Air suspension

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The Lincoln Town Car is one of the relatively few vehicles that utilize an air suspension system.

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 Citroën.

With a "leg up" on other companies, GM used its experience with commercial busses' air suspension to introduce systems for its car lines, beginning with the 1958 model year. Air bags 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, among others.

The air suspension designs from Land Rover, SsangYong 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) [1] for improved aerodynamic performance. Unfortunately, however, these systems turned out to be unreliable and in many cases ended up being retrofitted by aftermarket replacements [2] or conventional steel coil springs. [3]

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.

[edit] 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, unbolt the motor mounts and the bags will hold it up and make it bounce, the axle housing may be constrained laterally with either a Panhard bar 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" due to the fact that 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.