Chapman strut

The Chapman strut is a design of independent rear suspension used for light cars, particularly sports and racing cars. It takes its name from, and is best known for its use by, Colin Chapman of Lotus.

The characteristic feature of the Chapman strut is a long upright strut combining a coil spring and shock absorber, with a universal-jointed drive shaft itself forming the lower link of the suspension.

The design's origin lies with William Stout's 1932 Stout Scarab. This rear-engined car used swing axle independent rear suspension, with long near-vertical coilover struts from high mounting points on the space frame chassis. Stout had also been an aircraft designer and considered that the long-travel oleo struts made, 'the airplane landing gear [is] the easiest type of running gear for comfort yet devised.' The lower ends of the struts were attached to the swing axle casings by swivel bushes. Forward radius rods handled the longitudinal forces.

In 1947 Earle MacPherson patented the MacPherson strut for use on the Chevrolet Cadet. This was the first true strut suspension for cars: the stub axle was rigidly fixed to the strut, thus always keeping the same angle with it, rather than being attached through a swivelling joint as for Stout's Scarab.

MacPherson's strut took advantage of the unitary car bodies appearing in new factories after World War II: the strut requires a high top mounting point, which would be difficult to arrange for a ladder chassis, but is also advantageous for unitary construction as it distributes suspension loads over a greater volume of the bodyshell. New car designs were also lighter and faster than pre-war cars, a cheap mass-production car now having performance that would previously have been the preserve of an expensive sports car. Drivers demanded roadholding and comfort that required independent front suspension. MacPherson's strut was cheap to manufacture and reliable in service, having few moving joints and using the new telescopic shock absorbers, rather than the previous lever arms. MacPherson's strut also integrated an anti-roll bar, not only for control of body roll but also re-using it as an integral part of the suspension and so avoiding the need for a wishbone or radius rod in favour of a simple track control arm.

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