Anactive suspension is a type ofautomotive suspension that uses an onboardcontrol system to control the vertical movement of the vehicle'swheels andaxles relative to thechassis orvehicle frame, rather than the conventional passive suspension that relies solely on largesprings to maintain static support and dampen the vertical wheel movements caused by the road surface. Active suspensions are divided into two classes: true active suspensions, and adaptive or semi-active suspensions. While adaptive suspensions only varyshock absorber firmness to match changing road or dynamic conditions, active suspensions use some type ofactuator to raise and lower the chassis independently at each wheel.

These technologies allow car manufacturers to achieve a greater degree ofride quality andcar handling by keeping the chassis parallel to the road when turning corners, preventing unwanted contacts between the vehicle frame and the ground (especially when going over adepression), and allowing overall bettertraction andsteering control. An onboard computer detects body movement from sensors throughout the vehicle and, using that data, controls the action of the active and semi-active suspensions. The system virtually eliminatesbody roll and pitch variation in many driving situations includingcornering,accelerating andbraking. When used oncommercial vehicles such asbuses, active suspension can also be used to temporarily lower the vehicle's floor, thus making it easier for passengers to board and exit the vehicle.

Skyhook theory is that the ideal suspension would let the vehicle maintain a stable posture, unaffected by weight transfer or road surface irregularities, as if suspended from an imaginary hook in the sky continuing at a constant altitude above sea level, therefore remaining stable.

Since an actual skyhook is obviously impractical,^[1] real active suspension systems are based on actuator operations. The imaginary line (of zero vertical acceleration) is calculated based on the value provided by anacceleration sensor installed on the body of the vehicle (see Figure 3). The dynamic elements comprise only the linear spring and the linear damper; therefore, no complicated calculations are necessary.^[2][3]

A vehicle contacts the ground through the spring and damper in a normal spring damper suspension, as in Figure 1. To achieve the same level of stability as the Skyhook theory, the vehicle must contact the ground through the spring, and the imaginary line with the damper, as in Figure 2.Theoretically, in a case where thedamping coefficient reaches an infinite value, the vehicle will be in a state where it is completely fixed to the imaginary line, thus the vehicle will not shake.

Active suspensions, the first to be introduced, use separateactuators which can exert an independent force on the suspension to improve the riding characteristics. The drawbacks of this design are high cost, added complication and mass of the apparatus, and the need for frequent maintenance on some implementations. Maintenance can require specialised tools, and some problems can be difficult to diagnose.

Hydraulically actuated suspensions are controlled with the use ofhydraulics. The first example appeared in 1954, with thehydropneumatic suspension developed byPaul Magès atCitroën. The hydraulic pressure is supplied by a high pressureradial piston hydraulic pump. Sensors continually monitor body movement and vehicle ride level, constantly supplying the hydraulic height correctors with new data. In a matter of a few milliseconds, the suspension generates counter forces to raise or lower the body. During driving maneuvers, the encased nitrogen compresses instantly, offering six times the compressibility of the steelsprings used by vehicles up to this time.^[4]

In practice, the system has always incorporated the desirableself-levelling suspension andheight adjustable suspension features, with the latter now tied to vehicle speed for improvedaerodynamic performance, as the vehicle lowers itself at high speed.

This system performed remarkably well in straight ahead driving, including over uneven surfaces, but had little control over roll stiffness.^[5]

Millions of production vehicles have been built with variations on this system.

Colin Chapman developed the original concept of computer management of hydraulic suspension in the 1980s to improve cornering in racing cars. Lotus fitted and developed a prototype system to a 1985Excel with electro-hydraulic active suspension, but never offered it for sale to the public, although many demonstration cars were built for other manufacturers.

Sensors continually monitor body movement and vehicle ride level, constantly supplying the computer with new data. As the computer receives and processes data, it operates the hydraulic servos, mounted beside each wheel. Almost instantly, the servo-regulated suspension generates counter forces to body lean, dive, and squat during driving maneuvers.

In 1990, Nissan installed a hydraulic supported MacPherson strut based setup, called Full-Active Suspension that was used in the Nissan Q45 and President. The system used a hydraulic oil pump, a hydraulic cylinder, an accumulator and damping valve, which connected two independent circuits for the front and rear strut assemblies. The system would then recover motion energy to balance the car continuously.^[6] The system was revised and is now calledHydraulic Body Motion Control System, installed on theNissan Patrol andInfiniti QX80.

Williams Grand Prix Engineering prepared an active suspension, devised by designer-aerodynamicistFrank Dernie, for the team's Formula 1 cars in 1992, creating such successful cars that theFédération Internationale de l'Automobile decided to ban the technology to decrease the gap between Williams F1 team and its competitors.^[7]

Computer Active Technology Suspension (CATS) co-ordinates the best possible balance betweenride quality and handling by analysing road conditions and making up to 3,000 adjustments every second to thesuspension settings via electronically controlleddampers.

The 1999Mercedes-Benz CL-Class (C215) introducedActive Body Control, where high pressure hydraulic servos are controlled by electronic computing, and this feature is still available. Vehicles can be designed to activelylean into curves to improve occupant comfort.^[8][9]

Active anti-roll bar stiffens under command of the driver or suspensionelectronic control unit (ECU) during hard cornering. The first production car with this technology was theMitsubishi Mirage Cyborg in 1988.

In fully active electronically controlled production cars, the application of electric servos and motors married to electronic computing allows for flat cornering and instant reactions to road conditions.

TheBose Corporation has a proof of concept model. The founder of Bose,Amar Bose, had been working on exotic suspensions for many years while he was an MIT professor.^[10]

Electromagnetic active suspension uses linear electromagnetic motors attached to each wheel. It provides extremely fast response, and allows regeneration of power consumed, by using the motors as generators. This nearly surmounts the issues of slow response times and high power consumption of hydraulic systems. Electronically controlled active suspension system (ECASS) technology was patented by the University of Texas Center for Electromechanics in the 1990s^[11] and has been developed by L-3 Electronic Systems for use on military vehicles.^[12] The ECASS-equippedHumvee exceeded the performance specifications for all performance evaluations in terms of absorbed power to the vehicle operator, stability and handling.

• Michelin'sActive Wheel from 2004 incorporates an in-wheel electrical suspension motor that controls torque distribution, traction, turning maneuvers, pitch, roll and suspension damping for that wheel, in addition to anin-wheel electric traction motor.^[13][14]

• Audi activeelectromechanical suspension system introduced in 2017. It drives each wheel individually and adapts to the prevailing road conditions. Each wheel has an electric motor which is powered by the 48-volt main electrical system. Additional components include gears, a rotary tube together with internal titaniumtorsion bar and a lever which exerts up to 1,100 Nm (811.3 lb-ft) on the suspension via acoupling rod. Thanks to the front camera, the sedan detects bumps in the road early on and predictively adjusts the active suspension. Even before the car reaches a bump in the road, the preview function developed by Audi transmits the right amount of travel to the actuators and actively controls the suspension. The computer-controlled motors can sense imperfection on the road, and can raise the suspension up from the wheel which would go over the undulation, thus aiding the ride quality. The system will direct the motors on the outside to push up or pull down the suspension while cornering. This will result in a flatter drive and reduced body-roll around corners which in turn means more confident handling dynamics.^{[15][16][17][18][19][20][21]}

Adaptive or semi-active systems can only change theviscous damping coefficient of theshock absorber, and do not add energy to the suspension system. While adaptive suspensions have generally a slow time response and a limited number of damping coefficient values, semi-active suspensions have time response close to a few milliseconds and can provide a wide range of damping values. Therefore, adaptive suspensions usually only propose different riding modes (comfort, normal, sport...) corresponding to different damping coefficients, while semi-active suspensions modify the damping in real time, depending on the road conditions and the dynamics of the car. Though limited in their intervention (for example, the control force can never have different direction than the current vector of velocity of the suspension), semi-active suspensions are less expensive to design and consume far less energy. In recent times, research in semi-active suspensions has continued to advance with respect to their capabilities, narrowing the gap between semi-active and fully active suspension systems.

This type is the most economic and basic type of semi-active suspensions. They consist of a solenoid valve which alters the flow of the hydraulic medium inside theshock absorber, therefore changing the damping characteristics of the suspension setup. The solenoids are wired to the controlling computer, which sends them commands depending on the control algorithm (usually the so-called "Sky-Hook" technique).^{[citation needed]}

This type of system is used inCadillac'sComputer Command Ride (CCR) suspension system. The first production car^[22] was theToyota Soarer with semi-activeToyota Electronic Modulated Suspension, from 1983.

In 1985, Nissan introduced a shock absorber using a similar version, called "Super Sonic Suspension," adding an ultrasonic sensor that would provide information that a microcomputer would then interpret, combined with information from the steering, brakes, throttle, and vehicle speed sensor. The adjustment information signals would then modify the shock absorbers when a driver-controlled switch was placed in "Auto". The automatic adjustment could be limited if the switch was placed in "Soft," "Medium," or "Hard" settings. A modified version that didn't use the sonar module was also used, allowing the settings to be manually selected.^[23][24] This implementation is currently used industry-wide by a number of manufacturers, provided byMonroe Shock Absorbers called CVSAe, or Continuously Variable Semi-Active electronic.

In 2008, with the introduction of theNissan GT-R, "DampTronic" was jointly developed by Nissan and Bilstein. DampTronic provides three selectable driver settings that can also interact with theVehicle Dynamics Control technology to modify the transmission's shift points. The settings are labeled as Normal, Comfort, or R, and can be either set in Normal for automatic adjustment or the "R" setting for high-speed driving, while "Comfort" is for touring and a more compliant ride. The "R" mode enables the vehicle to utilize theyaw angle rate with a reduced steering angle for a crisper, more communicative steering, while the "Comfort" setting produces less vertical G-loading in comparison to the "Normal" or computer determined suspension setting.^[25]

Another method incorporates magnetorheological dampers with a brand nameMagneRide. It was initially developed by Delphi Corporation forGM and was standard, as many other new technologies, forCadillac STS (from model 2002), and on some other GM models from 2003. This was an upgrade for semi-active systems ("automatic road-sensing suspensions") used in upscale GM vehicles for decades. It allows, together with faster modern computers, changing the stiffness of all wheel suspensions independently. These dampers are finding increased usage in the US and already leases to some foreign brands, mostly in more expensive vehicles.^{[citation needed]}

This system was in development for 25 years. The damper fluid contains metallic particles. Through the onboard computer, the dampers' compliance characteristics are controlled by anelectromagnet. Essentially, increasing the current flow into the damper magnetic circuit increases the circuit magnetic flux. This in turn causes the metal particles to change their alignment, which increases fluid viscosity thereby raising the compression/rebound rates, while a decrease softens the effect of the dampers by aligning the particles in the opposite direction. If we imagine the metal particles as dinner plates then whilst aligned so they are on edge - viscosity is minimised. At the other end of the spectrum they will be aligned at 90 degrees so flat. Thus making the fluid much more viscous. It is the electric field produced by the electromagnet that changes the alignment of the metal particles. Information from wheel sensors (about suspension extension), steering, acceleration sensors - and other data, is used to calculate the optimal stiffness at that point in time. The fast reaction of the system (milliseconds) allows, for instance, making a softer passing by a single wheel over a bump in the road at a particular instant in time.^{[citation needed]}

By calendar year:

• 1954:Citroën Traction Avant 15-6H:,self-leveling Citroënhydropneumatic suspension on rear wheels.^{[citation needed]}
• 1955:Citroën DS, self-leveling Citroën hydropneumatic suspension on all four wheels.
• 1957:Cadillac Eldorado Brougham: premiere of self-leveling GMair suspension
• 1967:Rolls-Royce Silver Shadow Partial load bearing hydropneumatic suspension on all four wheels. Front system deleted in 1969^{[citation needed]}
• 1970:Citroën SM, self-leveling Citroën hydropneumatic suspension on all four wheels.^{[citation needed]}
• 1970:Citroën GS, self-leveling Citroën hydropneumatic suspension on all four wheels.
• 1974:Citroën CX, self-leveling Citroën hydropneumatic suspension on all four wheels.
• 1975:Mercedes Benz 450 SEL 6.9 Hydropneumatic suspension on all four wheels.
• 1982:Citroën BX, self-leveling Citroën hydropneumatic suspension on all four wheels.^{[citation needed]}
• 1979:Mercedes Benz W126 Hydropneumatic suspension on all four wheels as an option on the LWB v8 models
• 1983:Toyota Soarer: world first Electronically controlled (TEMS) that used a shock absorber control actuator (spring constant, variable attenuation force) installed^[26]
• 1985Mercedes Benz 190E 2.3-16 Partial load bearing hydropneumatic suspension on all four wheels as an option on the 16v model. Standard on the Evo 1 and Evo 2 models
• 1985: Nissan introduced ultrasoundsemi-active suspension sensing "Super Sonic Suspension" optionally on theCedric, Gloria andNissan Laurel that integrated actuators inside the MacPherson struts on the front and rear suspension.^[27]
• 1986:Jaguar XJ40, self-leveling suspension.^[28]
• 1986:Mercedes Benz W126 Hydropneumatic suspension on all four wheels with electronically controlled adaptive damping as an option on the LWB v8 models
• 1987:Mitsubishi Galant (sixth generation) - features Active Controlled Suspension (Dynamic ECS). The system enables a comfortable ride and handling stability by automatically adjusting the vehicle height and damping force.
• 1989:Citroën XM - self-levelling, semi-activeHydractive on all four wheels with automatically adjusted spring rates and dampeners.
• 1989:Mercedes Benz R129 Partial load bearing hydropneumatic suspension with automatically adjusted spring rates and dampers as an option (ADS)
• 1990:Infiniti Q45 andNissan President "Full-Active Suspension (FAS)", active suspension system
• 1990:Toyota Celica (ST183) Active Sports with fully Hydropneumatic active suspension and 4WS GT-R withToyota Electronic Modulated Suspension (TEMS) semi-active suspension
• 1992:Citroën Xantia VSX - self-levelling, semi-activeHydractive 2 on all four wheels, with automatically adjusted spring rates and dampeners.^{[citation needed]}
• 1993:Cadillac, several models with RSSroad sensing suspension. RSS was available in both standard andCVRSS (continuously variable road sensing suspension) systems. It monitoreddamping rates of theshock absorbers every 15milliseconds, selecting between two settings.^{[citation needed]}
• 1994:Toyota Celsior introduced firstSkyhook air suspension^[29]
• 1994:Citroën Xantia Activa - self-levelling, fully active Hydractive on all four wheels with hydraulic anti-roll bars and automatically adjusted spring rates and dampeners.
• 1998:Land Rover Discovery series 2 - Active Cornering Enhancement; an electronically controlled hydraulic anti-roll bar system was fitted to some versions, which reduced cornering roll.
• 1999:Mercedes Benz C215 Self leveling fully active hydraulicActive body control. Available on the S, CL and SL models
• 2000 Citroen C5 Hydractive 3 or Hydractive 3+
• 2002:Cadillac Seville STS, firstMagneRide^[30]
• 2004:VolvoS60 R andV70 R (Four-C, a short name for "Continuously Controlled Chassis Concept", semi-active)
• 2006 Citroen C6 - Hydractive 3+
• 2010:Alfa Romeo MiTo Cloverleaf (DNA System based onMaserati's Skyhook technology)
• 2012:Jaguar XF Sportbrake, self-leveling air suspension.^[31]
• 2013:Mercedes Benz W222: OptionalMagic body control. Self leveling fully active hydraulic system with road surface scanning electronics
• 2013:VolkswagenMk7 Golf R User-Selectable Electronically Controlled Shock Dampening (Dynamic Chassis Control (DCC))
• 2019:ToyotaAvalon Touring model (Adaptive Variable Suspension (AVS))
• 2025:NioET9 (SkyRide fully active suspension)^[32]

• Toyota Active Control Suspension
• Hydropneumatic suspension
• Active Body control

• Nye, Doug (1992).History of the Grand Prix Car: 1966-91. Hazleton Publishing.ISBN 0-905138-94-5.
• Cox, Ronald W. (1986).Electronics Developed for Lotus Active Suspension Technology. US: General Motors. Archived fromthe original on 2013-06-17. Retrieved2013-01-17.