the base of the cylinder, allow the fluid, or oil, to flow through as the unit expands and compresses. But when the compression is faster or greater, the oil can’t squeeze through the valve fast enough, and it becomes pressurized. The more the oil is pressurized, the more it resists the piston’s motion.
The size of the valve aperture helps determine the shock absorber’s rate — how firm or soft it is. Firmer shock absorbers make for a harsher ride but may improve handling and help to minimize body roll. There are variable-rate shock absorbers — both manual and electronically controlled. The manual ones have a setting that the owner or a mechanic can adjust. Electronically controlled types are the key to adaptive suspensions, the most sophisticated of which are computerized to vary individual shock firmness many times per second in response to conditions.
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