Strut

A strut is a suspension component that does double duty: it controls wheel movement like a damper while also serving as a load-bearing structural member that helps locate the wheel and support part of the vehicle's weight. This combination is what distinguishes it from a plain shock absorber. A conventional damper simply resists motion and carries no significant cornering or vertical load on its own, whereas a strut is built robustly enough to form part of the suspension's skeleton, standing in for the upper control arm that a double-wishbone layout would otherwise require.

The classic example is the MacPherson strut, in which a hydraulic damper is housed within a sturdy tube, a coil spring is mounted concentrically around it between an upper and lower seat, and the whole assembly is bolted to the steering knuckle at the bottom and to a reinforced point in the inner wing at the top. The lower end is located by a single transverse arm and an anti-roll bar, while the upper mount usually incorporates a bearing so the entire strut can rotate for steering. In this way one slim, vertical unit provides springing, damping, wheel location and, at the front, the steering pivot all at once.

Because it does so much in so little space, the strut is prized for its compactness and low cost. It frees up room within the engine bay and wheel arch, which is especially valuable on transverse front-wheel-drive cars where the powertrain already consumes much of the available width. With fewer separate links and joints than a double wishbone, it is cheaper to produce and assemble, which is why the strut dominates the front suspension of the vast majority of family cars and is also widely used at the rear.

The arrangement does carry inherent compromises. Because the strut leans and moves as a single unit, the camber and geometry change less favourably during suspension travel and body roll than a well-designed multi-link or short-long arm system, so the loaded tyre does not stay as square to the road. The tall installation also imposes side loads on the damper rod, which is one reason struts wear seals and bushings and can develop a characteristic top-mount knock. For these reasons performance and luxury vehicles often favour more sophisticated multi-link designs, accepting their greater complexity for sharper handling.

Servicing reflects the strut's integrated nature. Replacing the damper, spring or top mount usually means removing and dismantling the whole assembly, a job that requires a spring compressor to safely contain the energy stored in the coil. The strut sits at the heart of a family of related parts, drawing together the damper, the coil spring and the wheel-location function into one element, and understanding it is the key to understanding how most modern cars hold their wheels to the road.