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 and was first introduced in 1973 in the Austin Allegro. 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 Citroen, 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 oil, and then connecting the displacer to its neighbour on the other axle. This is unlike the Citroen 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.
Hydragas is a fluid/ gas based suspension system that is an alternative to conventional springs and dampers - although on the MGF, auxiliary dampers are actually employed.
The use of the Metro sub-frame brought with it that car’sHydragas suspension and the Metro’s unequal length double wishbone suspension geometry front and rear (the now un-used Metro rear subframe is a semi-trailing arm affair, near identical to that found on the Mini).
The consequence of this interconnection is that as the front suspension is compressed upwards, the rear Hydragas unit is pressurised, forcing the rear wheel to the ground.
The Hydragas units used in the present research project were from the new Rover MGF sports car, and featured a smaller-volume gas chamber and a damper valve of different construction.
Hydragas was introduced with the unloved Austin Allegro in the 1970s and is still in use on roads today fitted to the MGF.
Hydragas was never fitted to a production Mini, but Alex Moulton has some fascinating prototypes in his stable: a 1966 Cooper S fitted with Hydragas suspension and a 1980 Metro fitted with a more developed, fully interconnected and refined version of the system.
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