Air suspension

Air suspension replaces the steel spring with an air spring: a tough rubber bellows, shaped like a bag or a rolling lobe, that sits between the chassis and the axle. An engine-driven compressor fills a reservoir tank, and a levelling valve on the chassis, worked by a link to the axle, lets air in or out of each bellows. In India it is standard on Volvo and Scania luxury coaches, offered on some premium haulage tractors and tankers, and found under luxury cars from Mercedes-Benz and others, where the driver can also choose ride height and firmness from a button.

• More load is placed on the vehicle and the body sinks towards the axle.
• The levelling link tilts the valve, which opens and feeds more air from the tank into the bellows.
• The higher pressure lifts the body until the link returns the valve to neutral: ride height is back to normal.
• Remove the load and the valve vents air to bring the body down again.
• A conventional damper beside each bellows controls the bouncing.

Compressed air springs are naturally progressive, soft over small bumps and stiffer as they compress, which gives the gliding ride passengers notice on a Volvo bus and keeps a loaded truck level so the headlamps and brakes work as designed. For transporters the ability to lower the chassis to meet a loading dock, or to raise a lift axle, is a bonus. The drawbacks are cost and complexity: bellows perish, air lines leak, compressors fail, and a flat air spring leaves the vehicle sitting on its bump stops until it is repaired.