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 British rubber engineer Alex Moulton, and first used on the 1962 BMC project ADO16 under designer Alec Issigonis, later to be launched as the Morris 1100.
Cars with Hydrolastic suspension do, however, have a marked tendency to squat under acceleration, and to dive under braking (and for the rear end to sag under heavy loads).
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 problems (the Allegro version of Hydragas was found seriously wanting), it was gradually developed into an effective and efficient alternative to steel springs on later BL/Rover Group models such as the Austin Metro and MGF.
While the Metro was praised for its handling, and offered a significant improvement in ride quality over the Mini, it was criticised for its tendency to pitch and bounce on uneven roads - precisely the characteristics the interconnection was intended to remove.
The Hydragas system was dropped in favour of conventional suspension by Rover when BTR AVS sought to increase the price of the units substantially.
To ensure a safe and comfortable ride whilst using small wheels with high-pressure tyres Moulton fitted his bicycles with rubber suspension units similar to those used on the original Mini.
These are, in essence, miniaturised versions of the displacers used on Hydrolastic-equipped cars being a rubber cone spring with an internal fluid chamber to provide damping.