View source for Hydropneumatic suspension

{{Short description|Type of motor vehicle suspension system}}
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| caption1          = High position
| image2            = DS @ East Beach Cafe Low.jpg
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| caption3          = [[Citroën]] suspension sphere
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| caption4          = [[Challenger 2]], main battle tank of the [[British army]], uses hydropneumatic suspension for better crew comfort and increased firing accuracy.
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'''Hydropneumatic suspension''' is a type of [[motor vehicle]] [[suspension (vehicle)|suspension system]], invented by [[Paul Magès]], produced by [[Citroën]], and fitted to Citroën cars, as well as being used under licence by other car manufacturers. Similar systems are also widely used on modern [[tank]]s and other large [[military]] vehicles. The suspension was referred to as ''{{ill|Suspension oléopneumatique|fr|vertical-align=sup}}'' in early literature, pointing to oil and air as its main components.

The purpose of this system is to provide a sensitive, dynamic and high-capacity suspension that offers superior [[ride quality]] on a variety of surfaces. A hydropneumatic system combines the advantages of [[Hydraulic machinery|hydraulic]] systems and [[air suspension|pneumatic]] systems so that gas absorbs excessive force and liquid in hydraulics directly transfers force. The suspension system usually features both [[Self-levelling suspension|self-leveling]] and [[Height adjustable suspension|driver-variable]] [[ride height]], to provide extra clearance in rough terrain.

This type of suspension for automobiles was inspired by the pneumatic suspension used for aircraft landing gear, which was also partly filled with oil for lubrication and to prevent gas leakage, as patented in 1933 by the same company. The principles illustrated by the successful use of hydropneumatic suspension are now used in a broad range of applications, such as [[aircraft]] [[oleo strut]]s and gas filled automobile [[shock absorber]]s.

== Description ==
Hydropneumatic suspension is a type of [[motor vehicle]] [[suspension (vehicle)|suspension system]], invented by [[Paul Magès]], produced by [[Citroën]], and fitted to Citroën cars. The suspension was referred to as ''{{ill|Suspension oléopneumatique|fr|vertical-align=sup}}'' in early literature, pointing to oil and air as its main components.<ref name="air-techniques.fr/suspension-hydropneumatique">{{cite web |title=La suspension hydropneumatique du début à la fin chez Citroën |url=https://www.air-techniques.fr/air/blog/histoire/la-suspension-hydropneumatique-citroen |access-date=3 January 2023 |website=Le blog |publisher=Air-Techniques.fr |language=fr}}</ref><ref name="google/patent/EP0118335B1/fr">{{cite web |last1=Richard |first1=Denis |last2=Perineau |first2=Jean |date=4 June 1986 |title=Suspension oléopneumatique à amortissement total |url=https://patents.google.com/patent/EP0118335B1/fr |access-date=3 January 2023 |website=google patents |language=fr}}</ref>

The system was also used under licence by other car manufacturers, notably [[Rolls-Royce Limited|Rolls-Royce]] ([[Rolls-Royce Silver Shadow|Silver Shadow]]), [[BMW 5 Series (E34)]] Touring,  [[Maserati]] ([[Quattroporte]] II) and [[Peugeot]].{{cn|date=September 2025}} It was also used on [[Berliet]] trucks and has been used on [[Mercedes-Benz]] cars, where it is known as ''[[Active Body Control]].''<ref>{{cite book |url=https://link.springer.com/book/10.1007/978-3-8348-9493-9 |title=Fahrwerkhandbuch: Grundlagen, Fahrdynamik, Komponenten, Systeme, Mechatronik, Perspektiven |date=2008 |publisher=Vieweg+Teubner Verlag |isbn=978-3-8348-0444-0 |editor1-last=Heißing |editor1-first=Bernd |location=Wiesbaden |doi=10.1007/978-3-8348-9493-9 |access-date=3 January 2023 |editor2-last=Ersoy |editor2-first=Metin}}</ref> The [[Toyota Soarer]] UZZ32 "Limited" was fitted with a fully integrated four-wheel steering and a complex, computer-controlled hydraulic ''[[Toyota Active Control Suspension]]'' in 1991. Similar systems are also widely used on modern [[tank]]s and other large [[military]] vehicles.

==Effects==
The purpose of this system is to provide a sensitive, dynamic and high-capacity suspension that offers superior [[ride quality]] on a variety of surfaces.<ref name="daredifferentP75">{{cite book |last1=Reynolds |first1=John |title=Citroen: Daring to Be Different |date=2004 |publisher=[[Haynes Publishing]] |isbn=978-1-85960-896-8 |page=75 |language=en}}</ref> The suspension system usually features both [[Self-levelling suspension|self-leveling]] and [[Height adjustable suspension|driver-variable]] [[ride height]], to provide extra clearance in rough terrain.<ref>{{Cite web |title=Porsche? Releases Cayenne? Four-Wheel Drive Technical Specifications and New Photos |url=http://press.porsche.com/news/release.php?id=154 |url-status=dead |archive-url=https://web.archive.org/web/20150206234020/http://press.porsche.com/news/release.php?id=154 |archive-date=2015-02-06 |access-date=2015-01-29}}</ref> Hydropneumatic suspension has a number of natural advantages over steel springs, generally recognized in the auto industry.<ref name="wpi.edu">{{cite web |last1=Gardiner |first1=Nick |last2=Griesbach |first2=Doug |last3=McCann |first3=Connor |last4=McDonald |first4=Nick |date=April 26, 2012 |title=Project report: Ambulance Universal Chassis and Suspension |url=https://www.wpi.edu/Pubs/E-project/Available/E-project-042712-134653/unrestricted/FINAL_AUCAS_MQP_Report.pdf |url-status=bot: unknown |archive-url=https://web.archive.org/web/20150129062448/https://www.wpi.edu/Pubs/E-project/Available/E-project-042712-134653/unrestricted/FINAL_AUCAS_MQP_Report.pdf |archive-date=January 29, 2015 |access-date=January 3, 2023 |website=[[Worcester Polytechnic Institute]] |quote=We designed a bolt-on kit for aftermarket installation of hydropneumatic suspension into current ambulances using our 2004 Ford F-350 ambulance as a template.}}</ref> In a hydropneumatic system, gas absorbs excessive force, whereas liquid in hydraulics directly transfers force, which  combines the advantages of two [[technological]] [[principles]]:
* [[Hydraulic machinery|Hydraulic]] systems use [[Torque multiplier|torque multiplication]] in an easy way, independent of the distance between the input and output, without the need for mechanical gears or levers.
* [[air suspension|Pneumatic]] systems are based on the fact that [[gas]] is compressible, so equipment is less subject to shock damage.
[[Suspension (vehicle)|Suspension]] and springing technology is not generally well understood by consumers, leading to a public perception that hydropneumatics are merely "good for comfort".{{Citation needed|date=January 2024}} They also have advantages related to handling and control efficiency, solving a number of problems inherent in steel springs that suspension designers have previously struggled to eliminate.<ref name="initialdave.com">{{cite web |url=http://www.initialdave.com/cars/tech/suspensionbasics09.htm |title=Suspension Basics 9{{dash}}Hydropneumatic Springs |access-date=2015-01-29 |url-status=dead |archive-url=https://web.archive.org/web/20150129123613/http://www.initialdave.com/cars/tech/suspensionbasics09.htm |archive-date=2015-01-29 |website= InitialDave}}</ref> Although auto manufacturers understood the inherent advantages over steel springs, there were two problems. First, it was patented by the inventor, and second it had a perceived element of complexity, so automakers like [[Mercedes-Benz 600|Mercedes-Benz]], [[British Leyland]] ([[Hydrolastic]], [[Hydragas]]), and [[Lincoln (automobile)|Lincoln]] sought to create simpler variants using a compressed [[air suspension]].<ref>{{cite web |url=http://www.hydragas.co.uk/HydroService.pdf |title=Data |website=www.hydragas.co.uk |access-date=2015-01-29 |archive-date=2014-05-31 |archive-url=https://web.archive.org/web/20140531180541/http://hydragas.co.uk/HydroService.pdf |url-status=dead }}</ref><ref>{{cite web |url=http://www.initialdave.com/cars/tech/suspensionbasics08.htm |title=Suspension Basics 8{{dash}}Air Springs |access-date=2015-01-29 |url-status=dead |archive-url=https://web.archive.org/web/20150129123610/http://www.initialdave.com/cars/tech/suspensionbasics08.htm |archive-date=2015-01-29  |website= InitialDave}}</ref>

[[Citroën]]'s application of the system had the disadvantage that only garages equipped with special tools and knowledge were qualified to work on the cars, making them radically different from ordinary cars with common mechanicals.<ref>{{cite web|url=https://www.nbc.com/jay-lenos-garage/video/1971-citroen-ds/2839548?onid=159436#vc159436=1|title=1971 Citröen DS|date=12 January 2015|website=nbc.com}}</ref> France was noted for the poor quality of its roads after [[World War II]], but the hydropneumatic suspension as fitted to the [[Citroën DS|Citroën ID/DS]] and later cars reportedly ensured a smooth and stable ride there.<ref name="daredifferentP75" /><ref>{{cite web |url=http://landrovermena.com/2012/09/20/the-all-new-range-rover-unmatched-capability/ |title=The All-New Range Rover &#124; Unmatched Capability " Land Rover " www.landrovermena.com |access-date=2015-01-29 |url-status=dead |archive-url=https://web.archive.org/web/20150129075500/http://landrovermena.com/2012/09/20/the-all-new-range-rover-unmatched-capability/ |archive-date=2015-01-29 }}</ref><ref>{{cite web|url=http://activesuspensionsystems.com/lelx470susy.html|title=Lexus LX 470 suspension system|website=activesuspensionsystems.com|access-date=2015-01-29|archive-date=2015-01-29|archive-url=https://web.archive.org/web/20150129060322/http://activesuspensionsystems.com/lelx470susy.html|url-status=usurped}}</ref>

Hydropneumatic suspension offers no natural roll stiffness. There have been many improvements to the system over the years, including steel [[anti-roll bar]]s, variable ride firmness ([[Hydractive]]), and active control of body roll ([[Citroën Activa]]).<ref>{{cite book |title=Popular Science |date=1991 |publisher=Bonnier Corporation |page=[https://archive.org/details/bub_gb_eyPfgGGTfGgC/page/n30 29] |url=https://archive.org/details/bub_gb_eyPfgGGTfGgC |access-date=12 June 2018 |language=en}}</ref>

==Basic mechanical layout==
[[Image:Susp hydrp.jpg|right|thumb|230px|Blue: Nitrogen gas;  Gold: Hydraulic fluid under pressure from engine-driven pump]]
This system uses a belt- or camshaft-driven pump from the engine to pressurise a special [[hydraulic fluid]], which then powers the [[brake]]s, [[Suspension (vehicle)|suspension]] and [[power steering]].<ref name="initialdave.com"/><ref name="tramontana/citroen-guide">{{cite web |title=citroen guide |url=http://www.tramontana.co.hu/citroen/citroen%20guide.pdf |website=tramontana.co.hu |access-date=3 January 2023 |archive-url=https://web.archive.org/web/20040205050556/http://www.tramontana.co.hu/citroen/citroen%20guide.pdf |archive-date=2004-02-05}}</ref>  It can also power any number of features such as the [[clutch]], [[Headlamp#Directional headlamps|turning headlamps]] and even [[power windows]].<ref name="initialdave.com"/>

[[Nitrogen]] is used as the trapped gas to be compressed, since it is unlikely to cause corrosion. The actuation of the nitrogen spring reservoir is performed through an incompressible [[hydraulic fluid]] inside a suspension cylinder.<ref name=daredifferentP75 /> By adjusting the filled fluid volume within the cylinder, a leveling functionality is implemented.<ref name=daredifferentP75 /> The nitrogen gas within the suspension sphere is separated from the hydraulic oil by a rubber membrane.<ref name=daredifferentP75 />

==History==
[[Image:1954 Citroen 15CVH.JPG|right|thumb|230px|1954 [[Citroën Traction Avant]] 15CVH{{dash}}high position]]

Citroën first introduced this system in 1954 on the rear suspension of the [[Citroën Traction Avant|Traction Avant]].<ref>{{cite web|url=https://www.lesrendezvousdelareine.com/article-traction-avant-15cv-6-cylindres-suspension-hydraulique-la-reine-de-la-route-58048174.html|title=TRACTION AVANT 15cv, 6 cylindres, suspension hydraulique ! LA REINE DE LA ROUTE{{dash}}LES RENDEZ VOUS DE LA REINE|first=Les RENDEZ-VOUS de La|last=REINE|website=lesrendezvousdelareine.com|date=October 2010 }}</ref> The first four-wheel implementation was in the advanced [[Citroën DS|DS]] in 1955.<ref>{{cite web|url=https://www.supercars.net/cars/388.html|title=1955 Citroën DS 19{{dash}}Citroën|date=31 March 2016|website=SuperCars.net}}</ref>  This type of suspension for automobiles was inspired by the pneumatic suspension used for aircraft landing gear, which was also partly filled with oil for lubrication and to prevent gas leakage, as patented in 1933 by the same company.<ref>{{cite web |title=Shock absorbing device US 1918697 A |url=http://www.google.sc/patents/US1918697}}</ref> Other modifications followed, with design changes such as the 1960 "Double stage oleo-pneumatic shock absorber" patented by Peter Fullam John and Stephan Gyurik.<ref name="google.sc">{{cite web |title=Double stage oleo-pneumatic shock absorber |url=https://patents.google.com/patent/US2959410A/en |website=google.com}}</ref>

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 oil and air suspension to combine a new level of softness with vehicle control and [[self-levelling suspension|self-levelling]].<ref>{{cite web |url=http://www.geocities.com/MadisonAvenue/4430/bluebook.html |title=Citroën Faces – The men behind Citroën |website=The Citroën Source |access-date=2014-04-01 |quote=Magès was the man behind the hydropneumatic suspension of the DS. His ideas were discovered by Pierre Boulanger by accident, and Boulanger was fascinated by them, even though the Citroën technicians considered them hopeless. Boulanger employed Magès in the development department. A decision, he never would regret. Paul Magès was a curious man, and he consumed all literature concerning wheel suspension, suspension in general and braking systems. |archive-url=https://web.archive.org/web/20090803144320/http://geocities.com/MadisonAvenue/4430/bluebook.html |archive-date=2009-08-03 |url-status=dead }}</ref>
* 1954 [[Citroën Traction Avant|Traction Avant]] 15H: Rear suspension, using [[#Working fluid|LHS]] hydraulic fluid.
* 1955 [[Citroën 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.<ref>{{cite web|url=http://www.mycitroen.dk/library/ds/Red/Hydraulics/Chas'%20Hydraulic%20Course.pdf |title=Hydraulics info |website=www.mycitroen.dk/library }}</ref>
* 1960 The [[United States Patent and Trademark Office]] issues {{US patent|2959410A}} for a ''Double stage oleo-pneumatic shock absorber'' using concepts very similar to those developed earlier by Paul Magès{{dash}}Patent forms the basis for aircraft [[oleo strut]]s and gas-filled [[shock absorber]]s<ref name="google.sc"/>
* 1965 [[Rolls-Royce Limited|Rolls-Royce]] licenses Citroën technology for the suspension of the new [[Rolls-Royce Silver Shadow|Silver Shadow]]<ref>{{cite web|url=http://autoweek.com/article/car-life/project-car-hell-roller-budget-edition-silver-cloud-or-silver-shadow|title=Project Car Hell, Roller On a Budget Edition: Silver Cloud or Silver Shadow?|website=autoweek.com|date=15 December 2011}}</ref>
* 1967 The superior non-[[Hygroscopy|hygroscopic]] [[#LHS versus LHM|LHM]] mineral fluid is introduced
* 1969 [[Citroën M35]]: The Citroën M35 was a [[Coupe|coupé]] derived from the [[Citroën Ami|Ami 8]], and equipped with a [[Wankel engine]] and a [[hydropneumatic]] suspension. The bodies were produced by [[Heuliez]] from 1969 to 1971.
* 1969 [[National Highway Traffic Safety Administration]] legalizes LHM mineral fluid in the United States
* 1970 [[Citroën GS]]: Adaptation of the hydropneumatic suspension to a small car
* 1970 [[Citroën SM]]: [[Steering#Speed-sensitive steering|Variable speed auto-returning power steering]], dubbed [[DIRAVI]], and hydraulically actuated directional high beams. The beams of all six headlights are maintained parallel to the road surface by a hydraulic system separate from the directional long range high beams. The headlights' steering and leveling systems are totally separate from the central system that powers the suspension, steering and brakes and use a different fluid, a glycerine type.
* 1972 [[BMW E12]] 5-series released with optional hydropneumatic rear suspension. Coil springs are retained, though softer than conventional coils for the same car. This system was offered in most BMW 5-, 6-, and 7-series models, as well as the [[BMW E30|E30]] Touring ([[station wagon]]/estate), into the 1990s when it was replaced with an air suspension. Until late 1987, the hydraulic circuit was separate from the power steering, and the pump electrically powered.
* 1974 [[National Highway Traffic Safety Administration]] bans vehicles with [[height adjustable suspension]], impacting consumers in the [[United States]]. Ban repealed 1981.
* 1974 [[Citroën CX]]: The car was one of the most modern of its time, combining Citroën's unique hydro-pneumatic integral [[self-leveling suspension]] and speed-adjustable DIRAVI power steering (first introduced on the Citroën SM). The suspension was attached to sub frames that were fitted to the body through flexible mountings, to improve even more the ride quality and to reduce road noise. The British magazine ''[[Car Magazine|Car]]'' described the sensation of driving a CX as hovering over road irregularities, much like a ship traversing above the ocean floor.
* 1974 [[Maserati Quattroporte]] II: was on an extended Citroën SM chassis, available since Citroën had purchased the Italian company and was the only Maserati Quattroporte to feature hydropneumatic suspension and [[front-wheel drive]]
* 1975 The [[Mercedes-Benz 450SEL 6.9]] W116 replaces the [[air suspension]] of the ''[[Mercedes-Benz 300SEL 6.3|6.3]]'' with hydropneumatic suspension, 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.
* 1980 [[Mercedes-Benz W126]] 500SEL used hydropneumatic suspension as optional, later this system was available on 420SEL and 560SEL models.
* 1983 [[Citroën BX]], built as a 4WD in 1990<ref>{{Cite web |title=CITROEN bx GTI 4X4 |url=https://www.motorsportmagazine.com/archive/article/march-1990/61/citroen-bx-gti-4x4 |access-date=2022-11-29 |website=Motor Sport Magazine |date=7 July 2014 |language=en-GB}}</ref>
* 1984 [[Mercedes-Benz W124]] selected models of E class had this technology (rear only hydraulic suspension) height adjustable suspension and self-levelling suspension mixed with coil springs.
* 1987 [[BMW E30]] 3-series Touring ([[station wagon]]/estate) begins production in July, offering the same self-leveling hydropneumatic rear suspension as previous BMW, with the difference that the pump is a parallel circuit on the belt-driven steering assist pump, and shares its fluid. Starting in September, the [[BMW E32|E32]] 7-series (in production since June '86) switches to this pump from the previous electric pump. The [[BMW E34]] 5-series begins production in November, also with this new pump.
* 1989 [[Citroën XM]]: Hydractive Suspension, electronic regulation of the hydropneumatic system; sensors measure acceleration and other factors <ref name=XM>{{cite web|url=https://www.citroenet.org.uk/passenger-cars/psa/xm/xm-09.html |title=Citroën XM technical specification |publisher=Citroenet.org.uk |date=2000-06-10 |access-date=2018-06-29}}</ref>
* 1990 [[Peugeot 405]] Mi16x4: first Peugeot equipped with rear hydropneumatic suspension{{Citation needed|date=May 2013}}
* 1990 [[JCB Fastrac]] high speed agricultural tractor uses this system for its rear suspension.{{Citation needed|date=May 2013}}
* 1991 [[Toyota Soarer|Toyota Soarer UZZ32]] used hydraulic struts controlled by an array of sensors with yaw velocity sensors, vertical G sensors, height sensors, wheel speed sensors, longitudinal and lateral G sensors) that detected cornering, acceleration and braking force.
* 1993 [[Citroën Xantia]] used hydropneumatic, on 1995 Optional ''Activa'' ([[active suspension]]) system, eliminating body roll by acting on [[anti-roll bar]]s.<ref name=XM/> A ''Xantia Activa'' was able to reach more than 1g lateral acceleration, and still holds the record speed ({{convert|85|km/h|mph|abbr=on}}) through the [[moose test]] maneuver, due to its [[active anti-roll bar]]s.<ref name="Oppo">{{cite web|url=https://oppositelock.kinja.com/moose-tests-ranked-1681602389|title=Moose tests, ranked|publisher=G/O Media Inc.|date=24 January 2015|access-date=27 August 2020|archive-date=4 August 2020|archive-url=https://web.archive.org/web/20200804095914/https://oppositelock.kinja.com/moose-tests-ranked-1681602389|url-status=dead}}</ref> This test is conducted by the magazine ''[[Teknikens Värld]]'s'', as a test of avoiding a moose in the road. The second place car, [[Porsche 997]] GT3 RS was able to manage {{convert|82|km/h|mph|abbr=on}}.<ref name="drivetribe.com">{{Cite web|url = https://drivetribe.com/p/the-humble-champion-FSsUvLb_S9mQGtHkBMPFFQ?iid=BfUWANZdS3ivDjx53GocFQ|title = The humble champion|date = 29 November 2016|access-date = 17 November 2021|archive-date = 17 November 2021|archive-url = https://web.archive.org/web/20211117183616/https://drivetribe.com/p/the-humble-champion-FSsUvLb_S9mQGtHkBMPFFQ?iid=BfUWANZdS3ivDjx53GocFQ|url-status = dead}}</ref><ref name=XM/>
* 1995 [[Mercedes-Benz E-Class (W210)]] on estate (wagon) models on rear suspension used hydraulic suspension with spheres height adjustable suspension and self-levelling suspension mixed with coil springs.
* 1999 [[Mercedes-Benz CL-Class (C215)]] and [[Mercedes-Benz S-Class (W220)]] introduce optional [[Active Body Control]]{{dash}}an electronically controlled hydropneumatic system <ref name=StillRunning>{{cite web|url=https://stillrunningstrong.com/car-specs-problems/mercedes-benz/s-class-w220/|title=Mercedes-Benz S-Class (W220: 1998-2005)|author=|date=18 September 2018|access-date=16 November 2021}}</ref>
* 2001 [[Citroën 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. Hydractive 3+ was available on some models{{Citation needed|date=May 2013}}
* 2005 [[Citroën C6]]: An improved version of the C5 system known as Hydractive 3+ (also fitted to some C5 models), C6 with a V6 engine was fitted with AMVAR version of Hydractive 3+ (sometimes called Hydractive 4){{Citation needed|date=May 2013}}
* 2007 [[Citroën C5]] II: Hydractive 3+ as optional on Exclusive models. other versions of the car have normal spring suspension.
* 2008 [[JCB Fastrac]] high speed 7000 series agricultural tractors now use this system for front and rear suspension.{{Citation needed|date=May 2013}}
* 2019 [[Mercedes-Benz GLE-Class|Mercedes-Benz 450 GLE]] introduces e[[Active Body Control]] on a [[Sport utility vehicle]], discarding mechanical roll bars, notably enhancing performance.<ref name=RT2018>{{cite web|url=https://www.roadandtrack.com/new-cars/future-cars/a23100515/2019-mercedes-benz-gle-e-active-body-control-tech/|title=The 2019 Mercedes-Benz GLE 450 Has an Incredible Adaptive Suspension System{{dash}}The new Mercedes SUV can lean into corners and adjust each wheel's suspension tuning individually and on the fly.|author=CHRIS PERKINS|publisher=Road & Track|date=23 September 2018|access-date=16 November 2021}}</ref>
* 2023 [[BYD Auto]] introduces advanced active hydropneumatic suspension systems on the [[Yangwang]] U8 SUV and U9 sportscar. The suspension features the ability to drive with only three wheels fitted, and jump in the air while parked remaining level.<ref>{{Cite web |title=BYD Reveals DiSus Intelligent Body Control System, Exclusively for New Energy Vehicles |url=https://www.afp.com/en/news/1312/byd-reveals-disus-intelligent-body-control-system-exclusively-new-energy-vehicles-202304100052611 |access-date=2023-04-14 |website=AFP.com |date=17 April 2012 |language=en}}</ref>

==Functioning==
[[Image:Hydrosoft.gif|right|250px|thumb|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 Hydractive or Activa suspension, there may be as many as ten spheres. Spheres 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 (unit)|bar]], the bottom connects to the car's hydraulic fluid circuit. The high pressure pump, powered by the engine, pressurizes the hydraulic fluid (LHM – liquide hydraulique minéral) and an [[Hydraulic accumulator|accumulator sphere]] maintains a reserve of hydraulic power. This part of the circuit is at between 150 and 180 bars. It powers the front brakes first, prioritised via a security valve, and depending on type of vehicle, can power the steering, clutch, gear selector, etc.

Pressure flows from the hydraulic circuit to the suspension cylinders, pressurizing the bottom part of the spheres and suspension cylinders. Suspension works by means of a piston forcing LHM into the sphere, compressing the nitrogen in the upper part of the sphere; 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. Ride height correction (self leveling) is achieved by height corrector valves connected to the anti-roll bar, front and rear. When the car is too low, the height corrector valve opens to allow more fluid into the suspension cylinder (e.g., the car is loaded). When the car is too high (e.g. after unloading) fluid is returned to the system reservoir via low-pressure return lines. Height correctors act with some delay in order not to correct regular suspension movements. The rear brakes are powered from the rear suspension circuit. Because the pressure there is proportional to the load, so is the braking power.

== {{anchor|LHS versus LHM}} Working fluid ==
Citroën quickly realized that standard [[brake fluid]] was not ideally suited to high pressure hydraulics, and developed a special red-coloured [[hydraulic fluid]] named LHS ({{lang|fr|Liquide Hydraulique Synthétique}}), which they used from 1954 to 1967. The chief problem with LHS was that it absorbed moisture and dust from the air, which caused corrosion in the system. Most hydraulic brake systems are sealed from the outside air by a rubber diaphragm in the reservoir filler cap, but the Citroën system had to be vented to allow the fluid level in the reservoir to rise and fall, thus it was not hermetically sealed. Consequently, each time the suspension would rise, the fluid level in the reservoir dropped, drawing in fresh moisture-laden air. The large surface of the fluid in the reservoir readily absorbed moisture. Since the system recirculates fluid continually through the reservoir, all the fluid was repeatedly exposed to the air and its moisture content.

[[File:veerbol Xantia.jpg|thumb|LHM reservoir and green suspension sphere in a Citroën Xantia]]
To overcome these shortcomings of LHS, Citroën developed a new green fluid, ''LHM'' ({{lang|fr|Liquide Hydraulique Minéral}}). LHM is a [[mineral oil]], quite close to [[automatic transmission fluid]]. Mineral oil is hydrophobic, unlike standard brake fluid; therefore, water-vapour bubbles do not form in the system, as would be the case with standard brake fluid, creating a "spongy" brake feel. Use of [[mineral oil]] has thus spread beyond [[Citroën]], [[Rolls-Royce Limited|Rolls-Royce]], [[Peugeot]], and [[Mercedes-Benz]], to include [[Jaguar (car)|Jaguar]], [[Audi]], and [[BMW]].<ref name="citroen">{{cite web |last1=Jackson |first1=Tony |last2=Bardenwerper |first2=Mark L. |title=Revised Summary of Citroën Hydraulic Fluids |url=http://citroen.cappyfabrics.com/tony.html |website=citroen.cappyfabrics.com |date=March 2016 |access-date=2020-04-29 |archive-date=2020-02-15 |archive-url=https://web.archive.org/web/20200215083247/http://citroen.cappyfabrics.com/tony.html |url-status=dead }}</ref>

LHM, being a mineral oil, absorbs only an infinitesimal proportion of moisture, plus it contains corrosion inhibitors. The dust inhalation problem continued, so a filter assembly was fitted into the hydraulic reservoir. Cleaning the filters and changing the fluid at the recommended intervals removes most dust and wear particles from the system, ensuring the longevity of the system. Failure to keep the oil clean is the main cause of problems. It is also imperative to always use the correct fluid for the system; the two types of fluids and their associated system components are not interchangeable. If the wrong type of fluid is used, the system must be drained and rinsed with Hydraflush (Total's Hydraurincage), before draining again and filling with the correct fluid. These procedures are clearly described in DIY manuals obtainable from automotive retailers.

The latest Citroën cars with Hydractive 3 suspension have a new orange coloured ''LDS'' hydraulic fluid. This lasts longer and requires less frequent attention. It conforms to DIN 51524-3 for HVLP.<ref name=LDS/>

== Manufacturing ==
{{More citations needed section|date=June 2018}}
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, a type of polyurethane thermoplastic compatible with the LHM fluid. The moving parts of the system, ''e.g.'', suspension strut or steering ram, are sealed by contact seals 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. Height corrector, brake master valve and steering valve spools, and hydraulic pump pistons have extremely small clearances (1–3 micrometres) within their cylinders, permitting only a very low leakage rate. The metal and alloy parts of the system rarely fail, even after excessively high mileages, but the elastomer components (especially those exposed to the air) can harden and leak, typical failure points for the system.

Spheres are not subject to mechanical wear, but suffer pressure loss, due to the pressurised nitrogen diffusing through the membrane. They can, however, be recharged, which is cheaper than replacing them. When Citroën designed their Hydractive 3 suspension they redesigned the spheres with new nylon membranes, which greatly slow the rate of deflation. These are recognisable by their grey colouring.

Classic (non-saucer) green- (and grey-) coloured suspension spheres typically last between 60,000 and 100,000&nbsp;km. Spheres originally had a threaded plug on top for recharging. Newer ('saucer') spheres do not have this plug, but it can be retrofitted, enabling them to be recharged with gas. The sphere membrane has an indefinite life unless run at low pressure, which leads to rupture. Timely recharging, approximately every 3 years, is thus vital. A ruptured membrane means suspension loss at the attached wheel; however, ride height is unaffected. With no springing other than the (slight) flexibility of tyres, hitting a pothole with a flat sphere can bend the suspension parts or dent a wheel rim. In the case of main accumulator sphere failure, the high pressure pump is the only source of braking pressure for the front wheels. Some older cars had a separate front brake accumulator on power steering models.

The old LHS and LHS2 (coloured red) cars used a different elastomer in the diaphragms and seals that is ''not'' compatible with green LHM. The orange LDS fluid in Hydractive cars is also incompatible with other fluids.

== Legacy ==
The principles illustrated by the successful use of hydropneumatic suspension are now used in a broad range of applications, such as [[aircraft]] [[oleo strut]]s and gas filled automobile [[shock absorber]]s, first patented in the U.S. in 1934<ref>{{cite web |title=Hydropneumatic shock absorber US 1967641 A |url=http://www.google.sc/patents/US1967641}}</ref> by Cleveland Pneumatic Tool Co. Similar systems are also widely used on modern [[tank]]s and other large [[military]] vehicles.

=== Hydractive ===
'''Hydractive Suspension''' is an automotive technology introduced by [[Citroën]] in 1990. The prototype debuted in 1988 on the [[Citroën Activa]] concept. It describes a development of the 1954 hydropneumatic suspension design using additional electronic sensors and driver control of suspension performance. The driver can make the suspension stiffen (sport mode) or ride in outstanding comfort (soft mode). Sensors in the steering, brakes, suspension, throttle pedal and gearbox feed information on the car's speed, acceleration, and road conditions to an on-board computer, which in turn activates or deactivates an extra pair of suspension spheres on the circuit, to enable either a more smooth supple ride or tighter handling in corners. On the Activa and Activa 2, the car leaned inwards by one degree in turns{{dash}}Citroën acknowledged that this was somewhat of a marketing gimmick, and that a lean of zero degrees was optimal.<ref name=TV0391.44>{{cite magazine | ref = RC91 | title = Citroën fjädrar sig igen | trans-title = Citroën springs back up | language = sv | magazine = [[Teknikens Värld]] | publisher = Specialtidningsförlaget AB | location = Stockholm, Sweden | page = 44 | date = 1991-02-07 | issue = 2 | volume = 43 | first = Robert | last = Collin }}</ref>

An additional, perhaps unexpected, benefit of active suspension is that fuel consumption and tire wear is lowered overall. The negative camber designed into most suspensions in order to maximize the size of the contact patch when turning leads to tire scrub, which wears out tires and increases fuel consumption.<ref name=TV0391.44/>

==== Hydractive 1 and Hydractive 2 ====
[[Citroën]] Hydractive (and later Hydractive 2) suspension was available on several models, including the [[Citroën XM|XM]] and [[Citroën Xantia|Xantia]], which had a more advanced sub-model known as the [[Citroën Activa|Activa]]. The first Hydractive suspension systems (now known as Hydractive 1) had two user presets, ''Sport'' and ''Auto''. In the ''Sport'' setting the car's suspension was always kept in its firmest mode. In the ''Auto'' setting, the suspension was switched from soft to firm mode temporarily when a speed-dependent threshold in accelerator pedal movement, brake pressure, steering wheel angle, or body movement was detected by one of several sensors.<ref name="XM"/>

In Hydractive 2, the preset names were changed to ''Sport'' and ''Normal''. In this new version the ''Sport'' setting would no longer keep the suspension system in firm mode, but instead lowered the thresholds significantly for any of the sensor readings also used in ''Normal'' mode, allowing for a similar level of body firmness during cornering and acceleration, without the sacrifice in ride quality the ''Sport'' mode in Hydractive 1 systems had caused.

Whenever the Hydractive 1 or 2 computers received abnormal sensor information, often caused by malfunctioning electrical contacts, the car's suspension system would be forced into its firm setting for the remainder of the ride.

Starting with Xantia model year 1994 and XM model year 1995, all models featured an additional sphere and valve that together functioned as a pressure reservoir for rear brakes because of new hydraulic locks, letting the car retain normal ride height for several weeks without running the engine. Correctly called the SC/MAC sphere, it often became known as the 'anti-sink' sphere, because of its ability to better maintain rear suspension height.

==== Hydractive 3 ====
The 2001 [[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 orange synthetic hydraulic fluid named LDS fluid in place of the green LHM [[mineral oil]] used in millions of hydropneumatic vehicles.<ref name=LDS>{{cite web|url=http://www.lubadmin.com/upload/produit/FichePDF/lang_1/4008.pdf|title=LDS Fluid data sheet|website=lubadmin.com|access-date=2014-10-20|archive-date=2014-08-04|archive-url=https://web.archive.org/web/20140804134424/http://www.lubadmin.com/upload/produit/FichePDF/lang_1/4008.pdf|url-status=dead}}</ref>

A further improved Hydractive 3+ variation was for cars with top engines on the [[Citroën C5]] and in 2005 was standard on the [[Citroën C6]]. Hydractive 3+ systems contain additional spheres that can be engaged and disengaged via a ''Sport'' button, resulting in a firmer ride.

The Hydractive 3 hydraulic suspension has 2 automatic modes:
* Motorway position (lowering by 15&nbsp;mm of the vehicle height above 110&nbsp;km/h)
* Poor road surface position (raising by 13&nbsp;mm of the vehicle height below 70&nbsp;km/h)

The BHI of the Hydractive 3 suspension calculates the optimum vehicle height, using the following information:
* Vehicle speed
* Front and rear vehicle heights

The 3+ Hydractive hydraulic suspension has 3 automatic modes:
* Motorway position (lowering by 15&nbsp;mm of the vehicle height above 110&nbsp;km/h)
* Poor road surface position (raising by 13&nbsp;mm of the vehicle height below 70&nbsp;km/h)
* ''Comfort'' or ''dynamic'' suspension (variation of suspension firmness)

The BHI of the 3+ Hydractive suspension calculates the optimum vehicle height, using the following information:
* Vehicle speed
* Front and rear vehicle heights
* Rotation speed of steering wheel
* Angle of rake of steering wheel
* Vehicle's longitudinal acceleration
* Vehicle's lateral acceleration
* Speed of suspension travel
* Movement of the accelerator throttle

C5 I (2001–2004)
* Hydractive hydraulic suspension 3: [[PSA EW/DW engine|EW7J4]] and [[PSA EW/DW engine|DW10TD]] engines.
* Hydractive hydraulic suspension 3+: [[PSA EW/DW engine|EW10J4]], [[PSA EW/DW engine|EW10D]], [[PSA ES engine|ES9J4S]] and [[PSA EW/DW engine|DW12TED4]] engines.

C5 II (2004–2007)
* Hydractive hydraulic suspension 3: EW7J4, [[PSA EW/DW engine|EW10A]], [[Ford DLD engine|DV6TED4]] and [[PSA EW/DW engine|DW10BTED4]] engines.
* Hydractive hydraulic suspension 3+: [[PSA ES engine|ES9A]] and DW12TED4 engines (prior to RPO No 10645).

C6 (2005–2012)
* Hydractive hydraulic suspension 3+: Standard on all models.

C5 III X7 (2007–2017)
* Hydractive hydraulic suspension 3+: Depends on country and trim.

==See also==
* [[Hydrolastic]]{{dash}}a type of automotive suspension system used in many [[Automobile|car]]s produced by [[British Leyland]] and its successor companies.
* [[Hydragas]]{{dash}}is an improved form of Hydrolastic, using nitrogen-pressurised gas springs, rather than rubber.
* [[Hydraulic recoil mechanism]]{{dash}}uses the same principal for artillery.
* [[Oleo strut]]{{dash}}suspension for most large aircraft, using the same physical properties of air and hydraulic fluid.
* [[Active Body Control]]{{dash}}ABC, is the [[Mercedes-Benz]] [[brand]] name used to describe hydropneumatic fully [[active suspension]], that allows control of the vehicle body motions and therefore virtually eliminates [[car handling|body roll]] in many driving situations including [[Cornering force|cornering]], [[accelerating]], and [[braking]].
* [[Air suspension]]{{dash}}a type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. The air pressure inflates the bellows, and raises the chassis from the axle.
* [[Electronic Air Suspension]] (EAS) is the air suspension system installed on the second version of the [[Range Rover]]. Five suspension heights are offered by this system.

==References==
{{Reflist}}

== External links ==
* [https://web.archive.org/web/20170423065748/http://citroen.tramontana.co.hu/technical-guide The Citroën Technical Guide]

{{Citroën}}
{{Powertrain}}

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[[Category:Automotive suspension technologies]]
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