Hydropneumatic suspension

Citroën Hydropneumatic Suspension

Company type	Technology / Vehicle suspension system
Industry	Automotive
Founder	Paul Magès
Products	Hydropneumatic suspension systems
Owner	Citroën

Hydropneumatic suspension represents a vehicle suspension technology developed by Paul Magès, manufactured primarily by Citroën, and incorporated into their automobiles, with licensing extended to other producers. This system merges hydraulic and pneumatic elements—utilizing gas to cushion impacts and hydraulic fluid to convey forces—resulting in exceptional ride comfort and responsive handling. It commonly features both automatic self-leveling and driver-selectable ride height adjustments, allowing improved ground clearance across diverse driving conditions.

Drawing inspiration from aircraft pneumatic landing gear partially lubricated with oil, the underlying concept is similarly applied in devices like aircraft oleo struts and gas-charged automotive shock absorbers.

Description

Conceived by Paul Magès and produced by Citroën, hydropneumatic suspension (Suspension oléopneumatique) combines pressurized air and hydraulic oil to create a highly responsive, robust suspension system. This technology has been licensed to various manufacturers, including Rolls-Royce—notably for the Silver Shadow model—BMW (5-Series e34 Touring), Maserati (Quattroporte II), and Peugeot. It has also found application in commercial vehicles such as Berliet trucks and influenced systems like Mercedes-Benz’s Active Body Control.

In 1991, the Toyota Soarer UZZ32 "Limited" incorporated a fully integrated four-wheel steering system alongside a computer-regulated hydraulic Toyota Active Control Suspension, showcasing the adaptability of hydropneumatic technology. This suspension type is extensively utilized in contemporary military vehicles, including armored tanks, to enhance operational performance and occupant comfort.

Effects

Offering a highly sensitive and dynamic suspension with significant load capacity, this system ensures outstanding ride quality over uneven terrain. It generally integrates both automatic leveling and driver-controlled ride height, providing additional clearance for challenging environments. Hydropneumatic suspension delivers distinct benefits compared to traditional steel springs by combining two fundamental technologies:

• Hydraulic mechanisms allow force multiplication without relying on mechanical gears or linkages.
• Pneumatic components employ compressible gas to mitigate shock impacts.

Beyond merely enhancing comfort, hydropneumatics improve vehicle handling and control precision, addressing performance limitations inherent in steel spring systems. Despite these advantages, its widespread adoption was constrained by patent protections and perceived system complexity. Alternative simpler compressed air suspensions were developed by manufacturers such as Mercedes-Benz, British Leyland (Hydrolastic, Hydragas), and Lincoln.

Citroën’s version required specialized maintenance tools and expertise, distinguishing their vehicles from conventional counterparts. In post-World War II France, where road infrastructure was often substandard, the hydropneumatic suspension featured in the Citroën ID/DS models delivered a notably smooth and stable driving experience.

While inherently lacking roll stiffness, advancements including steel anti-roll bars, adjustable firmness systems (Hydractive), and active body roll management (Citroën Activa) have progressively improved its dynamic performance.

Fundamental mechanical configuration

The system relies on an engine-driven pump, powered either by a belt or camshaft, to generate pressure in a specialized hydraulic fluid. This pressurized fluid operates various components including the brake system, suspension, and power steering. Additionally, it can control functions such as the clutch, steering-responsive headlights, and power windows.

For the compressed gas medium, Nitrogen is utilized due to its minimal tendency to cause corrosion. The nitrogen-filled spring reservoir is activated by incompressible hydraulic fluid contained within a suspension cylinder, allowing for suspension height adjustment through fluid volume variation. A flexible rubber diaphragm separates the nitrogen gas from the hydraulic oil inside the suspension sphere.

Development history

Citroën first implemented this technology in 1954 on the rear suspension of the Citroën Traction Avant. Its initial application across all four wheels was introduced with the innovative Citroën DS model in 1955. The suspension concept drew inspiration from pneumatic landing gear used in aircraft, a design patented by Citroën in 1933. Further advancements include the 1960 patent for a "Double stage oleo-pneumatic shock absorber."

Significant historical points are as follows:

• During WWII, Paul Magès, a Citroën employee without formal engineering credentials, developed an oil-and-air suspension system that combined comfort and control with automatic leveling.
• In 1954, the Traction Avant 15H featured rear suspension utilizing LHS hydraulic fluid.
• The 1955 Citroën DS debuted a system where suspension, power steering, brakes, and gearbox/clutch were driven by high-pressure hydraulics supplied by a belt-driven 7-piston pump.
• A US patent granted in 1960 covered the double-stage oleo-pneumatic shock absorber, which laid groundwork for aircraft oleo struts and gas-filled shock absorbers.
• Rolls-Royce acquired licensing rights in 1965 to employ Citroën’s technology for the Silver Shadow’s suspension.
• The introduction of LHM mineral fluid in 1967 marked an improvement with its non-hygroscopic properties.
• In 1969, Heuliez produced the Citroën M35 coupé featuring a Wankel engine coupled with hydropneumatic suspension.
• The same year saw US regulatory approval for LHM mineral fluid.
• By 1970, hydropneumatic suspension was adapted for the compact Citroën GS; simultaneously, the Citroën SM introduced speed-sensitive DIRAVI power steering and hydraulically controlled directional high beams using a separate glycerine-based fluid.
• BMW incorporated optional hydropneumatic rear suspension in its E12 5-series starting in 1972, continuing through various models until air suspension replaced it in the late 1980s.
• The US banned height-adjustable suspensions in 1974 (a ban lifted in 1981); during this period, the Citroën CX combined hydropneumatic self-leveling suspension with DIRAVI power steering to enhance ride comfort.
• After acquiring Maserati, Citroën equipped the Maserati Quattroporte II (1974) with hydropneumatic suspension based on an extended SM chassis.
• From the mid-1970s through the 1980s, Mercedes-Benz integrated hydropneumatic suspension systems into vehicles such as the 450SEL 6.9 and W126 series; BMW maintained self-leveling hydropneumatic rear suspensions on E30 and E34 models using shared hydraulic circuits.
• In the late 1980s to early 1990s, Citroën launched Hydractive Suspension featuring electronic control on the XM; Peugeot introduced rear hydropneumatic suspension on the 405 Mi16x4; JCB Fastrac tractors adopted hydropneumatic technology for enhanced ride quality.
• The early 1990s saw Toyota Soarer UZZ32 employ sensor-regulated hydraulic struts to improve dynamic handling.
• Mid-1990s brought optional Activa active suspension on the Citroën Xantia, which eliminated body roll through anti-roll bars and achieved exceptional lateral acceleration and moose test results.
• Between the late 1990s and early 2000s, Mercedes-Benz implemented electronically controlled Active Body Control on CL-Class and S-Class models; concurrently, Citroën C5 and C6 featured Hydractive systems with electric sensors for height adjustment.
• From the mid-2000s onward, Hydractive technology remained available as an option on newer Citroën vehicles; JCB Fastrac expanded hydropneumatic applications to both front and rear suspensions.
• In 2019, Mercedes-Benz introduced eActive Body Control on GLE-Class SUVs, eliminating mechanical anti-roll bars to enhance performance.
• By 2023, BYD Auto launched advanced active hydropneumatic suspensions on its Yangwang U8 SUV and U9 sports car models, incorporating three-wheel drive functionality and stationary level-jumping capabilities.

Functioning

At the heart of the system lie the spheres—typically five or six in number—acting as pressure reservoirs and suspension components: one assigned to each wheel, a primary accumulator, and occasionally an independent brake accumulator. Advanced Hydractive or Activa versions may incorporate as many as ten spheres. Each sphere consists of a hollow metal shell containing a flexible Desmopan rubber diaphragm that separates the upper chamber filled with nitrogen gas (pressurized up to 75 bar) from the lower chamber holding hydraulic fluid (LHM), which connects to the vehicle’s hydraulic network. A high-pressure pump driven by the engine pressurizes the LHM between 150 and 180 bar, initially supplying the front brakes via a safety valve, and depending on model configuration, also powering steering, clutch, or gear selector mechanisms.

Hydraulic pressure is transmitted to suspension cylinders and spheres, compressing the nitrogen gas to generate spring force. Suspension damping is achieved as LHM passes through a bidirectional leaf valve that regulates movement within the suspension. Ride height adjustments are controlled by height corrector valves linked to anti-roll bars; these valves modify fluid volume in suspension cylinders in response to load variations, maintaining consistent ride height while incorporating a delay to prevent overreaction to typical suspension travel. Rear braking derives its hydraulic power from the rear suspension circuit, with braking force modulated proportionally according to load.

Working fluid

Citroën initially employed LHS (Liquide Hydraulique Synthétique), a red synthetic hydraulic fluid used from 1954 until 1967. However, due to reservoir venting necessary for fluid level changes, LHS absorbed moisture and airborne contaminants, leading to corrosion problems.

To overcome this, Citroën introduced LHM (Liquide Hydraulique Minéral), a green mineral oil similar in composition to automatic transmission fluid. Unlike conventional brake fluids, LHM is hydrophobic, preventing moisture bubbles that would otherwise impair braking efficiency. Its adoption extended beyond Citroën to manufacturers such as Rolls-Royce, Peugeot, Mercedes-Benz, Jaguar, Audi, and BMW.

Although LHM contains corrosion inhibitors, it still requires filtration of dust particles; therefore, filters were integrated into reservoirs. Periodic fluid replacement and filter maintenance are vital for system durability. Using inappropriate fluids can cause component damage; consequently, systems must undergo thorough flushing with Hydraflush before refilling with the correct hydraulic fluid.

The latest Citroën models equipped with Hydractive 3 suspension utilize an orange LDS hydraulic fluid that offers extended service intervals and reduced maintenance needs while complying with DIN 51524-3 HVLP standards.

Manufacturing

High-pressure components are fabricated from narrow-diameter steel tubing connected to valve control assemblies via Lockheed-style pipe unions sealed with Desmopan—a polyurethane thermoplastic compatible with LHM fluid. Moving elements such as suspension struts and steering rams rely on contact seals between cylinder and piston surfaces to ensure pressure integrity. Additional plastic and rubber components include return tubes from valves like brake control or height corrector valves, which also collect leaking fluid around suspension push-rods. Critical components including height corrector spools, brake master valve spools, steering valve spools, and hydraulic pump pistons maintain extremely tight clearances (1–3 micrometres), minimizing leakage. Metal and alloy parts seldom fail even after extensive mileage; however, elastomer elements exposed to air tend to harden and develop leaks, representing frequent points of failure.

Spheres themselves do not undergo mechanical wear but gradually lose nitrogen pressure as gas diffuses through the membrane; they can be recharged at a lower cost compared to replacement. Citroën’s Hydractive 3 suspension introduced spheres with nylon membranes that significantly reduce deflation rates and are visually distinguished by their grey color.

Traditional green and grey spheres typically last between 60,000 and 100,000 kilometers. Original spheres included a threaded plug for nitrogen recharging; newer 'saucer' style spheres lack this feature but can be retrofitted for gas recharge capability. The membrane has an indefinite lifespan unless subjected to low pressure operation causing rupture. Regular recharging approximately every three years is essential. A ruptured membrane results in suspension failure on the affected wheel but does not impact ride height control. Without spring support beyond tire elasticity, a deflated sphere may damage suspension components or wheel rims when encountering road irregularities. Failure of the main accumulator sphere leaves the high-pressure pump as the sole source of front brake pressure; certain older models featured separate front brake accumulators on power steering variants.

Older vehicles using LHS or LHS2 (red-colored) fluids employed elastomers incompatible with green LHM fluid. Similarly, the orange LDS fluid used in Hydractive models is not compatible with other hydraulic fluids.

Legacy

The fundamental concepts of hydropneumatic suspension have been adapted for various uses, including aircraft oleo struts and gas-charged shock absorbers in automobiles, with the first U.S. patent granted in 1934 to Cleveland Pneumatic Tool Co. Comparable systems are extensively implemented in modern armored vehicles such as tanks and other heavy military machinery.

Hydractive

Hydractive Suspension is a technology launched by Citroën in 1990, originally showcased on the 1988 Citroën Activa concept car. Building upon the 1954 hydropneumatic suspension design, it incorporates electronic sensors and driver input to adjust suspension stiffness—enabling a sportier setting or a softer, more comfortable ride. Data from steering, braking, suspension, throttle, and transmission sensors is processed by an onboard computer that engages or disengages additional suspension spheres to enhance either handling or ride quality. The Activa models were capable of leaning one degree inward during cornering; however, Citroën identified that a zero-degree lean was ideal.

An added commercial benefit of active suspension systems is improved fuel efficiency and reduced tire degradation. Conventional negative camber leads to increased tire scrubbing, which accelerates wear and raises fuel consumption—issues mitigated by the Hydractive system.

Hydractive 1 and Hydractive 2

Citroën introduced Hydractive suspension on vehicles such as the XM and Xantia, with the sophisticated Activa variant. The original Hydractive 1 system provided two modes: Sport (for firm suspension) and Auto (which automatically alternated between soft and firm settings based on sensor inputs including accelerator position, brake force, and steering angle).

Hydractive 2 refined these modes into Sport and Normal. In contrast to Hydractive 1’s fixed firm setting in Sport mode, Hydractive 2 lowered sensor thresholds so that firm handling was engaged during cornering and acceleration without sacrificing as much ride comfort.

Both Hydractive 1 and 2 defaulted to firm suspension if sensor faults were detected to ensure safety.

Starting with the 1994 Xantia and 1995 XM models, an extra sphere and valve assembly known as the SC/MAC or 'anti-sink' sphere was incorporated. This component acted as a pressure reservoir for the rear brakes, allowing the rear ride height to remain stable for extended periods without engine operation—thereby improving reliability and customer satisfaction.

Hydractive 3

The 2001 Citroën C5 introduced an advanced iteration with Hydractive 3 suspension, which electronically maintains normal ride height even when the engine is switched off. This version also replaced the traditional green LHM mineral oil with orange synthetic LDS hydraulic fluid.

An upgraded Hydractive 3+ variant was launched on higher-end engines of the Citroën C5 and became standard equipment on the Citroën C6 from 2005 onward. This system activates extra spheres via a Sport button to provide a firmer driving experience.

Hydractive 3 offers two automatic height adjustment modes:

• Motorway mode: lowers vehicle height by 15 mm when exceeding 110 km/h
• Rough road mode: raises vehicle height by 13 mm below speeds of 70 km/h

Its central control unit (BHI) optimizes suspension height based on vehicle speed as well as front and rear ride heights.

Hydractive 3+ introduces a third mode allowing selection between comfort or dynamic firmness levels while utilizing additional sensor inputs:

• Vehicle speed
• Front and rear suspension heights
• Steering wheel rotation speed and angle
• Longitudinal and lateral acceleration forces
• Suspension travel velocity
• Accelerator pedal movement

Applications by model:

C5 I (2001–2004)

• Hydractive 3: EW7J4, DW10TD engines
• Hydractive 3+: EW10J4, EW10D, ES9J4S, DW12TED4 engines

C5 II (2004–2007)

• Hydractive 3: EW7J4, EW10A, DV6TED4, DW10BTED4 engines
• Hydractive 3+: ES9A, DW12TED4 engines (prior to RPO No 10645)

C6 (2005–2012)

• Hydractive 3+: Standard equipment across all variants

C5 III X7 (2007–2017)

• Hydractive 3+: Availability depends on market region and trim level

External links

• Citroën Technical Manual