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Typical Air suspension setup for a trailer axle (exhibition display unit)

The Lincoln Town Car is one of the relatively few production passenger cars that utilize an air suspension system

Air suspension is a type of vehicle suspension powered by an electric or engine driven air pump or compressor. This pump pressurizes the air, using compressed air as a spring. Air suspension is often used in place of conventional steel springs, and in heavy vehicle applications such as buses and trucks. If the engine is left off for an extended period, the vehicle will gradually settle to the ground. The purpose of air suspension is to provide a smooth, constant ride quality and in most cases it is self-leveling. Air pressurizes the bag and in turn raised the chassis from the axle. Although traditionally called air bags, the correct term is air bellows.


Whilst not using high pressure mineral oil (as does the Citroën system), the system aims to achieve a result similar to the hydropneumatic suspension arrangement introduced in 1954 by Citroën.

In 1901 William W. Humphreys patented (#673682) a 'Pneumatic Spring for Vehicles'. The design consisted of a left and right air spring longitudinally channeled nearly the length of the vehicle. The channels were con-caved to receive two long pneumatic cushions. Each one was closed at one end and provided with an air-valve at the other end.[1]

An early attempted implementation of air suspension was by Messier[2][not in citation given] in the 1920s.

Immediately following World War II, William Bushnell Stout built one last prototype Stout Scarab, called the Stout Scarab Experimental.[3] It was shown in 1946 and was more conventional in appearance than the prewar Scarabs, although still equipped with a rear engine. It was 2-door and featured a wraparound windshield. It featured the world's first fiberglass body, and like its metal counterparts, it was monocoque, built up out of only eight separate pieces. More importantly, it featured the world's first fully functioning air suspension system, previously developed by Firestone. It never went into production.

With a "leg up" on other companies[citation needed], GM used its experience with commercial bus air suspension to introduce systems for its automobile lines, introducing it as standard equipment on the Cadillac Eldorado Brougham in the 1957 model year.[4] The following year it was offered as optional equipment on all Cadillacs, and in 1959 it was made standard equipment on all Eldorados. Air bags at each wheel replaced standard coil springs, and had sensors to keep the car level under load and in turns. It was too slow to react in sudden maneuvers. Period reviews rated the air suspension somewhat superior in ride quality, but not dramatically. Some reliability issues plagued these systems as well.

Cadillac discontinued air suspension after the 1960 model year. Air suspension would not return to standard production until Lincoln introduced it as standard equipment on the Lincoln Continental Mark VII in the 1984 model year.

Vehicles that use air suspension today include models from Maybach, Rolls-Royce, Lexus, Jeep Grand Cherokee, Cadillac (GM), Mercedes-Benz, Land Rover/Range Rover, SsangYong, Audi, Subaru, Volkswagen, and Lincoln and Ford, among others. Citroën now feature Hydractive suspension, a computer controlled version of their Hydropneumatic system, which features sport and comfort modes, lowers the height of the car at high speeds and continues to maintain ride height when the engine is not running.

The air suspension designs from Land Rover, SsangYong, Subaru and some Audi, VW, and Lexus models, feature height adjustable suspension controlled by the driver, suitable for clearing rough terrain. The Lincoln Continental and Mark VIII also featured an air suspension system in which the driver could choose how sporty or comfortable they wanted the suspension to feel. These suspension settings were also linked to the memory seat system, meaning that the car would automatically adjust the suspension to individual drivers. The control system in the Mark VIII lowered the suspension by about 25 mm (1 inch) at speeds exceeding about 100 km/h (60 mph)[5] for improved aerodynamic performance. Due to the many advantages air suspensions provide, and with the advancement of new materials and technologies, these systems are being designed in to many future platforms. This is especially important as car manufacturers strive to improve gas mileage by reducing weight and utilizing active suspension technology to maximize performance. The electric car maker Tesla Motors plans to offer "Active Air Suspension" as an option on the Tesla Model S as a means to automatically lower the vehicle for optimized aerodynamics and increased range while allowing the user to change the setting as desired.[6]

Pneumatic spring on a semitrailer

In addition to passenger cars, air suspension is broadly used on semi trailers, trains (primarily passenger trains) and buses, which are all transportation sectors that helped pioneer the use and design of air suspension. One application was on EMD's experimental Aerotrain (GM)|Aerotrain.

Custom applications

Over the last decade or so air suspension has become extremely popular in the custom automobile culture: street rods, trucks, cars, and even motorcycles may have air springs. They are used in these applications to provide an adjustable suspension which allows vehicles to sit extremely low, yet be able rise to a level high enough to maneuver over obstacles and inconsistencies in the roadways (and parking lots). These systems generally employ small, electric or engine-driven air compressors which sometimes fill an on-board air receiver tank which stores compressed air for use in the future without delay. High-pressured industrial gas bottles (such as nitrogen or carbon dioxide tanks used to store shielding gases for welding) are sometimes used in more radical air suspension setups. Either of these reservoir systems may be fully adjustable, being able to adjust each wheel's air pressure individually. This allows the user to tilt the vehicle side-to-side, front-to-back, in some instances "hit a 3-wheel" (contort the vehicle so one wheel lifts up from the ground) or even "hop" the entire vehicle into the air. When a pressure reservoir is present, the flow of air or gas is commonly controlled with pneumatic solenoid valves. This allows the user to make adjustments by simply pressing a momentary-contact electric button or switch.

The installation and configuration of these systems varies for different makes and models but the underlying principle remains the same. The metal spring (coil or leaf) is removed, and an air bag, also referred to as an air spring, is inserted or fabricated to fit in the place of the factory spring. When air pressure is supplied to the air bag, the suspension can be adjusted either up or down (lifted or lowered).

For vehicles with leaf spring suspension such as pickup trucks, the leaf spring is sometimes eliminated and replaced with a multiple-bar linkage. These bars are typically in a trailing arm configuration and the air spring may be situated vertically between a link bar or the axle housing and a point on the vehicle's frame. In other cases, the air bag is situated on the opposite side of the axle from the main link bars on an additional cantilever member. If the main linkage bars are oriented parallel to the longitudinal (driving) axis of the car, the axle housing may be constrained laterally with either a Panhard rod or Watt's linkage. In some cases, two of the link bars may be combined into a triangular shape which effectively constrains the vehicles axle laterally.

Often, owners may desire to lower their vehicle to such an extent that they must cut away portions of the frame for more clearance. A reinforcement member commonly referred to as a C-notch is then bolted or welded to the vehicle frame in order to maintain structural integrity. Specifically on pickup trucks, this process is termed "notching" because a portion (notch) of the cargo bed may also be removed, along with the wheel wells, to provide maximum axle clearance. For some, it is desirable to have the vehicle so low that the frame rests on the ground when the air bags are fully deflated.

Common air suspension problems

Air bag or air strut failure is usually caused by wet rot, due to old age, or moisture within the air system that damages it from the inside. Air ride suspension parts may fail because rubber dries out. Punctures to the air bag may be caused from debris on the road. With custom applications, improper installation may cause the air bags to rub against the vehicle's frame or other surrounding parts, damaging it. The over-extension of an airspring which is not sufficiently constrained by other suspension components, such as a shock absorber, may also lead to the premature failure of an airspring through the tearing of the flexible layers. Failure of an airspring may also result in complete immobilization of the vehicle, since the vehicle will rub against the ground or be too high to move.

Air line failure is a failure of the tubing which connects the air bags or struts to the rest of the air system, and is typically DOT-approved nylon air brake line. This usually occurs when the air lines, which must be routed to the air bags through the chassis of the vehicle, rub against a sharp edge of a chassis member or a moving suspension component, causing a hole to form. This mode of failure will typically take some time to occur after the initial installation of the system, as the integrity of a section of air line is compromised to the point of failure due to the rubbing and resultant abrasion of the material. An air-line failure may also occur if a piece of road debris hits an air line and punctures or tears it.

Compressor failure is primarily due to leaking air springs or air struts. The compressor will burn out trying to maintain the correct air pressure in a leaking air system. Compressor burnout may also be caused by moisture from within the air system coming into contact with its electronic parts.

In Dryer failure the dryer, which functions to remove moisture from the air system, eventually becomes saturated and unable to perform that function. This causes moisture to build up in the system and can result in damaged air springs and/or a burned out compressor.[7]

Use on Coaches and Buses

Most factory standard coaches have a system called ferry lift. This allows the air suspension to be raised above the normal ride height level to originally aid loading and unloading the vehicle on and off ferries due to their steep ramps and risk of grounding out, but can be used on rough ground or on steep crests. Although the ferry lift may be installed on some buses, the Kneel Down facility is more common on public transport buses. This allows air to be released from the suspension system to decrease the step that passengers have to climb to enter the bus as they usually level out to kerb level. The Kneel Down facility is also used when using the built in wheel chair ramps.

See also


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