Despite advances in modern technology, the fundamental issue of wheel detachment continues to be a major concern for the commercial vehicle industry.
When a wheel becomes detached from a moving bus, coach, truck or trailer, it can accelerate to 150 km/h. This has been likened by academics to a bouncing bomb, as the vehicle wheel soars to a height of up to 50 m before dropping and potentially colliding with other road users at an equivalent force of around ten tonnes.
The damage this can cause was evidenced in May when a wheel came off an HGV in Lancashire and collided with an oncoming car.
A Transport Research Laboratory (TRL) report into wheel loss on commercial vehicles, published for the Department for Transport in 2010, estimated that each year there were between 7,500 and 11,000 wheel fixing incidents. Of these, some 150 to 400 resulted in wheel detachments, with 50 to 134 causing damage, ten to 27 leading to injury, and between three and seven resulting in a fatality.
There are many reasons why a wheel may come off a moving commercial vehicle.
Consider a standard heavy truck wheel fixing for a moment. There are typically ten studs and nuts holding that wheel assembly on to the axle. The clearances that allow studs and rim holes to line up are generous so there is potential for movement -- and you are reliant on nut heads and enough torque to hold the wheel in place.
As the wheel turns, it is subject to thermal expansion, vibration and axial forces. These forces load on to one or two studs in sequence rather than all ten simultaneously. The effect is that those generous tolerances present a less-than-perfect mechanical set-up.
The TRL report suggested the standard design of wheel fixings required a thorough maintenance regime to "adequately reduce the risk of detachment". However, even the best-maintained vehicles can still be prone to mechanical failure or human error.
Fleet operators have been trying to resolve the issue for many years, usually with brightly coloured indicator tags that alert the driver of a loosening wheel nut. But these tags detect potential wheel losses on stationary vehicles only. They are also reliant on the driver conducting their walk-around vehicle inspection which, as we know, doesn't always happen.
It's clear a new approach is needed. The industry has to look at the problem from a different angle if incidents like the one in Lancashire are to be prevented.
Given the most recent research is nearly ten years old, the DfT should perhaps commission a further study to ascertain what, if anything, has changed.