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Hydrolastic and Hydragas suspension systems
Moulton Developments, The Hall, Bradford-on-Avon, Wiltshire, UK
Hydrolastic and Hydragas suspension systems
associated engineer
Alexander Eric Moulton
date  1955 - 1962
UK era  Modern  |  category  Machinery, industrial  |  reference  ST829609
photo  © British Motor Industry Heritage Trust Film & Picture Library
Hydrolastic suspension is a combination of rubber springs and interconnected fluid-filled chambers — cushioning a vehicle’s occupants from bumps and dips. First used in the Morris 1100 ADO16, manufactured by BMC and released in 1962, the suspension design was later refined into the Hydragas system by replacing rubber dampers with compressed gas cushions. Both systems have been phased out of car manufacture.
The forerunner of the Hydrolastic system dates back to 1952, when Alexander Arnold 'Alec' Issigonis (1906-88, knighted 1969) was designing a new saloon car, the Alvis TA350. He asked his friend and rubber spring specialist Alexander Eric Moulton (1920-2012) to devise all-independent suspension for the vehicle.
The result may have been inspired by the Citroen 2CV suspension, with its longitudinal horizontal coil springs in the floor pan. The Alvis had rubber cones at each wheel that when compressed displaced water in pipes connecting the front and back suspension units on each side. However, Alvis didn't have the resources to produce the TA350 in sufficient numbers to compete in the car market and the project was cancelled in 1953.
In November 1955, Issigonis left Alvis and joined the British Motor Corporation Limited (BMC, formed 1952), which retained Moulton’s company Moulton Developments as a suspension consultant.
The system of interconnected fluid suspension that Moulton started to develop, which he and Issigonis called Hydrolastic, resulted from an iterative process of designing, making, testing and evaluating the results. As he remarked some years afterwards, "we had no analogy to fall back on; indeed the devices were more akin to biological organs than engineering mechanisms".
The way it works is that it keeps a moving vehicle level by linking, with small diameter pipes, the front and rear suspension sub-frames, and it uses space-saving integrated fluid damping rather than the conventional arrangement of separate springs and dampers. The sub-frames are fitted with short-stroke displacer units on either side (four per vehicle, located near the wheels). Each unit consists of a reinforced diaphragm to push hydraulic fluid from a chamber through a rubber cushion, or damping valve, into a rubber drum, or 'cheese', which acts as a bounce spring in shear and compression.
When a front wheel is deflected by going over a bump, for example, fluid is displaced to the front suspension, pressurising the interconnecting pipe, stiffening the rear wheel damping (on the same side) and lowering the wheel. If the front and rear wheels hit bumps or dips simultaneously, the fluid suspension stiffens under the combined motion, acting as a damper and transferring the load to the rubber springs.
Hydrolastic suspension practically eliminates any pitching of a vehicle, and doesn't hinder a suspension sub-frames' range of movement. Because the connecting pipes are of small bore, fluid flow is restricted more as a vehicle's speed increases, resulting in a smooth ride at low speeds and a steady ride at higher speeds.
The system was intended for use on BMC's Mini car, designed by Issigonis and launched in August 1959. However, the system wasn't ready in time. Minis did not have Hydrolastic suspension until 1964, and then only for a few years because of the expense.
The first production car to feature Moulton’s Hydrolastic suspension was the Morris 1100, also designed by Issigonis and known as the ADO16. It was launched on 15th August 1962 (see photo above: Moulton and Issigonis at the launch). Like the Mini it had front-wheel drive and its engine, a BMC 1098cc A series, was mounted transversely to maximise interior space. The front displacers were fitted vertically and the rear ones horizontally.
BMC went on to introduce other models based on the ADO16 template and incorporating the new suspension system. Those with 1100 engines included the twin-carburettor MG (1962), the Austin/Vanden Plas Princess 1100 (1963), the Wolseley 1100 and Riley Kestrel (both 1965), and the three-door estate car Austin Countryman/Morris Traveller (1966). In 1967, the 1300 model was produced with an engine capacity of 1275cc.
In 1967, Anthony 'Tony' Best joined Moulton Developments. He was at the forefront of the company’s progression from iterative design to controlled testing and in-depth monitoring, and ultimately predictive modelling — from paper to computer. He led the process that refined Hydrolastic into Hydragas suspension.
The layout and installation of both systems is the same, and the external appearance similar. The difference lies inside the displacer unit. A Hydragas displacer has no rubber damper valve. Above the hydraulic fluid chamber is a cylindrical or spherical chamber filled with nitrogen gas under high pressure, separated from the fluid by a membrane or diaphragm.
The chambers are usually metal, and nitrogen is used because it’s an inert gas. The nitrogen bubble acts as both spring and damper, while the fluid is free to flow between the front and rear suspension. Using a compressed gas spring results in a rising-rate performance, just like a rubber spring in fact, resulting in a more comfortable ride without a wallowing effect, especially at the full extent of a bounce.
In January 1968, BMC’s successor, British Motor Holdings Ltd, merged with the Leyland Motor Corporation to form the British Leyland Motor Corporation. It was headed by Charles Spencer 'Spen' King (1925-2010), previously with Rover, which was also absorbed into British Leyland. Under his tenure Hydragas was fitted on the Austin Allegro in 1973 and on British Leyland’s new marque, the Princess, in 1975.
In 1978, the Austin Maxi was launched with Hydrolastic suspension, though later models were modified to use Hydragas suspension.
In October 1980, the Austin Mini Metro was launched as an intended replacement for the Mini. Though a hatchback rather than a saloon, the Metro has a similar suspension sub-frame layout, A series engine, front-wheel drive and manual gearbox to the Mini. However, its Hydragas suspension was installed with telescopic dampers and transverse connection pipes, contrary to Moulton’s design. To prove the superiority of his original system, Moulton purchased a Metro and fitted longitudinally interconnected Hydragas.
In 1982, the Austin Ambassador (only in production for two years) included a highly developed version of Hydragas, with improved steel valves. In 1986, Hydragas suspension was used in the Porsche 959 rally car that entered the Paris to Dakar Rally.
After British Leyland executives had driven Moulton’s reconfigured Metro, presumably noticing the considerable difference in ride comfort, the new Rover Metro (the Rover 100) was produced from 1990 with interconnected gas suspension. It also had a new K series engine.
When the MGF open-topped sports car was launched in 1995, it too was fitted with Hydragas suspension. The last new car to be fitted with Hydragas was an MGF that came off the production line in 2002 — thought to be the 12 millionth car to have Moulton-designed suspension, 43 years after the Mini was launched.
Problems associated with the suspension were the loss of hydraulic fluid and the diffusion of nitrogen gas, both of which seemed to affect the MGF’s rear suspension more often than the front, resulting in a loss of suspension ride height (and cushioning). When the MGF was replaced by the MGTF (2002), Hydragas was abandoned in favour of conventional coil spring suspension.
Design: Moulton Developments, Bradford-on-Avon
Contractor: British Motor Corporation, Longbridge, Birmingham
Research: ECPK
"Dr Alex Moulton CBE — A Man of Conviction" by Dan Farrell, Moulton Bicycle Company, 1st March 2013
"Motor Vehicle Engineering" by Tom Denton, 2nd edition, Thomson Learning, 2002
reference sources   DNB

Hydrolastic and Hydragas suspension systems