New tyre indicators: A safer journey with clearer warnings
A tyre's tread depth is crucial for its performance on wet surfaces and snow. That's why there's a TWI indicator on tyres, which shows when the tyre is worn out. Some manufacturers have introduced new, simpler, and more precise indicators.
The TWI indicator has been a solution used for years. It consists of leaving low blocks between the tread elements. If the tread wears down to its height, the tyre should be replaced. This system works well for summer tyres. Many drivers—quite sensibly—are willing to replace winter tyres at much lower wear levels than summer tyres. That's where the numbers placed on the tyre tread come in handy.
The first numbers of this kind appeared on Nokian tyres, but now we can also see them from manufacturers like Continental and Goodyear. Other manufacturers are also starting to use them. These mysterious numbers are tread groove depth markings in millimetres. The numbers on the tyre indicate the depth of the grooves and disappear as the tread wears down.
The smallest visible number is 4. This is intentional because a tread depth of 4 millimetres (about 5/32 inches) no longer provides adequate grip, especially for winter tyres but also summer ones. When we see only the "four", it indicates that the tyre is still usable, but we should consider replacing it. When the last number disappears, the tyre is only suitable for replacement.
Why is the tread depth so important?
Tread grooves act like reservoirs for water at the contact point with the road. When a car drives on a wet surface, these grooves create space for water that would otherwise remain under the tyre. If a tyre were completely smooth, the water couldn't be removed, leading to a loss of grip.
The more grooves a tyre has, the more water it can store, which increases driving safety. Grooves are more efficient when they are deeper and wider and the more there are on the tyre. This is why winter tyres often have a "herringbone" pattern, which allows for more grooves at the contact point with the surface than a longitudinal pattern would.
The concept is similar to understanding that the diagonal is always longer than any side in a rectangle. It contacts the asphalt surface in a rectangular-like form.
The width and number of grooves are determined by the tyre manufacturer, and we cannot change this by choosing a specific model. For users, the key aspect is the depth of the grooves, which changes as the tyre wears. Earlier tyre replacement leads to better performance on wet roads, while delay can have negative consequences.
To better understand how important tread grooves are, it's worth looking at motorsport, where tyre grooving adapts them to wet or dirty conditions. During rainy races, teams use tread grooving, which results in worse grip on dry surfaces. This action increases the number of grooves or their width, aiming to increase their overall capacity at the contact point with the road.
