Hydrolastic is a type of space-efficient automotive suspension system used in many cars produced by British Motor Corporation (BMC) and its successor companies.

Invented by famous British rubber engineer Alex Moulton, and first used on the 1962 BMC project ADO16 under designer Alec Issigonis (of Mini fame), later to be launched as the Morris 1100. [cite book
title=Creativity and Concept Design
publisher=Open University Worldwide
id=ISBN 0749259086


The system replaces the separate springs and dampers of a conventional suspension system with integrated, space efficient, fluid filled, displacer units, which are interconnected between the front and rear wheels on each side of the vehicle.

Each displacer unit contains a rubber spring, and damping is achieved by the displaced fluid passing through rubber valves. The displaced fluid passes to the displacer of the paired wheel, thus providing a dynamic interaction between front and rear wheels.

ervicing difficulties

The system was not popular with home mechanics, as it had to be re-pressurized after a chassis or subframe replacement. Even welding the sub-frame to repair rust damage typically required the system to be drained. The only way to re-pressurize was to pay a properly equipped mechanic, usually at a BMC dealer. Inevitably the charge for this service was higher than many cared to pay.


Jon Pressnell suggests in his book that the hydropneumatic suspension of the Citroën DS motivated Issigonis and Moulton, who were at the time of the launch of the DS working on a rubber and fluid suspension system themselves. Pressnell also suggests that the complexity of the Citroën system encouraged Moulton to develop a much simpler system. [cite book
title=Citroen DS: The Complete Story
author=Jon Pressnell
publisher=The Crowood Press
id=ISBN 1861260555

In a magazine interview for 'CAR' magazine in the late 1980s, Dr Moulton stated that he and Issigonis had also studied the Citroën 2CV in the 1950s. They particularly wished to address the comical lack of roll stiffness of that car with the system that they were designing.

Later History

Hydragas is a type of automotive suspension system used in many cars produced by British Leyland and its successor companies.

Invented by famous British automotive engineer Alex Moulton, Hydragas is an evolution of the previous Hydrolastic system. It was first introduced in 1973 in the Austin Allegro and was later fitted to the 1975 Princess and it's successor, the 1982 Austin Ambassador. Both systems attempt to address the ride-handling compromise of car suspension by interconnecting the suspension of the front and rear of the car in some way. Hydragas attempted to perform the same function and advantages as the famous hydropneumatic system developed by Citroën, but without its attendant complexity.

The heart of the system are the displacer units, which are pressurised spheres containing nitrogen gas. These replace the conventional steel springs of a regular suspension design. The means for pressurising the gas in the displacers is done by pre-pressurising a hydraulic fluid, and then connecting the displacer to its neighbour on the other axle. This is unlike the Citroën system, which uses hydraulic fluid continuously pressurised by an engine-driven pump and regulated by a central pressure vessel.

Despite early teething problems (the Allegro version of Hydragas was found seriously wanting), it was gradually developed into a very effective and efficient alternative to steel springs on later BL/Rover Group models such as the Austin Metro and MGF.


Cars using the Hydrolastic suspension system:
* BMC 1100
* Mini (from 1964 to 1971)
* Austin 1800
* Austin Maxi
* Austin 3-Litre
* Austin Metro
* Rover Metro
* Rover 100
* Huffaker Indianapolis cars 1964-1969 "MG Liquid Suspension Special"

External links

* [http://www.hydragas.co.uk/ The Hydragas Register] - a website helping owners of hydragas and hydrolastic cars to find parts and services in their local area
* [http://www.hydragas.co.uk/discuss The Hydragas Forum] - a discussion forum for those interested in discussing the workings of the system
* [http://www.avtokolor.ru/ Hydragas pump-up service] - service for hydrolastic and hydragas suspension in Moscow, Russia


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