MacPherson Strut

The MacPherson strut is a type of independent suspension, used overwhelmingly at the front of cars, that combines the damper and the coil spring into a single vertical strut which also forms a structural part of the suspension. Devised by the American engineer Earle S. MacPherson and brought to prominence in the early 1950s, it was conceived as a way to make independent front suspension cheaper, lighter, and more compact than the double-wishbone arrangements that preceded it. Its elegance lies in making the damper unit do double duty as a load-bearing member, which removes the need for an upper control arm altogether.

In its usual form the strut comprises a telescopic damper enclosed within a tube, with the coil spring mounted concentrically around it on spring seats. The top of the strut is attached to the body through a bearing-mounted mounting that allows it to swivel for steering, while its lower end connects to the wheel hub or steering knuckle. The wheel's position is located below by a single transverse control arm, often combined with an anti-roll bar acting as a locating link. As the wheel rises and falls the entire strut moves with it, the spring compressing and the damper controlling the motion.

The design's chief advantages are practical. By stacking the spring and damper into one unit and dispensing with the upper arm, it occupies little space across the car, which frees up width in the engine bay for transversely mounted engines and front-wheel-drive transaxles. It uses fewer parts, is straightforward to manufacture and assemble, and is comparatively light. These attributes made it the natural choice for mass-produced cars, and it remains by a wide margin the most common front-suspension layout in the world.

There are inherent compromises. Because the strut is tall and the wheel's geometry is governed largely by the single lower arm and the strut's own movement, the designer has less freedom to control camber change through the suspension's travel than a double wishbone allows. As the strut compresses, the wheel can lean in a way that reduces the tyre's grip in hard cornering, so the MacPherson strut is generally regarded as offering less precise geometry control than a wishbone system. Its height can also intrude into the engine bay or boot area.

For the great majority of road cars these trade-offs are well worth accepting, and careful tuning of the geometry, bushes, and mounting can yield excellent results. The MacPherson strut sits within the wider family of independent suspension, using a coil spring and a damper as its essential elements, and it stands as the practical everyday counterpart to the more sophisticated and costly double-wishbone and multi-link designs.