By Jim Kerr
My first recollection of air suspensions goes back to the 1958 Chevrolet, but there are likely earlier examples. Similar to today’s vehicles, the metal coil springs were replaced with rubber bladders that could be filled with air to hold the car up. Unfortunately, the rubber bladder technology was in its infancy and too many Chevy owners came out to find their parked cars sitting right on the ground. This look may be good for a lowrider but not for everyday driving, so almost all were converted back to regular metal spring suspension again.
Modern rubber technology has improved the durability of those rubber bladders or air bags as they are more commonly referred to. Many of the big highway trucks and trailers use them instead of leaf springs. Grain haulers even use the gauge pressures to determine the load on their trailer. Even the cabs on some of these trucks are suspended by air bags so that the driver receives the same smooth ride as their load does.
Air makes a great spring. When compressed only a little, it is very flexible. The heavier the load placed on it, the more it compresses and becomes stiffer. This variable rate spring can be accomplished in steel coil springs by changing the wire diameter and coil sizes over the
length of a spring, but air bags make it so much easier. Air suspension adds comfort to passenger vehicles, with a few additional advantages.
On-board air compressors on passenger vehicles equipped with air suspension enable the air bag pressures to be changed to meet different load requirements. The simplest systems use air bags in the rear only. These are typically called automatic level control and use a sensor
between the suspension and body to monitor body height. Place a heavy load in the back and the body height drops. The sensor signals a module that turns on the compressor and pumps the air pressure up until the body is at the correct height again. All the systems I have seen have a timer on the compressor so that it will only pump for a few minutes if
there is a leak in the system. This protects the compressor from overheating. If the compressor seems to be running excessively long each time you start the vehicle, then the system should be checked for leaks.
Air hoses and fittings are the most common source of leaks but occasionally a rubber bladder will fail on older high mileage vehicles. Instead of a separate rubber air bag as found on big trucks, most passenger vehicles build the air bag into the shock or strut assembly and when one leaks the complete unit is changed.
Many luxury vehicles such as Land Rover, Mercedes, Jaguar, Porsche and Lincoln use air suspension at all four corners not only for ride comfort but for vehicle handling. As vehicle speed increases, the vehicle’s suspension computer reduces air pressure slightly to lower the vehicle. This lowers the centre of gravity for better stability and reduces aerodynamic drag to improve fuel economy. Suspension sensors monitor wheel movement and the computer can vary air pressure for rough roads or potholes, keeping the tire on the pavement for increased traction. Some systems even sense when the vehicle is going around a corner and increase the air pressure on one side of the vehicle to keep it level in the corner.
Lowering the vehicle when parked also works well on higher SUV’s so passengers can get in the vehicle easier. Open a door and the systems stop lowering however. You wouldn’t want to bend that door on a high curb, would you?
Another advantage of an air suspension is that it can be used to raise the vehicle for rough road driving. Driver-operated manual inputs signal the computer to turn on the compressor and the vehicle rises. Some can lift several inches. The Range Rover will use the air suspension to automatically drop the wheels an additional three inches if the vehicle is in a difficult traction situation, so that more load is placed on the slipping wheel.
Hot rodders are the latest group to utilize air bag technology. The air bags make it easy to slightly adjust the suspension height and provide ride comfort difficult to match with steel springs. Important to hot rodders, they also look neat on exposed suspensions, so they can build an image and ride at the same time.