As you motor down the road, does your vehicle invoke motion sickness from the bobbing and weaving it continues to do, long after you’ve passed over that pothole or speed bump? Does it adopt a nose-up attitude worthy of a wheelstander each time you take off, or affect a gnarly slammed stance upon braking? If you answered yes to any of these, check your shocks or struts, pronto.

The shock absorber, and the related strut, represent parts that aren’t typically seen or even thought about, but whose job is crucial in keeping cars and trucks stable, comfortable, and safe. In broad terms, shocks and struts change kinetic (movement) energy to thermal (heat) energy through friction; they’re also–and more accurately–described as dampers, because they control excess suspension action as your wheels roll.

Unlike the tires, whose tread becomes visibly shallower as they wear, shocks and struts rarely physically show the deterioration that use and years compound. It’s therefore important to spot the signs of failing dampers, and to understand what these components do and how they differ, should you choose to upgrade them from standard replacement units for improved performance on the street, at the track, or off-road.

What’s the Difference?Shock absorbers have come a long way since the late 1800s, when their concept originated with dry, solid-material friction: to absorb suspension movement, rubber and bendable metal coils kept tension via compression, stretching, or bending. Fluid friction was a major advance in the early 1900s, when double-action rotary shocks were supplanted by lever arm, and then telescopic, or tubular, shocks. And the introduction of gas-charged telescopic shocks moved damping technology still further.

An offshoot of the shock absorber is the strut, the most common version patented in the late 1940s by Earle MacPherson. This component, often used by automakers because of its space-efficient design, consists of a shock absorber cartridge located in a tubular housing that can support a coil spring and connects to the hub or axle on the bottom; it’s linked to the body/frame by a lower control arm or wishbone (and, in front-wheel applications, a steering tie rod). Another type used in some rear-wheel drive racing and street applications is the Chapman strut, named for Lotus founder Colin Chapman. This design features a coil spring surrounding a shock absorber, and the tubular shaft’s lower connections to the body/frame are a driveshaft and radius arm.

Read on