Why do Some Trucks and Buses Have Plastic Arrows on the Wheels?

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Unlike the small cars and trucks we see on the road every day, semi’s and buses are very big vehicles that travel a lot. Most semi’s/trucks or buses spend an average of 110,000 miles on the road every year. And it’s no surprise that during the journey they go through serious wear and tear. Although transmission and engine failures would be the first thing someone would think of, more than common, tires endure a lot of wear and tear. A tire failure while traveling 70 mph can have serious consequences and can put a lot of lives at risk. That is why some commercial trucks or buses have those tiny plastic arrows on the wheels.

What are those arrows, exactly? Why are they there? What do they do?

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Image: Herkie/Wikimedia

Those little plastic arrows are called “loose wheel nut indicators”. When mechanics first put on the wheels, or balance the wheels, and tighten all the nuts properly, these little plastic arrow things are placed on them. There are several methods but the two popular methods to use loose wheel nut indicators are the “buddy system” and the “chain method”. The one above is called the “chain method”.

In the chain method, the arrows point to the center of the next nut, and so on, forming an unbroken, flowing chain-of-arrows effect. So, if one of the wheel nuts comes loose, the arrow will move, breaking the flowing chain-of-arrows effect, letting the driver know that they need to inspect the wheel immediately.

In the buddy system, the arrows point at one another.

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Image: Wikimedia

The buddy system only works on wheels with an even number of wheel bolt holes. Whatever method a mechanic prefers, they both provide easy, one-glance checking to see if any of the bolts have come loose. Before the driver starts his journey, he/she can do a quick walk-around and immediately be able to tell if any critical wheel nuts are loose. This cheap plastic indicator also helps reduce maintenance cost since it eliminates the need for expensive electronic equipments.

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Image: Wikimedia

Once the driver notices that the loose nut indicator has moved, the nuts need to be retorqued by a qualified person using the proper tools. If a wheel has been replaced but not retorqued, a red indicator can be used. This is removed when the wheel is properly retorqued. This step should always be performed by a qualified technician since over torquing or under torquing can have serious consequences.

For instance, some drivers think that over torquing a wheel nut is the best option since the tighter is better. Over torquing actually stretches the studs or threads beyond their ability to respond, eventually causing them to fail. It can also result in cracked, seized or cross-threaded nuts and cracked wheels. Additionally, these plastic indicators also protect the nuts from dirt, rain, and other elements that could damage them and cause them to loosen.

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