Wheel fitment: This is a sensitive issue to write about!
There are many strong opinions (probably more than there are facts), and serious consequences when things go wrong. From what I can tell, the human is usually the root cause of most wheel losses.
It could be the component condition assessment undertaken by the technician or the fitment process being incorrect. Although not the root cause, it could be that the driver fails to adequately inspect their bus on first use, which could and should identify a problem before it results in wheel loss.
Many manufacturers specify a wheel nut torque value with quite a broad range. It is odd how many vehicle OEMs use the same axles, hubs and wheels, yet specify different torque values. These differences can be up to 100Nm.
As wheel studs are used many times, it is important that they remain within their elastic zone. If they were to be taken beyond it and slightly into plastic deformation, it could result in a much more consistent and stable clamping force, but who wants to replace wheel studs after every use?!
It is generally (but not always) recommended to lightly lubricate threads and nut collars. This makes a meaningful difference to the clamping force achieved. The resistance in the nut moving on the stud reduces significantly, meaning that the torque applied increases the clamping and therefore the tensile load on the stud.
Most wheel fitment procedures will entail ensuring that all mating faces are in good condition, fitting the wheel and running the nuts up to a torque lower than the final setting, and then using a torque wrench to apply the final torque to the nuts in a specified pattern. Most will require another torque check after 24 hours or a certain distance.
Some companies require a further torque check shortly after the initial fitment or a test drive, which is to deal with any torque relaxation. From research I have done, this achieves very little, as the amount of relaxation will be less than the tolerance range of the torque wrench, and any relaxation occurs almost immediately after the target torque is achieved. If you are concerned about torque relaxation, you may as well run around the nuts with the torque wrench twice on initial fitment.
Stud, wheel and hub face condition are all important. Even if the wheel and hub faces are clean, any raised or pitted surfaces due to corrosion will result in an uneven clamping pressure. Particularly with flat or flange-type nuts, the high forces imposed on the wheels during acceleration, braking and cornering can cause the wheel to move slightly and the nuts to loosen slightly.
This is more prevalent on the nearside because the forces at play mean the wheel is more likely to move in the direction to slacken the nut. The inertia of the wheel on the nearside then encourages the nuts to loosen further.
With shanked nuts typically used on alloy wheels where the threaded shank of the nut goes through the stud holes of the wheel, coupled to the lower weight of an alloy wheel, movement is less likely. The stretching of studs on every fitment and torque check, and the loads they experience in use, means that studs will fatigue over time. Any corrosion or damage will shorten their useful life.
All that said, if followed correctly, most companies’ wheel fitment procedures are adequately robust. The most effective thing for an operator to prevent wheel loss is to do everything reasonably possible to ensure the procedure is followed correctly.
There are few adequate electronic maintenance systems that promote compliance of wheel fitment and help the technician to demonstrate their compliance. Most I have seen are rather lacking in this area, leaving operators to use old paper-based systems to manage wheel fitment and subsequent torque checks.
And finally, perhaps manufacturers can bring back reverse threads on nearside wheel studs?
