Air suspension is a type of vehicle suspension powered by an engine-driven or electric air pump or compressor. This pump pressurizes the air, using compressed air as a spring. Air suspension replaces conventional steel springs. If the engine is left off for an extended period, the car will settle to the ground. The purpose of air suspension is to provide a smooth ride quality and in some cases self-leveling. While not using high-pressure mineral oil, the system aims to achieve a result similar to the hydropneumatic suspension arrangement introduced in 1954 by Citron.
With a “leg up” on other companies, GM used its experience with commercial buses’ air suspension to introduce systems for its car lines, beginning with the 1958 model year. Airbags at each wheel replaced the 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, however. Period reviews rated the air suspension somewhat superior in ride quality, but not dramatically. Some reliability issues plagued these systems, as well. Thus, as an option, air suspension was short-lived in that era.
Vehicles that use air suspension today include models from Maybach, Rolls-Royce, Lexus, Mercedes-Benz, Land Rover/Range Rover, SsangYong, Audi, Subaru, Volkswagen, and Lincoln and Ford, among others.
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 the individual driver. The control system in the Mark VIII also lowered the suspension by about 25 mm (1 inch) at speeds exceeding about 100 km/h (60 mph) for improved aerodynamic performance. Unfortunately, however, these systems turned out to be unreliable and in many cases ended up being retrofitted with aftermarket replacements or conventional steel coil springs.
In addition to passenger cars, air suspension is broadly used on semi-trailers and buses, which are both transportation sectors that helped pioneer the use and design of air suspension. An unusual application was on EMD’s experimental Aerotrain.
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 to 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 onboard 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 vary 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 airbag, 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 airbag 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 vehicle’s 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:
Airbag 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 by 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 overextension of an air spring which is not sufficiently constrained by other suspension components such as a shock absorber may also lead to the premature failure of an air spring through the tearing of the flexible layers.
Airline 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 airlines, 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 be formed. 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 the airline is compromised to the point of failure due to the rubbing and resultant abrasion of the material. An airline failure may also occur if a piece of road debris hits an airline 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.