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Citroën hydraulic system

Citroën's hydropneumatic suspension, despite a reputation for complexity, complication and unreliability is actually very simple in operation and very reliable and furthermore, offers an unrivalled level of comfort coupled with excellent handling and grip.

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. 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 an excellent explanation of how the system works, visit Toaph's site which employs didactic GIFs to great effect..

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.

Schematic of suspension system of Hydractive 2 equipped Xantia

1. Computer
2. Steering wheel movement and speed of movement sensor
3. Accelerator sensor - reads accelerator movement and rate of movement
4. Brake sensor
5. Speed sensor
6. Body movement sensor
7. Electrovalve
8. Stiffness regulator
9. Extra sphere
10. Front suspension sphere
11. Rear suspension sphere

Above the Hydractive 2 system fitted to high specification variants of the Xantia and XM.

The system switches from "soft" to "firm" modes according to a number of parameters programmed into the on board computer.

Sensors provide the necessary input to the computer to determine which mode is appropriate.

A central sphere on each axle is switched in and out of circuit to alter the amount of suspension travel and damping.

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 SC-CAR or Activa system builds on Hydractive 2 by incorporating hydraulic jacks to counter body roll.

Hydractive 2 is also fitted to most XM models.

Below right the rear suspension of the Xantia comprises trailing arms connected to an anti roll bar with height corrector attached.

The rear spheres lie flat which permits an unobstructed boot floor with no suspension unit intrusions.

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 reliable.

In 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.

Prior to the advent of cheap electronics, a totally hydraulic anti lock braking system had been built, together with zero roll suspension.

Electronics provided a cheap, low weight solution for active suspension and anti lock brakes.

Schematic of the anti lock braking system on a Xantia fitted with Hydractive 2 suspension

1. Front brakes accumulator
2. Front to rear brake line
3. Brake servo with front to rear compensator
4. Hydraulic distribution unit
5. Cogged wheel
6. Brake rotation sensor
7. Computer
8. Dashboard warning light 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.

1. Electromagnetic valve
2. Stiffness regulator
3. Additional spheres
4. Front spheres
5. Rear spheres
6. Additional dampers
7. Dampers
8. Central computer
9. Sensors

Top left - suspension in "soft" state; the solenoid valve (1) is energised, the slide valve opens allowing hydraulic fluid to flow between the between the suspension cylinders and the spheres (4 and 3) via the dampers (6). All six spheres are in use.

Bottom left - suspension is in "firm" mode; the solenoid valve (1) is not energised, the slide valve takes up a position which blocks the movement of hydraulic fluid between the two main spheres (4) on each axle and isolates the additional spheres (3) from them both.

Above - a normally suspended car undertaking the slalom test


High pressure hydraulics are the jewel in Citroën's crown. The hydropneumatic suspension system offers an unparalleled level of comfort and control although it must be admitted that it is not perfect. Non-Hydractive or Activa cars suffer from considerable body roll (the basic system is poor in roll stiffness), the ride tends to be harsh at low speeds, the hydraulic system is very good at transmitting road noise into the cabin, some people suffer from motion sickness in an hydropneumatic Citroën, there is a tendency for pitching if one is heavy footed when driving a Citroën and some drivers dislike the feeling of being divorced from the road surface. The system can be caught out by hump backed bridges - the body rises from the road surface as the apex of the bridge is reached, the height corrector then decides the car body is raised too high and reduces ride height as the vehicle descends the other side of the bridge - the body then crashes into the suspension bump stops. On the plus side, one can drive far quicker on indifferent road surfaces than might be believed possible (this goes a long way to explaining the success of the marque in motorsport) and handling is almost unaffected by a change in ride height caused by a full load. Additional benefits include an absence of diagonal pitching, the constant ride height ensures headlamp beams are always properly aligned, irrespective of load and aerodynamics remain unaffected by load thanks to a constant angle of attack. More prosaic but nevertheless useful benefits include the ability to drive on terrain that is normally restricted to off-road vehicles, the ability to raise or lower the suspension to assist in loading or unloading the vehicle, to assist in changing a wheel, to facilitate towing and to make washing of the car easier.

Above - the Xantia Activa undertaking the same slalom test at the same speed - note the total absence of body roll

The C5 features a decentralised system employing separate systems for power steering and braking while future developments may feature a pump for each road wheel and a totally active system (the Activa is, in truth, reactive, albeit its reactions are so quick as to almost deserve the appellation) which reads the road surface ahead of the car and provides precisely the correct amount of suspension travel.

Above a schematic of the DS showing the suspension system

Below a publicity shot for the DS showing the interaction between hydraulics, represented by water and air represented by the ballons.

Above a schematic of the DS showing the braking system

Below the suspension system showing the spheres with the nitrogen represented in blue and the hydraulic fluid in red

Citroën hydraulic system

© 1999 Julian Marsh updated 2001