Loose wheel nut indicators are small pointed tags, usually made of fluorescent plastic, commonly fixed to lug nuts of large vehicle wheels. The tag rotates with the nut, so that if the nut becomes loose, it shifts noticeably out of alignment with other tags. If too many lug nuts are loosened, the wheel can detach.

But these systems have a major flaw – they rely on the driver identifying the loose nuts before setting off. If the driver forgets the inspection, or if the wheel loosens during a journey, the driver may not realise until it’s too late.

What’s more, if nuts are not tightened correctly or the studs or nuts break, the driver will be unaware whilst travelling at speed – which can potentially lead to wheel loss occurring even with these mechanical devices fitted.

Wheel loss can also occur with kerbing (a regular occurrence in the bus and multi-drop urban distribution sector). If the torque wrench used to tighten nuts is out of calibration or stud fatigue occurs, it can also trigger a wheel loss accident.

In our experience, mechanical systems can be used only to complement a full maintenance programme – but the industry is still missing a true all-round safety solution. That’s why we’ve developed MICHELIN Wheel Safe.

Wheel fixings can fail or loosen for a variety of different reasons. The IRTE and FTA have identified the most likely causes as:

1. The fixing is insufficiently tightened and thus allows the wheel to fret and wear
2. The fixing is over-tightened, causing stretched/broken studs or causing studs to be pulled through the hub. It is often an unconscious act for drivers and maintenance staff to over-torque wheels, in the misguided belief that exerting extra pressure on a wheel fixing will prevent them from coming loose. Either way it will result in stud failure due to the fact the elastic limit of the material is exceeded
3. Failure to regularly check tightness of wheel fasteners
4. Examples of elongation of stud holes, wheel movement, wear to studs and a cracked wheel
5. Fitment of incompatible wheels
6. Fitment of unserviceable wheels and components
7. Incorrect lubrication of threads and interfaces. Lack of initial clamping force can be due to high friction loss which can be resolved by the use of appropriate lubrication of the wheel nuts, studs and washers
8. Excessive lubrication or incorrect lubrication applied to the mating surfaces of wheels
9. Severe corrosion and/or wasting of wheel studs
10. Studs not pressed fully into the hub or loose back nuts
11. Incorrectly assembled components
12. Incorrect assembly, from not following the correct torque and retorque procedure
13. Paint, rust/scale or dirt on mating surfaces
14. Worn wheel spigots
15. Fractured or seized wheel nuts washers
16. Brake drum securing screws/bolts loose, holding wheel off the hub face
17. Inaccurately calibrated torque wrench or calibration date expired
18. Incorrect use of air impact tools. This is a common cause of both under-torquing and over-torquing because the actual torque applied depends on a wide range of variables and is not measurable
19. The power of air impact tools also often leads to problems such as cross-threading and cracked nuts and washers
20. Incorrect tightening sequence
21. Temperature effects: The clamp load can vary during service due to changes in temperature of the components
22. Settlement from any other factor eventually causing the clamping load to become inadequate

In summary, the IRTE and FTA concluded that the assembly can only accommodate a very small amount of settlement. The wheel studs are very stiff and the amount of extension caused by the tightening torque is limited, particularly if the stud tension is lower than intended in the first place. Nut movement can be indicated whereas settlement is difficult to detect visually.