1954 Citroën Traction Avant 15CVH - high position

Diagram of the Hydractive system, showing centre spheres and stiffness valves

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Suspension & Steering

Hydropneumatic is a type of automotive suspension system invented by Citroën and fitted to Citroën cars, as well as being adapted by other car manufacturers, notably Rolls-Royce, Mercedes-Benz and Peugeot. It was also used on Berliet trucks. more...

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Similar systems are also used on some military vehicles.

The purpose of this system is to provide a soft, comfortable, yet well-controlled ride quality. Its nitrogen springing medium is approximately six times more flexible than conventional steel, so self-leveling is incorporated to allow the vehicle to cope with the extraordinary suppleness provided. France was noted for poor road quality in the post-war years, so the only way to maintain relatively high speed in a vehicle was if it could easily absorb road irregularities.

While the system has inherent advantages over steel springs, generally recognized in the auto industry, it also has an element of complexity, so automakers like Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln have sought to create simpler variants.

This system uses a belt or camshaft driven pump from the engine to pressurise a special hydraulic fluid, which then powers the brakes, suspension and power steering. It can also power any number of features such as the clutch, turning headlamps and even power windows. The suspension system usually features driver-variable ride height, to provide extra clearance in rough terrain.

The suspension setup is referred to as 'oléopneumatique' in early literature, pointing to oil and air as its main components.

There have been many improvements to this system over the years, including variable ride firmness (Hydractive) and active control of body roll (Activa). The latest incarnation features a simplified single pump-accumulator sphere combination.

The system had one key negative impact on the inventor, Citroën - only specialist garages were qualified to work on the cars - making them seem radically different from ordinary cars with common mechanicals.

Auto manufacturers are still trying to catch up with the combination of features offered by this 1955 suspension system, typically by adding layers of complexity to an ordinary steel spring mechanical system.

History

Citroën first introduced this system in 1954 on the rear suspension of the Traction Avant. The first full implementation was in the advanced DS in 1955.

Major milestones of the hydropneumatics design were:

During World War II, Paul Magès, an employee of Citroën, with no formal training in engineering, secretly develops the concept of an air/oil suspension to combine a new level of softness with vehicle control and self-leveling; 1954 Traction Avant 15H: Rear suspension, using LHS hydraulic fluid.; 1955 DS: Suspension, power steering, brakes and gearbox/clutch assembly powered by high pressure hydraulic assistance. A belt driven 7-piston pump, similar in size to a power steering pump generates this pressure when the engine is running.; 1962 Morris introduces the BMC ADO16 {'1100'} with hydrolastic suspension; 1964 Mercedes-Benz introduces the 600 with air suspension designed to avoid Citroën patents; 1965 Rolls-Royce licenses Citroën technology for the suspension of the new Silver Shadow; 1966 Mercedes-Benz introduces the 6.3 also with air suspension; 1967 The superior LHM mineral fluid is introduced; 1970 GS: Adaptation of the hydropneumatic suspension to a small car; 1970 SM: Variable speed auto-returning power steering, dubbed DIRAVI, and hydraulically actuated directional high beams; 1974 The Mercedes-Benz 450SEL 6.9 becomes the first hydropneumatic Mercedes-Benz automobile, with the pump driven by the engine's timing chain instead of an external belt. This adaptation was used only for the suspension. Power steering and brakes were conventional hydraulic- and vacuum-powered, respectively.; 1983 Citroën BX, built as a 4WD in 1990; 1989 XM: electronic regulation of the hydropneumatic system; sensors measure acceleration and other factors; 1990 Peugeot 405 Mi16x4: first Peugeot equipped with rear hydropneumatic suspension; 1990 JCB Fastrac high speed agricultural tractor uses this system for its rear suspension.; 1993 Xantia: Optional 'Activa' (active suspension) system, eliminating body roll by acting on torsion bars. An 'Activa' equipped Xantia was able to reach more than 1 g lateral acceleration; 2001 C5: Hydractive 3 removes the need for central hydraulic pressure generation; combined pump/sphere unit for the suspension only and with electric height adjustment sensors; 2005 C6: An improved version of the C5 system known as Hydractive 3+; 2007 JCB Fastrac high speed 7000 series agricultural tractors now use this system for front and rear suspension.;