Hydropneumatic suspension

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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. 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. Citroën thus encountered the same issue as Apple Computer did - building a proprietary system deprives you of positive network externalities.

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.


1954 Citroën Traction Avant 15CVH - high position

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
  • 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: No more central hydraulic pressure generation; combined pump/sphere unit for the suspension only and with electric height adjustment sensors


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

At the heart of the system, acting as pressure sink as well as suspension elements, are the so called 'spheres', five or six in all; one per wheel and one main accumulator as well as a dedicated brake accumulator on some models. On later cars fitted with antisink or Activa suspension, there may be as many as nine spheres. They consist of a hollow metal ball, open to the bottom, with a flexible desmopan rubber membrane, fixed at the 'equator' inside separating top and bottom. The top is filled with nitrogen at high pressure, up to 75 bar, the bottom connects to the car's LHM fluid circuit. (See hydraulic accumulator). The high pressure pump powered by the engine pressurizes the circuit and an accumulator sphere. This part of the circuit is between 150 and 180 bars. It powers the front brakes first, prioritised via a security valve, and depending on type, can power steering, clutch, gearchange etc.

Pressure goes from this circuit to the wheel spheres, pressurizing the bottom part of the spheres and rods connected to the wheel suspension. Suspension works by the rod pushing LHM into the sphere, compacting the nitrogen in the upper part of the sphere, the damping is provided by a two-way 'leaf valve' in the opening of the sphere. LHM has to squeeze back and forth through this valve which causes resistance and controls the suspension movements, it is the simplest damper and one of the most efficient. Car height correcting works by height correctors connected to the anti-roll bar, front and rear. These height correctors allow for more fluid to travel under pressure to the rod/sphere system when detecting that the suspension is lower than its expected ride height (e.g. the car is loaded) and when the car is too high (e.g. after unloading). Height correct/ors act with some delay in order not to correct regular suspension movements. Rear brakes are powered from the rear suspension spheres. Because the pressure there is proportional to the load, so is the braking power.

Fluid Types

Citroën quickly realized that standard brake fluid was not ideally suited to high pressure hydraulics. They invented a new, green fluid, LHM. LHM stands for Liquide Hydraulique Minéral and is a mineral oil, quite close to automatic transmission fluid. Mineral oil is not hygroscopic (ie will not absorb water from the air) unlike standard brake fluid, so therefore gas bubbles do not form in the system, as used to be the case with standard brake fluid, creating a 'sprongy' brake feel. Use of mineral oil has thus spread beyond Citroën, Rolls-Royce, Peugeot, and Mercedes-Benz, to include Jaguar, Audi, and BMW.


The whole high pressure part of the system is manufactured from steel tubing of small diameter, connected to valve control units by Lockheed type pipe unions with special seals made from desmopan rubber, a type of rubber compatible with the LHM fluid. The moving parts of the system e.g. suspension strut or steering ram are sealed by extremely small tolerances between the cylinder and piston for tightness under pressure. The other plastic/rubber parts are return tubes from valves such as the brake control or height corrector valves, also catching seeping fluid around the suspension push-rods. The metal and alloy parts of the system rarely fail even after excessively high mileages but the rubber components (especially those exposed to the air) can harden and leak, typical failure points for the system.

Spheres are subject to no mechanical wear but suffer pressure loss, mostly from nitrogen naturally diffusing through the membrane. They typically keep between 60,000 and 100,000 km. Spheres originally used to have a valve on top and be rechargeable. Newer spheres do not have this valve anymore, but it can be retrofitted. Though a rechargeable sphere has a longer lifespan, the membranes will eventually wear out though this can take over 20 years. A ruptured membrane means suspension loss at the attached wheel, however ride height is unaffected. Or in the case of the accumulator sphere, reliance on the high pressure pump as the only source of braking pressure to the front wheels.


  • The suspension is self-leveling and ride-height is adjustable; this provides aerodynamic benefits because of the stable ride-height and extra clearance over rough terrain.
  • The ride comfort is excellent (the ride is described as floating above the road surface) but the suspension never 'wallows', giving precise handling and road-holding (like a sports car)
  • Large loads do not seriously affect the dynamics of the suspension system and handling is not affected thereby.
  • Compact suspension design.
  • Maintenance for trained mechanic is relatively easy.
  • Inexpensive in mass production; for vehicles that would otherwise have a conventional power steering pump, hydropneumatic suspension adds no new equipment and in many cases results in a lower unsprung mass.
  • Upon body roll, the pressure exerted between the tyres of the same axle is not subject to the same differential as on some other cars; the pressure in one suspension strut equals the pressure in the other through Pascal's law, potentially giving the 'light' tyre more footprint pressure.
  • Can be conveniently interconnected in the roll plane to improve roll stiffness and thus roll stability limit, especially for heavy vehicles.
  • Can be connected in the pitch plane to improve braking dive and traction squat.
  • If they are interconnected in the three-dimensional full car model, the interconnected hydro-pneumatic suspension could realize enhanced roll and pitch control during excitations arising from steering, braking/traction, road input and crosswind, as with the Hydractive arrangement
  • Flexibility in the suspension strut design in the interconnected suspension system to realize desirable vertical, roll and pitch properties for different types of vehicles.
  • Horizontal orientation of the rear suspension cylinders below the level of the boot floor means that the full width of the boot is available for loads.
  • Mechanical steel spring suspension systems that try to replicate some of the inherent advantages of hydropneumatic suspension (multilink, adjustable shock absorbers) end up more complex to build and maintain than the straightforward hydropneumatic layout.
  • People who are prepared to carry out simple maintenance can acquire a luxury car for a fraction of the cost as hydropneumatic suspension scares potential buyers and dealers.


  • Service requires a specifically trained mechanic.
  • Hydropneumatic suspension systems are expensive to repair or replace, if poorly maintained.
  • Older designs of hydropneumatic suspension systems can lead to significant body roll.
  • Failure of the hydraulic system will cause a drop in ride height and, possibly, the failure of suspension completely.
  • The novelty (and unfamiliarity) of the hydropneumatic suspension system leads to people being wary of using, purchasing or trusting it.


Hydractive Suspension is a new automotive technology introduced by the French manufacturer Citroën in 1990. It describes a development of the 1955 Hydropneumatic suspension design using additionally electronic sensors and driver control of suspension performance. The driver can make the vehicle stiffen (sport mode) or ride in outstanding comfort (soft mode). Sensors in the steering, brakes, suspension, throttle pedal and gearbox to feed information on the car's speed, acceleration, and road conditions to on-board computers. Where appropriate - and within milliseconds - these computers switched an extra pair of suspension spheres in or out of circuit, to allow the car a smooth supple ride in normal circumstances, or greater roll resistance for better handling in corners. This development keeps Citroën in the forefront of suspension design, given the widespread goal in the auto industry of an Active Suspension system. All auto suspension is a compromise between comfort and handling. Auto manufacturers try to balance these aims and locate new technologies that offer more of both.

Citroën hydractive (Hydractive 1 and Hydractive 2)suspension was available on several models, including the XM and Xantia, which had a more advanced sub-model known as the Activa.

The 2003 Citroën C5 has continued development of Hydractive suspension with hydractive 3.. Compared to earlier cars, the C5 stays at normal ride height even when the engine is turned off for an extended period, through the use of electronics. The C5 also uses a new, incompatible orange fluid, rather than the familiar green LHM mineral oil used in millions of hydropneumatic vehicles.


  • The accumulator sphere typically has enough pressure left for multiple hard brakings (up to 50), even if the engine (and attached high pressure pump) stops working.
  • Hydropneumatic cars can drive on 3 wheels. One can remove one rear wheel, and the car will be able to compensate by pressurising the other 3 wheels higher.
  • Maseratis in the 1970s borrowed some parts of the system for braking and power steering, but the only Maserati that did use a hydropneumatic suspension was the Quattroporte II. Citroën owned Maserati at this time and produced the 'Citroën-Maserati' SM.
  • Rolls-Royce cars used the system in combination with a standard suspension.
  • The first hydropneumatic cars used a different liquid called LHS (liquide hydraulique synthétique). This was thought to have superior properties over LHM, but it turned out to eat the rubber of the return tubes when aging and absorb atmospheric moisture turning acidic, thoroughly immobilising thousands of 50s cars. This probably contributed to uneasiness of the systems reliability.
  • Hydropneumatic parts are finely machined to the micrometre level of tolerance - a level not otherwise seen in auto manufacture.
  • The "poor man's" alternative to hydropneumatic is British Leyland's Hydragas system, which uses pre-pressurised spheres which are interconnected between axles, and various air suspension arrangements.
  • In 1974, the USA outlawed cars that lower as the hydraulic system depressurises.
  • Over 9 million vehicles have been produced with this type of suspension.

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