Air suspension (pneumatic suspension)

With air suspension (pneumatic cushioning) the spring forces are resisted by a chamber filled with compressed air, which is formed either by a cylinder containing a piston which can move inside it or which may, alternatively, be designed as a compressible spring bellows. With this arrangement a suitably yielding suspension, well adjusted to the load, can be obtained, and the movement of the car body thereby reduced (more particularly when travelling in bends).

As a result, riding comfort and safety are substantially increased. Depending on the type of suspension concerned, the air forming the cushion has a pressure of 3 to 9 atm. (45-130 lb/in.2). The piston is connected directly to the sprung masses, which try to push the piston farther into the cylinder.

This causes the air in the latter to become more highly compressed and thus resist this movement. The metal bellows, which can absorb movements only in the direction of its longitudinal axis, is used mainly in conjunction with rigid axles. With this kind of suspension it is necessary to provide special guiding devices in order to obviate the occurrence of transverse forces. Each bellows is connected to an auxiliary air vessel which ensures that there is always a sufficient quantity of air in the bellows. In the case of the roll bellows a piston-shaped movable body is directly connected to the auxiliary air vessel by the bellows.

It can also absorb transverse forces and can therefore be used in combination with independent wheel suspension and swinging half-axles. The installation principle of air suspension systems: The road clearance is automatically always kept constant by means of the air equalising valve (height regulator). The latter is connected to the axle through a linkage system.

When the load on the axle increases, the linkage rods will cause the piston of the height regulator to rise, so that air from the compressor and storage vessel can flow into the bellows, until the car body has risen so high that the inflowing air is cut off again by the piston in the height regulator. When the load on the axle decreases, or when the body lifts, the piston in the height regulator opens a hole through which air from the bellows can escape into the atmosphere, until the car body has regained its normal position and the piston in the regulator has returned to its middle position.

The height regulator can be provided with a damping device, which produces a certain time lag in the response of the regulator. The regulating system will then not respond to jolts of short duration. In addition, damping prevents oscillation of the regulator about its normal position, i.e., continual alternation between admission of air into the pneumatic spring and escape of air.