thumb|[[Lotus Twelve]]
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.
Origins
Stout Scarab
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.
MacPherson strut
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. This was developed as a 1.5-litre Formula 2 in 1957, but re-engined in 1958 it also competed in Formula 1. The connection between the hub carrier and the shock absorber base was made rigid, making the spring unit into a strut and also controlling the camber angle. Using the drive shaft itself to control the rear track meant that the shaft could be of fixed length, avoiding the need for a sliding spline joint and its stiction problems.
Chapman applied for a patent on his strut design on 13 June 1957.
The Chapman strut also appeared on the Lotus Sixteen of 1958. In 1959 this design was revised to use a lower wishbone. This in turn required a return to a sliding joint in the drive shaft.
Comparison to the MacPherson strut
The Chapman strut is often confused with the MacPherson strut, as both use a near-vertical upper strut, The distinction is that the MacPherson strut always requires a lower track control arm where the Chapman uses the drive shaft and a light radius rod.
As it requires a drive shaft, the Chapman strut can only be used at the driven end of the vehicle. For the Lotus designs to which it was applied, this was their rear. MacPherson struts are most commonly used for front suspension and the strut axis forms the kingpin bearing, for steering movement. There is little reason why a Chapman strut could not be used similarly, although there is no record of this being tried on a front-wheel or four-wheel drive car. Both designs are struts, where the strut base is rigidly attached to the hub carrier.
Chapman did use MacPherson struts for the front suspension of the Lotus 17. These were unsuccessful, owing to the stiction problems that Chapman had worked to avoid with the sliding drive shaft joint. A wishbone conversion kit was later developed by ex-Lotus engineer Len Terry.
Applications
thumb|[[Lotus 16 Grand Prix car<br>Note the height of the rear strut]]
Racing cars
:* Lotus Twelve
:* Lotus 15
:* Lotus Sixteen
:* Lotus 17
Road cars
thumb|left|[[Lotus Elan#1960s model|Lotus Elan backbone chassis]]
Lotus road cars after the Twelve also used the Chapman strut for rear suspension. These included the fibreglass platform-chassis Elite and the backbone chassis Elan. Chapman struts, and their wide separation of load paths into the chassis, were a good fit to a stressed-skin structure such as the Elite and may have been an influence on its choice.
:* Lotus Elan
:* Lotus Europa
Successor designs
thumb|upright|[[Lotus 18 with wishbone suspension]]
The successful mid-engined Lotus 18 of 1960 no longer had the high bodywork behind the driver's seat on which to mount the tops of the struts. Instead it used a wishbone rear suspension with wide-based tube lower wishbones carrying coilover shocks. The upper wishbones though were, once again in the style of the Chapman strut, using the driveshafts themselves and a front radius rod. This is similar to the Jaguar independent rear suspension, which also uses the driveshafts as an upper control arm.
The 2016 all wheel drive version of the Fiat 500X crossover based on the Jeep Renegade uses multi-link Chapman strut suspension at the rear, with MacPherson struts at the front. Although the driveshafts are a suspension link, making this a Chapman strut, there are additional transverse links as well.