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
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?
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.
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
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