Suspension, steering and wheels on heavy vehicles

A heavy vehicle must carry large and varying loads while maintaining road contact, steering precisely, and protecting cargo and chassis from vibration. Suspension, steering, and wheel mounting are therefore a central area of expertise, and faults here often affect both handling and tire and component wear.

Most modern lorries and trailers use air suspension instead of classic leaf springs. The system consists of air bellows mounted between the chassis frame and the axle which absorb the vertical movements from road irregularities. Because air bellows can only absorb vertical forces they are supplemented by reaction rods that absorb forces from cornering and braking and keep the axle in place in the vehicle's longitudinal direction. Air suspension also allows level control so the loading height can be adapted when loading at a ramp.

• 01Check air suspension for cracks, wear and air loss at least every six months or at each service inspection.
• 02Check reaction rods and bushings for wear, which can cause imprecise steering and increased tire wear.
• 03Check level control valve function so suspension maintains correct vehicle height under load.

The steering on a heavy vehicle transfers the driver's steering movement via a steering column to the wheels. Because the forces are large, almost all lorries are equipped with hydraulic power steering, and some heavy models also have a steered rear axle to reduce turning radius. Wear in the steering's joints and ball joints can result in imprecise or loose steering, and because steering is directly safety-critical, control of it is included in the legally required inspection and maintenance process.

Wheel bolts on heavy vehicles must be re-tightened to a precise torque according to the manufacturer's instructions — loose bolts can in the worst case lead to a wheel coming loose while driving. The tires carry large loads and must therefore be regularly checked for tread depth, damage and correct air pressure, which is important for both safety and fuel consumption. Uneven tire wear is often a symptom of problems with suspension, steering geometry or brakes and should always be investigated further rather than just replaced.

Suspension and steering are one of the areas where preventive inspection pays back directly — a small leak discovered in time is cheap to fix compared to the wear and risk it would otherwise develop into.