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Suspension system for the Mini
Moulton Developments, The Hall, Bradford-on-Avon, Wiltshire, UK
Suspension system for the Mini
associated engineer
Alexander Eric Moulton
date  1957 - 1959
era  Modern  |  category  Machinery, industrial  |  reference  ST829609
photo  © geni (cc-by-sa/2.0) vis Wikimedia Commons
The rubber suspension system for the Mini car was devised by Alex Moulton as a lightweight alternative to metal springs. The Mini was Britain’s solution to a need for economic cars prompted by fuel shortages after the Suez Crisis (1956). Its small wheels and innovative suspension gave the car agility and road-holding ability.
Britain was subject to petrol rationing between September 1939 and May 1950, during and after World War II (1939-45), and again from December 1956 to May 1957 as a consequence of the Suez Crisis. In the 1950s, very small vehicles, or 'bubble cars', became popular for their low fuel consumption. Many were manufactured by German companies Heinkel, Messerschmitt and BMW and some by British company Peel Engineering.
Most of these miniature cars had only three wheels, so insurance and road tax cost less (they often counted as motorcycles) but the space inside was cramped. Clearly there was a gap in the market for a fuel-efficient small vehicle with four wheels and room for passengers.
In 1952, the British Motor Corporation Limited (BMC) was formed by the merger of Morris and Austin. Its president, Leonard Percy ‘Len’ Lord (1896-1967, knighted 1954, 1st Baron Lambury 1962) was keen to provide a high quality British-made alternative to the European microcars as quickly as possible.
In 1955, Alexander Arnold 'Alec' Issigonis (1906-88, knighted 1969) returned to BMC after three years at Alvis. With William John ‘Jack’ Daniels (1912-2004) and Chris Kingham, plus a team including Charles Arthur Griffin (1918-99), Alexander Eric Moulton (1920-2012), Doug Adams (c1920-2015), John Sheppard (1922-2015) and John Cutler, he embarked on the design of a lightweight compact car for the mass market.
In October 1956, Lord stipulated the dimensions of the car should be no more than 3m × 1.2m × 1.2m with passenger accommodation 1.8m long and, to keep costs down, use an existing BMC engine. In July 1957, a prototype (XC9003) was operational and the new car, the ADO15, passed for production. The launch in Italy of the little Fiat 500 Nuova in July 1957 may have given the BMC project added impetus.
Perhaps the most innovative element of the ADO15 is its suspension system, designed by Moulton. Traditionally, suspension systems consist of springs — coil, torsion or leaf — and shock absorbers, or dampers. Springs allow a car's wheels to react to uneven bits of road without cracking the chassis, and shock absorbers damp the vibration caused by the kinetic energy released from the springs.
Drawing on his background in rubber manufacturing (the family firm made rubber railway springs and shock absorbers), Moulton had already used the material in cars. Working with Issigonis at Alvis, they had developed a car suspension system of rubber springs with fluid interconnection. Unfortunately, the fluid-filled system could not be produced quickly enough to fit the ADO15 so Moulton offered dry rubber springing instead.
He had trialled a rubber system on a Morris Minor in 1953, using cone springs working in compression and shear, harnessing rubber’s ability to store more energy per unit mass than any other type of material used for springs (such as metal). The car covered 1,609km on a rolling road without problems, which was significant, as cars of the day often broke down around 800km.
The Moulton system adopted for ADO15 is sometimes called 'cone and trumpet' suspension. The cone is the rubber 'spring' at the end of the suspension strut or 'trumpet', which connects to a suspension arm, and the arm to a telescopic damper. The dampers are fitted with bushes and the rubber springs seated in nylon ball joint sockets.
Each wheel has an individual cone and trumpet assembly. While all four dampers are mounted vertically, the front suspension has vertical cones and trumpets (above the top wishbones) but the rear suspension has horizontal cones and trumpets to save space.
The cone springs, which have a short travel only, are stressed in compression and shear, inducing load in the suspension pivots and spring abutments. The strain distribution in the rubber is designed to be uniform throughout. The system is progressive, or variable rate. Its stiffness changes, so that passing over slight bumps the suspension is soft but over larger obstructions the suspension is stiff.
Testing the system in the earlier prototype car revealed that its unitary construction (the chassis and car body form a unit) couldn't withstand the high loading imposed by the cone and trumpet system. Instead, front and rear steel sub-frames were designed to carry the drivetrain and suspension. This decision simplified production too: the sub-frames and shell could be produced separately and joined later.
Wheel size was important, since small wheels turn faster than large ones under the same power. Plus small wheels are manoeuvrable as well as taking up less space. The new car had tiny tyres, 132mm wide, fitted to wheels just 254mm in diameter.
The engine used was a water-cooled BMC A-series, with four inline cylinders. It is front-wheel drive, then an uncommon choice. The engine is mounted transversely to occupy and sits directly above the transmission, with the carburettor to the rear. Its original 948cc capacity proved a little more powerful than intended for an economy car, so it was reduced to 848cc with a stroke of 68mm instead of 76mm, giving a top speed of about 120kph (75mph).
The finished car, complete with 9 litres of fuel but no payload weighs 571.5kg, 60 percent of which rested on the front wheels. With four people on board, the weight could increase by more than 30 percent.
Production of the ADO15 began at Longbridge on 3rd April 1959 and at Cowley on 8th May, and the car was launched on 26th August the same year, in patriotic shades of red, white and blue. Originally named the Morris Mini Minor and the Austin Seven, it soon became known as the Austin Mini and later simply Mini. It's the best-selling British car ever made, and at least 5.3 million Minis (possibly as many as 6 million) were produced worldwide between 1959 and 2000.
The Engineer magazine commented on 28th August 1959, "The suspension is hard, as expected on lightweight vehicles, allowing perceptible roll but very little pitch, both highly damped". It also noted the Mini’s "brakes … were progressive", and the "clutch is light, but not so light as to be out of harmony with the throttle".
Soon after the Mini’s launch, racing supremo John Newton Cooper (1923-2000) suggested Issigonis should produce a sport model. Issigonis had built his own racing car, the Lightweight Special, in 1938 with rubber loop suspension but was reluctant at first to make a sporty version of the Mini. Cooper, who designed the world championship-winning Formula 1 cars of 1959 and 1960, persisted.
The Mini Cooper, with a more powerful 997cc engine, went on sale in September 1961. In 1963, the Mini Cooper S was released. In August 1964, the Mini Cooper S was updated to produce two models suitable for racing circuits, with engines of 970cc and 1275cc capacity. The smaller engine was discontinued in 1965, but a car with the larger engine was made until 1971. A new version of the Mini Cooper was relaunched in 1990.
In 1964, the Mini was fitted with Moulton’s Hydrolastic interconnected fluid-filled suspension. Though heavier and more expensive to manufacture, it did give the car a softer more comfortable ride. In 1971, with BMC now absorbed into British Leyland, the Hydrolastic system was withdrawn from the Mini to reduce costs. The earlier rubber cone suspension was re-instated.
In 1975, Moulton developed and improved the Mini’s dry rubber suspension, giving the system softer rubber springs with adjustable top plates. The improved components were later available in kit form.
In 1994, BMW gained control of Rover Group (formerly British Leyland) and took over manufacturing Mini cars. Airbags were fitted to comply with European legislation. Production halted six years later and the last Mini was completed on 4th October 2000.
BMW rebranded the car as MINI, keeping the overall shape of the original vehicle but making it bigger and more luxurious. The first of the new generation models, aimed at the premium car market, was launched in July 2001.
Designer: Moulton Developments Ltd, Bradford-on-Avon
Contractor: BMC, Longbridge, Birmingham
Rubber cones: Dunlop Rubber Company
Drive shafts: Hardy Spicer Ltd
Research: ECPK
bibliography
"A Lifetime in Engineering", interview with Alex Moulton and John Pinkerton, 24th October 1998, Lit Verlag, Berlin, 2007
"Simplified Small Car", The Engineer, Vol.208, No.5405, pp.140-3, 28th August 1959
http://news.bbc.co.uk
www.architectsjournal.co.uk
www.aronline.co.uk
www.caranddriver.com
www.ft.com
www.institution-engineering-designers.org.uk
www.motorsportmagazine.com
www.moultonbuzz.com
www.oecc.ca
reference sources   DNB
Location

Suspension system for the Mini