How semi-trailer air suspension works

Basic operating principle

An air spring sits between suspension components and the frame. When load increases and the frame tends to lower, the height control system changes the amount of air in the springs to restore the specified position.

When load decreases, some air is released. Ride height is therefore maintained by regulation, not by a constant fixed pressure.

The regulatory definition of air suspension

GOST 33987-2016 defines air suspension as a system in which pneumatic spring elements provide at least 75% of the spring effect. It also recognises suspension equivalent to air suspension, with separate criteria.

This definition matters for vehicle assessment and application of mass and axle load rules. It does not mean that any system containing an air bag qualifies as air suspension without examining its design.

Main components

• air springs — elastic elements;
• trailing arms — guide axle movement and transmit forces;
• dampers — suppress oscillations;
• height control valve or electronic control system;
• air reservoirs, lines and valves;
• axle lift mechanism where provided.

Sespel’s industrial experience

Sespel’s air suspension range includes different brake systems and component layouts. For example, AB09W0-037-TB uses a drum brake, while AD09W0-037-TB uses a disc brake. Both specifications list a TB combined radial/axial bearing and a W0 welded suspension arm.

This variety shows that air suspension is selected as a system matched to the axle, brakes and operating conditions, rather than as an individual air spring.

Load distribution

In a multi-axle group, the pneumatic system helps equalise vertical loads. The result depends on installation height, arm geometry and pneumatic circuit condition.

Incorrect height or a faulty component can redistribute load, affecting tyre wear and suspension life.

Why air spring pressure is not a direct measure of axle load

Pressure relates to load, but conversion depends on effective air spring area, arm leverage, suspension height and the geometry of the axle group. A universal “pressure × area = axle load” formula that ignores kinematics gives only a rough estimate.

Load distribution is checked by weighing or using the vehicle’s calibrated load measurement system. Changing ride height changes arm geometry and the air spring operating point, so load distribution may become incorrect even with a sound pneumatic circuit.

• maintain the manufacturer’s specified transport height;
• do not change height control linkage length without following the adjustment procedure;
• after repairs, check symmetry and actual axle loads;
• do not use system pressure as the sole indication that the vehicle is not overloaded.

Why dampers are needed

An air spring provides elasticity but insufficient damping. The damper performs that function.

Worn dampers increase oscillation, impair tyre contact with the road and raise dynamic loads on mountings.

Signs requiring diagnosis

• a leaning semi-trailer or unstable ride height;
• noticeable pressure loss;
• uneven tyre wear;
• excessive bouncing after road irregularities;
• damaged air springs;
• unusual noises from joints and mountings.

Maintenance considerations

Check leak-tightness, air springs, mountings, joints, dampers and the height control system. A mechanical fault cannot be compensated for by arbitrary system adjustment.

After repairs affecting geometry, check operating parameters against documentation for the specific suspension.