By Gene Markel
Passenger car air helper springs have been around since the early fifties. Fifty years ago (1958 model year), GM introduced air suspension as an option for all of its passenger cars. It featured a single cylinder belt driven compressor from Bendix Westinghouse. Goodyear supplied its rolling lobe and sleeve type Super-Cushion air springs. The option lasted for two model years.
In the late 1960s, Mercedes-Benz applied air suspension to their SL sedans and limousines. Air springs returned in 1974 for the rear suspension of the new GMC Motor Home RV26. This time the springs came from Firestone and the compressor was driven by an electric motor. The mid 1980s saw a revival of air suspension at all four corners for Ford and other manufacturers.
Then there are air shocks, which were introduced in the 1960s. Air suspension systems came in to wide use on class 8 heavy-duty truck and bus applications in the early 1980s. Today, air shocks and struts are part of the suspension on many vehicles to compensate for back seat passengers and luggage.
What makes air springs different?
There are significant differences between a coiled steel spring and an air spring. Engineers use the term “hysteresis” to describe it. Hysteresis is the loss of mechanical energy that occurs as a spring is cycled from loaded and unloaded and is proportional to the amount of deflection.
The rubber products that make up the air spring do not react with the same speed as the steel coil spring. This makes the air spring slower to return to its uncompressed state. This difference in air spring reaction time requires different shock absorber valve configurations, when compared to a conventional coil spring to produce the same ride characteristics. All air spring suspensions are more complex than the conventional steel spring in that they require a compressor, solenoid valves, controls and plumbing.
Air Spring Types
Most passenger car original equipment air springs are of the rolling lobe, or tapered sleeve. Rolling lobe and tapered sleeve air springs have a cylindrical sleeve that is rolled inward and attached to a mounting that is smaller in diameter than the sleeve. An end plate is attached to the other end.
A convoluted air spring is constructed with one or more donut-shaped chambers that are sealed by end plates. For convoluted springs with more than one chamber, a band is used to contain each chamber. Air struts are a combination of a shock absorber and air spring.
Compressor and Dryer
Most passenger and light truck compressors are a diaphragm-type that supplies an oil free air supply to the springs. A piston-type compressor is available for custom systems. The compressor is designed for intermittent service to inflate the air springs. Running the compressor for extended periods can over heat the compressor and damage the diaphragm or piston. It is very important to ensure that the source of air for the compressor is clean and as dry as possible.
The air in air springs makes them more vulnerable to damage and malfunctions than a conventional steel coil spring. When air is compressed, the water vapor contained in the air is condensed into a liquid. If there is no means of removing the water from the system, it will find its way to all parts of the system causing corrosion damage or freezing.
Most systems have a dryer that is connected to the compressor outlet to absorb the water entering the system. The dryer contains a moisture absorbing desiccant such as silica gel. The desiccant can hold a given amount of water and once the desiccant is saturated with water it will allow water to pass into the system.