Backround Information on Wheel-fixings

Wheels are secured to commercial vehicles by wheel-nuts. Improperly tightened nuts can lead to a wheel flying off, which the UK Department of Transport says causes as many as 3,000 accidents and 10 deaths annually in the UK alone.

Wheels fly because of a failure of their nuts and/or studs. Many factors contribute to these failures, but they all concern one thing - stud tension.

A wheel-stud or bolt is like an elastic band; if you stretch it, it pulls back. The force of this pullback is called tension. The further you stretch the stud, the more tension is generated. This tension pulls the wheel against the hub and generates friction, which holds the wheel in place.

Set too low a tension:

• Loose nuts
• Wheel suffles on hub

Set too high a tension:

• Snapped studs
• Fatigue in studs

Any of the above will lead to a flying wheel.

Settlement, or bedding-in, is a compression of the mating surfaces of the assembly. This takes place soon after the assembly is tightened up. This compression means the stud is not stretched as far and so the tension is reduced. This reduction in tension could lead to a flying wheel.

There is only a small 'safe tension zone' of between 220kN and 260kN. Above 220kN to ensure enough friction is generated and below 260kN to prevent fatigue in the stud.