Wheel loss: ‘Merely a case of physics’

I see the subject of wheel loss syndrome has reared its head again in your recent article from Croner-i.

I think Croner-i should stick to law. While its advice is sound enough, it does not seem to understand the root cause of the problem, which is forgivable, but then neither does DVSA, which is less so. There is no syndrome. The errant wheel nuts are merely obeying the laws of physics. If you fasten a wheel which is rotating anti-clockwise with fixings that undo anti-clockwise, you are just asking for the wheel to fall off.

This is due to precession. As the wheel rotates, it flexes under load, and this flexing, combined with the effects of braking and acceleration, and repeated heat cycling, will tend to undo the wheel nuts, with predictable consequences. That is why it is invariably nearside rear wheels that come adrift. The front wheel only experiences braking forces – the rear wheels have to contend with both acceleration and braking, and they have more mass to retain.

The principle of precession is not immediately apparent. Left-hand bicycle pedals, counter-intuitively, have a left-hand thread nut. The drag from the bearings will tend to undo the nut, but the forces of precession tend to tighten it, and they are stronger.

The pen pushers at DVSA call wheel loss a ‘syndrome’ and scratch their heads in bafflement when the answer should be obvious to any competent engineer: Fit left-hand thread nuts on nearside wheels and the problem goes away.

This used to be standard practice on commercial vehicles and quality cars, and wheels never used to fall off. Then we adopted the European system of spigot-mounted wheels held on with right-hand nuts, and now we have ourselves a ‘syndrome’. If the rules were made by engineers rather than bureaucrats, this would be a non-issue.

Hugo Miller