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What are the yellow plastic indicators on truck and bus wheels?

Wheel-off incidents – where a wheel comes off a vehicle while it’s moving – can cause property damage, injury and even death. It’s more common than you realise. In 2010 a bus passenger was killed when a wheel came off a truck on Auckland’s Southern Motorway, bounced over the median barrier and smashed through the bus’s windscreen.

The wheels of the truck had been replaced but the wheel nuts were not retorqued properly. They gradually worked loose and the driver didn’t notice this on the pre-start inspection (checking the wheels properly is something you learn on a pre-trip inspection course).

Looking at the wheel nuts alone is not a reliable way of determining whether the nuts are loose, though. You can notice if there is a rust mark running down from the nut, but there’s little else unless the nut is so loose that you can see it has moved on the thread.

Streaks running out from the nut are an indication your wheel nuts have been loose a while!

There is a cheap, effective way of telling when a wheel nut has come loose, though, and it’s done with plastic indicators that are fitted over the wheel nuts. For general use, they come in yellow, red, orange and light green.

Banana wheel nut indicator
The plastic on these Zafety Lug Locks holds the nuts and stops them from moving. They only work on wheels with even numbers of wheel nuts.
Pointing at one another
Plastic arrow indicators can be used on wheels with any number of nuts. They point in a certain direction. If the wheel nut comes loose, they will be pointing away from the other nut. In this case, the top right nut has had two indicators added.
Lug nuts indicated in a clockwise pattern
However, you can use a system like above where each wheel nut points to the next one around in a clockwise direction.
One of the lug nuts has come loose
If a wheel nut is coming loose then you’ll see it like with the top indicator. It’s no longer pointing at the indicator to the right, so it’s obviously come loose.

This system of using plastic indicators means that, at a glance, you can see whether the wheel needs attention.


It also is a way to tell if there’s a problem with a sticking brake or a wheel hub that is dry as this will cause excessive heat in the wheel which will discolour and warp or melt the plastic.


A red indicator (or an indicator of a different colour) is used if a wheel has been changed but the nuts have not been retorqued.

red wheel nut indicator
This wheel shows one red indicator in front of a yellow indicator which means all the nuts need to be checked. The yellow indicator on the bottom left is showing the nut might have moved a little, perhaps as it hasn’t been retorqued properly. The driver, on doing the pre-trip inspection, must check with the transport operator or whoever changed the wheel to see what action needs to be taken. Torquing truck wheels requires some training and the right tools. If all the indicators are red, this does not mean it needs to be retorqued, just simply that they have used red indicators.

These lug nut indicators are not mandatory.

Some drivers only put a few indicators on a wheel. They are not expensive so there’s no reason not to put them on all the nuts.

For transport companies with a dedicated tyre department, the lug nuts allow the mechanic or tyre technician to instantly see if any nuts need checking, saving time.

On some trailer wheels, a separate long plastic strip might be added to the wheel, from one nut, that extends to the sidewall of the tyre. This is to provide visual reference that the wheel is turning and helps diagnose sticking brakes on trailers that have many axles.

When won’t wheel nut indicators prevent wheel-off incidents?

A wheel-off incident can be a wheel-end failure, meaning the whole set of tyres comes off. This is a failure inside the hub, which is almost impossible to see on a pre-trip inspection. This sort of wheel-off incident would be caused by either mechanical failure, or the person who performed the last service did not check the axle nut was tight, or the locking ring was installed properly.

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Darren has written over 3000 articles about driving and vehicles, plus almost 500 vehicle reviews and numerous driving courses. Connect with him on LinkedIn by clicking the name above

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