Chapman strut
Encyclopedia
Sports car
A sports car is a small, usually two seat, two door automobile designed for high speed driving and maneuverability....
and racing cars. It takes its name from, and is best known for its use by, Colin Chapman
Colin Chapman
Anthony Colin Bruce Chapman CBE was an influential British designer, inventor, and builder in the automotive industry, and founder of Lotus Cars....
of Lotus
Lotus Cars
Lotus Cars is a British manufacturer of sports and racing cars based at the former site of RAF Hethel, a World War II airfield in Norfolk. The company designs and builds race and production automobiles of light weight and fine handling characteristics...
.
The characteristic feature of the Chapman strut is a long upright strut combining a coil spring and shock absorber
Shock absorber
A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. It is a type of dashpot.-Nomenclature:...
, with a universal-joint
Universal joint
A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion...
ed drive shaft itself forming the lower link of the suspension.
Stout Scarab
The design's origin lies with William StoutWilliam Bushnell Stout
William Bushnell Stout was an inventor, designer whose work in automotive and aviation fields was notable. Stout designed an aircraft that eventually became the Ford Trimotor and was an executive at the Ford Motor Company.-Early years:William Bushnell Stout was born March 16, 1880 in Quincy,...
's 1932 Stout Scarab
Stout Scarab
The Stout Scarab is a unique 1930s U.S automobile produced in small numbers by Stout Engineering Laboratories and later by Stout Motor Car Company of Detroit, Michigan...
. This rear-engined car used swing axle
Swing axle
A swing axle is a simple type of independent suspension first used in early aircraft , such as the Sopwith and Fokker, usually with rubber bungee and no damping....
independent rear suspension, with long near-vertical coilover
Coilover
A coilover is an automobile suspension device. "Coilover" is short for "coil spring over strut". It consists of a shock absorber with a coil spring encircling it. The shock absorber and spring are assembled as a unit prior to installation, and are replaced as a unit when the shock absorber has...
struts from high mounting points on the space frame chassis. Stout had also been an aircraft designer and considered that the long-travel oleo strut
Oleo strut (aircraft suspension)
An oleo strut is an air–oil hydraulic shock absorber used on the landing gear of most large aircraft. It cushions the impacts of landing and while taxiing and damps out vertical oscillations.-Operation:...
s 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 1949 Earle MacPhersonEarle S. MacPherson
Earle Steele MacPherson was an automotive engineer, most famous for developing the MacPherson strut in the 1940s.-Biography:Earle S. MacPherson was born in Highland Park, Illinois in 1891, and attended the University of Illinois...
patented the MacPherson strut
MacPherson strut
The MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot. It is widely used in modern vehicles and named after Earle S. MacPherson, who developed the design.-History:...
. 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 monocoque
Monocoque
Monocoque is a construction technique that supports structural load by using an object's external skin, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork...
car bodies appearing in new factories after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
: the strut requires a high top mounting point, which would be difficult to arrange for a ladder chassis, but is also advantageous for a monocoque 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 arm
Lever arm shock absorber
Lever arm shock absorbers were the first form of hydraulic shock absorber or damper used for car suspension. They appeared in the 1930s and were most commonly used in the 1950s and 1960s, but were replaced by telescopic shock absorbers in the 1970s...
s. 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.
The MacPherson strut appears to have had little influence on Chapman at this time (although he would later use it unsuccessfully for the front suspension of his Lotus 17
Lotus 17
The Lotus 17 was a sports car racing model built by Lotus Cars in 1959. Designed by Len Terry, it was built in response to the Lola Mk1 which was dominant against the previous Lotus model, the 11...
). Double wishbone suspension
Double wishbone suspension
In automobiles, a double wishbone suspension is an independent suspension design using two wishbone-shaped arms to locate the wheel. Each wishbone or arm has two mounting points to the chassis and one joint at the knuckle. The shock absorber and coil spring mount to the wishbones to control...
was already well-established for the front of high-performance cars. A front strut would also have required much greater height than was available in a racing car.
Goggomobil
In 1957, the lightweight GoggomobilGoggomobil
Goggomobil was a series of microcars produced in the Bavarian town Dingolfing after World War II by Glas.Glas produced three models on the Goggomobil platform: the Goggomobil T sedan, the Goggomobil TS coupé, and the Goggomobil TL van...
used a similar suspension design to the Scarab. Much simpler though, the Goggomobil had bare drive shafts that also acted as the suspension links, with suspension forces passing through the inboard Hooke joint
Universal joint
A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion...
s. The swing axle shafts were only carried by the trailing radius rods, rather than an axle casing, and the long struts were shortened to a simple coilover shock absorber.
Chapman's design
In 1956 ChapmanColin Chapman
Anthony Colin Bruce Chapman CBE was an influential British designer, inventor, and builder in the automotive industry, and founder of Lotus Cars....
was using de Dion rear axles for his Lotus
Lotus Cars
Lotus Cars is a British manufacturer of sports and racing cars based at the former site of RAF Hethel, a World War II airfield in Norfolk. The company designs and builds race and production automobiles of light weight and fine handling characteristics...
racing cars and also the Vanwall
Vanwall
Vanwall was a Formula One motor racing team that competed in the 1950s. Founded by Tony Vandervell, the Vanwall name was derived by combining the name of the team owner with that of his Thinwall bearings produced at the Vandervell Products factory at Acton, London...
. These had initially used transverse leaf spring
Leaf spring
Originally called laminated or carriage spring, a leaf spring is a simple form of spring, commonly used for the suspension in wheeled vehicles...
s, but later changed to coilover shock absorbers. Chapman had identified some of the de Dion's disadvantages as including its high unsprung weight
Unsprung weight
In a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels or tracks , and other components directly connected to them, rather than supported by the suspension...
, the need for sliding splines in the rear driveshafts, and also (a typical Chapman fixation) its insensitivity to changes in weight as fuel is consumed. The first Lotus Twelve Formula 2 cars used the same system, although the de Dion's mass was a drawback for a lightweight single-seater racing car.
At a 750 Motor Club
750 Motor Club
750 Motor Club is an auto racing club in the UK. It was founded in 1939 and was originally for Austin 7 fans . It later led to racing and the 750 Formula where specials were raced...
meeting in 1957, Chapman saw the Goggomobil system and was impressed by its Lotus-like simplicity and light weight. The use of the drive shafts for both drive and as a suspension component appealed to Chapman, who always favoured solutions that could make one component do double duty.
Chapman struts were introduced in Lotus' first single-seater car, the Lotus Twelve. This was developed as a 1.5 litre Formula 2 in 1957, but re-engined in 1958 it also competed in Formula 1. This same car also introduced Lotus' wobbly-web wheel
Wobbly-web wheel
The wobbly-web wheel is a form of metal disk wheel where the disk is 'wobbled' into spokes. This provides a stiffer, lightweight wheel.Wobbly-web wheels are best known through their iconic use on Lotus racing cars of the late 1950s and 1960s.- Theory :...
.
There were two differences from these precursors to the Chapman strut. As the Lotus Twelve had inboard disk brake
Inboard brake
An inboard braking system is an automobile technology wherein the disk brakes are mounted on the chassis of the vehicle, rather than directly on the wheel hubs...
s, there was no need to pass the braking torque through the radius rods. Chapman also avoided the swing axle and its camber
Camber angle
thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
changes with suspension travel, in favour of a drive shaft with two universal joints. 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
Stiction
Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the term "static friction", perhaps also influenced by the verb "stick"....
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 strutMacPherson strut
The MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot. It is widely used in modern vehicles and named after Earle S. MacPherson, who developed the design.-History:...
, as both use a near-vertical upper strut, The distinction is that the MacPherson strut always requires a lower track control armAlthough some strut suspensions use lower wishbones, MacPherson's patent covered the use of the anti-roll bar for longitudinal control as well, thus allowing a simpler one-bearing track control arm, rather than a broad-based wishbone with two inner bearings. 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
Lotus 17
The Lotus 17 was a sports car racing model built by Lotus Cars in 1959. Designed by Len Terry, it was built in response to the Lola Mk1 which was dominant against the previous Lotus model, the 11...
. 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
Racing cars
Road cars
Lotus road cars after the Twelve also use the Chapman strut for rear suspension. These included the fibreglass platform-chassis Elite and the backbone chassis Elan.The mid-engined Europa
Lotus Europa
The Lotus Europa was a two door mid-engined GT coupé built by Lotus Cars from 1966 to 1975. In 2006 Lotus began production of a totally new, Lotus Elise-derived design, a mid-engined GT coupé named Europa S....
used a modified version of the Chapman strut. Owing to the need to isolate the engine and transaxle
Transaxle
In the automotive field, a transaxle is a major mechanical component that combines the functionality of the transmission, the differential, and associated components of the driven axle into one integrated assembly....
from the chassis in order to reduce noise and vibration, the rubber mounts led to an excessive sideways movement of the driveshafts. In this case, sideways location of the hubs was controlled by an enlarged diagonal radius rod to the front. Rubber sandwich mounts were developed that allowed vertical movement whilst controlling lateral movement. Similar vibration problems were encountered with the Elite, but as these only had differential vibration to control rather than engine vibration, it could be managed by suitable design of the rubber mounts.
- Lotus Elite
- Lotus Elan
- Lotus EuropaLotus EuropaThe Lotus Europa was a two door mid-engined GT coupé built by Lotus Cars from 1966 to 1975. In 2006 Lotus began production of a totally new, Lotus Elise-derived design, a mid-engined GT coupé named Europa S....
Successor designs
The successful mid-engined Lotus 18Lotus 18The Lotus 18 was a race car designed by Colin Chapman for use by Lotus in Formula Junior, Formula Two, and Formula One. It was the first mid-engined car built by Lotus and was a marked improvement over Chapman's early and only moderately successful front-engined formula cars, the 12 and 16. It was...
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.