Hydragas

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.

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. When a front wheel encounters a bump fluid is transferred to the corresponding rear displacer then lowers the rear wheel, hence lifting the rear, minimising pitch associated with the bump. Naturally the reverse occurs when it is a rear wheel that encounters a bump. This effect is particularly good on small cars as their shorter wheelbases are more affected by pitching.

However, the key improvement over conventional suspension is that the front/rear interconnection allows the vehicle to be stiffer in roll than in pitch. Hence it is possible to design a compliant suspension - giving a comfortable ride - without suffering a penalty in terms of excessive roll when cornering. In roll, there is no transference of fluid from the displacers, and hence its internal pressure rises. The only "give" in the suspension occurs because of the inherent flexibility of the rubber springs. These are naturally stiff.

In pitch, as described above, fluid is displaced front to rear, and hence the pressure in the system stays effectively the same, and thus the suspension is much more compliant.

The design of the displacer units, and the way in which they are mounted means that as the suspension is compressed, the (roughly spherical) displacer deforms, and hence presents a larger area to the mounting plates. The pressure in the system is thus acting over a larger area, and hence applying additional force. This gives the suspension a sharply rising rate even in pitch, so that there is a strong tendency to return to equilibrium. Without this rising rate there would be no effective pitch resistance at all.

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Wikipedia