Air suspension

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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 compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. This in turn inflates the bellows, and raises the chassis from the axle.

Air suspension is often used in place of conventional steel springs, and in heavy vehicle applications such as buses and trucks. The purpose of air suspension is to provide a smooth, constant ride quality, but in some cases is used for sporty suspensions. Modern electronically controlled systems in automobiles and light trucks almost always feature self-leveling along with raising and lowering functions. Although traditionally called air bags or air bellows, the correct term is air spring (although these terms are also used to describe just the rubber bellows element with its end plates).


While 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.

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. It also featured the world's first fully functioning air suspension system, previously developed by Firestone. It never went into production.

General Motors 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 bellows 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 so.

An optional air suspension system was available on the 1959 Rambler Ambassadors and for all "Cross Country" station wagon models.[5] The "Air-Coil Ride" utilized an engine-driven compressor, reservoir, air bags within the coil springs, and a ride-height control, but the troublesome nature of the system outweighed its benefits and American Motors discontinued the system after only one year. Cadillac also discontinued air suspension after the 1960 model year.

Air suspension would not return to standard production on American-built cars until Lincoln Motor Company introduced it as standard equipment on the Lincoln Continental Mark VII in the 1984 model year.

Mercedes Benz equipped W112 Chassis series cars, as well as 300SE sedans and Coupes/Cabriolets with air suspension since 1962. The system used a Bosch main valve (distributing the air pressure) with two axle valves on the front axle and one valve of the rear axle. These controlled an air spring on each wheel axle. This was entirely different from the GM system in that the airspring used a bag mounted on a cone. As the car load increases on the bag it rolls down the cone and this in turn increases the air pressure in the bag. Because of the cone shape the suspension is infinitely variable. The axle valves do three jobs; they are fed reduced air pressure to the front and keep the bag supplied with sufficient air to keep the ride height constant. When the load is relieved they release air back to the car's air dryer. Later versions, such as the W109, included a ride height adjustment feature. The main valve has an extra setting the W112 cars did not have — the ability to raise the car up to 50 mm above the normal ride height. The rear valve is fed full air pressure from the reservoir in front, which in turn is kept filled by a single-cylinder air compressor powered by the engine. In 1964, Mercedes introduced its W100 Chassis car, the 600 Grosse or Grand Mercedes, which remained in production until 1984. The air springs on these are bigger version of those found on the W112 and W109 cars. On the 600 the air also powers the brake servo.

Vehicles that use air suspension today[when?] include models from Maybach, Rolls-Royce, Lexus, Jeep, Ram, Cadillac (GM), Mercedes-Benz, Porsche, Land Rover/Range Rover, SsangYong, Audi, Subaru, Volkswagen, Lincoln, Ford, and Tesla, among others. Citroën now[when?] 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, Chrysler, Subaru, Audi, Volkswagen, Tesla, Porsche, and Lexus models feature height adjustable suspension controlled by the driver, suitable for making it easier to enter the vehicle, clear bumps, or clear 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. Porsche has taken this to the next level on the Panamera with a system that changes the spring rate and damping settings, among other changes, for their sport/track modes. The Mark VIII 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)[6] for improved aerodynamic performance. One way automakers strive to improve gas mileage is by utilizing active suspension technology. Tesla Motors offers an optional "Active Air Suspension" on the Model S to lower the vehicle for aerodynamics and increased range.[7]

Pneumatic spring on a semitrailer

In addition to passenger cars, air suspension is broadly used on semi trailers, trains (primarily passenger trains) and buses. One application was on EMD's experimental Aerotrain.

Custom applications[edit]

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 manoeuver over obstacles and inconsistencies on paved surfaces. 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.

Cut-way of a deflated commercial truck air spring bellows. 'A' is the rubber bellows element.

Common air suspension problems[edit]

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. However, most modern automotive systems have overcome many of these problems.

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, although this is unlikely to occur in normal road use. It does occur in harsh off-road conditions but it still not common if correctly installed.

Air fitting failure usually occurs when they are first fitted or very rarely in use. Cheap low quality components tend to be very unreliable. Air fittings are used to connect components such as bags, valves, and solenoids to the airline that transfers the air. They are screwed into the component and for the most part push-in or push-to-fit DOT line is then inserted into the fitting.

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. This is far more likely to occur with low specification compressors with insufficient duty cycle which are often purchased due to low cost. For redundancy in the system two compressors are often a better option.[8]

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.[9]

Use on coaches and buses[edit]

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 curb level. The Kneel Down facility is also used when using the built in wheel chair ramps.

See also[edit]