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 some cases it is self-leveling. 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).
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 to 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.
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