When vehicles carry heavy loads, their suspension is compressed, causing the rear of the vehicle to be lower than normal.
As a result, steering becomes lighter, the alignment of the headlights becomes too high, and the compression length of travel of the suspension over bumps is reduced, causing discomfort to passengers.
To reduce these effects, a manually adjustable air spring can be incorporated into each rear shock absorber. The air spring consists of a flexible rubber cylinder which seals the outside of the upper and lower halves of the shock absorber.
The shock absorber is a standard hydraulic type, providing normal dampening action, but when a heavy load is placed on the rear of the vehicle, the rubber air cylinder can be pressurized to assist the suspension springs.
By changing the air pressure in the cylinder, the ride height can be adjusted, as well as the stiffness of the suspension.
Compressed air in the pneumatic cylinder can absorb smaller road shocks, and provide better ride characteristics than just stiff springs alone.
The rubber air cylinder is connected to a filling valve by a flexible plastic hose. Air from a tire pump or a hand unit forces more air into the rubber cylinder, allowing the suspension to support more weight.
The maximum air pressure setting must not be exceeded as this can damage the shock absorber and its mounting points on the vehicle frame.
When the load is removed, the extra air is released through a filling valve, which allows the suspension to return to its original settings.
A minimum air pressure must be maintained in the cylinder to prevent tearing of the rubber as it collapses internally with shock absorber action.