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Filmed by: Tomaž Kožar Jesenice
The system takes advantage of the fact that you can compress a gas but cannot compress a fluid. Thus gas acts as the springing medium while the hydraulic fluid does all the clever things such as providing damping and levelling. Unlike the hydrolastic system fitted to certain BMC and BLMC cars, the Citroën system relies on an engine-driven pump to pressurise the hydraulic system and it is this power source which enables self-levelling, variable ride height, assisted jacking and zero roll (in the Activa Xantia) and also allows for fully powered braking systems and power steering too.
The system provides a soft, comfortable, yet well-controlled ride. The nitrogen springing medium is approximately six times more flexible than a conventional steel system, so self-leveling is incorporated to allow the vehicle to cope with the extraordinary suppleness provided. In the early fifties, France was noted for particularly poor road quality and therefore the only way to maintain a relatively high speed in a vehicle was if it could easily absorb road irregularities. It was this need that also drove the development of the 2CV's interconnected suspension system.In the DS, the high pressure hydraulic system also operated the clutch and gear change. With the introduction of the XM came electronic control of the suspension system - called Hydractive and a refined version of this system was fitted to high specification Xantias and to later XMs. The Hydractive system allowed for variable damping and automatic switching between soft and firm modes which allowed an unparalleled combination of ride comfort and good handling.
Hydropneumatic suspension was first shown in 1952 when it was fitted to the rear of the 15CV H Traction Avant . This was a foretaste of the system that was fitted to the astonishing DS 19 of 1955. Variants of the suspension have been fitted subsequently to the GS of 1970, the SM of the same year, the CX , the BX , the XM and the Xantia . Other cars fitted with the system were the M 35 Wankel-engined prototype, the GZ Birotor , some variants of the H van and various Rolls Royce and Mercedes-Benz cars where it was used under licence.
For a reproduction of a booklet issued by Citroën in the mid sixties, click here
For an excellent technical guide in Adobe Acrobat format covering all Citroën's hydraulic systems, click here and follow the link to Technical Guide.Below left the Xantia front suspension comprises MacPherson type struts with the Hydropneumatic suspension spheres mounted on top. Each sphere contains a diaphragm behind which a quantity of nitrogen is trapped. A height corrector is attached to the anti roll bar and when a load is placed in the car, the car body sinks. This movement is registered by the height corrector which opens a valve to admit hydraulic fluid under pressure to "lengthen" the hydraulic strut and thereby re-establish the correct ride height. A control inside the car allows the ride height to be varied by the driver thereby aiding wheel changes.The hydraulic system acquired a reputation for unreliability when it was prematurely released on the DS. There were problems with seals and the fluid originally used was intensely hygroscopic (it absorbed water) which caused oxydisation and early DS drivers frequently discovered their pride and joy resting on its haunches in a pool of fluid. The introduction of LHS improved matters but it was not until a mineral oil LHM or Liquide Hydraulique Minéral was introduced in the mid sixties that the system became thoroughly reliableIn the DS, the hydraulic system provided power for the clutch, gearchange and steering. Additional uses of this system included the fully powered self centring steering system (DIRAVI or Varipower) fitted to the SM and CX, levelling of the headlamps fitted to the SM and a number of design studies were undertaken to investigate hydraulic operation of windscreen wipers, an air dam brake and even the opening and closing of windows.Anti dive suspension is incorporated since the rear brakes take their fluid from the rear suspension which pulls the tail down under heavy braking. On all hydraulic Citroëns (with the exception of some early ID 19s), the balance between front to rear braking effort is modified by the respective loads placed on the front and rear suspension.
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Citroën C5 Hydractive demonstration on poorly surfaced roadby semperveritas12,322 views
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