A strut is a structural component designed to resist longitudinal compression. Struts provide outwards-facing support in their lengthwise direction, which can be used to keep two other components separate, performing the opposite function of a tie. They are commonly used in architecture and engineering, for instance as components of an automobile chassis, where they can be passive braces to reinforce the chassis and/or body, or active components of the suspension.
An automotive suspension strut combines the primary function of a shock absorber (as a damper), with the ability to support sideways loads not along its axis of compression, somewhat similar to a sliding pillar suspension, thus eliminating the need for an upper suspension arm. This means that a strut must have a more rugged design, with mounting points near its middle for attachment of such loads.
The most common form of strut in an automobile is the MacPherson strut. MacPherson struts are often purchased by the automakers in sets of four completed sub-assemblies: these can be mounted on the car bodies as part of the manufacturers' own assembly operations. The MacPherson strut combines a shock absorber and a spring in a single unit, by means of which each wheel is attached to the car body.
Struts were commonly used in early aircraft to support wings, stabilizers, A-frames or triangle control frames (TCF) in gliders and hang gliders, and landing gear. Starting from 1930s they were mostly replaced with cantilever constructions, and became rarely used, mostly in light aircraft. Applications of struts are used in cars called struts in aircraft, these structural members are frequently loaded in tension, but may vary between tension and compression as the aircraft maneuvers.