Confusion surrounds latest wheel standard
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• Doubts have been raised about some of the advice on wheel-fixing maintenance in a revised British Standard published this month (13S AU50 part 2, section 7A, 1995: Code of practice for the selection and care of tyres and wheels for commercial vehicles).
The standard advises that wheel nuts be re-checked after the vehicle has stood for 30 minutes or has covered 4080km. But in trials with four artics in the Bass brewery fleet some nuts still worked loose after 160km despite adherence to the standard's recommendations. Checks after another 160km showed no further loss of tightness. Wheel nuts were tightened to the torques recommended by Volvo and SAF. In checks 15 minutes after fitting, new studs and nuts needed further tightening, but not the old ones.
The latest British Standard still avoids recommending a rationalised tightening torque. It merely advises following "manufacturer's torque value".
As these vary between 450 and 800Nm for 10-stud fixings, there is considerable scope for error. The standard does not even recommend that manufacturers set torque values that will induce the 22 tonnes of stud tension necessary to assure durable tightness over all operating conditions.