Navigating tyre safety: New tread depth indicators unveiled

Adequate tread depth in tyres is extremely important, especially when driving on wet surfaces and, in the case of winter tyres, also on snow. Manufacturers place a TWI indicator that shows the tyre's wear level. Recently, a new solution has been implemented: numbers on the tread.

As we see in the photos, Nokian first introduced such markings. Continental and Goodyear also use them, so they can be found on tyres from various manufacturers. The mysterious numbers indicate the tread groove depth in millimetres. Disappearing numbers indicate worn tread.

The smallest visible number is 4 mm. This value suggests that a tread thickness of 4 mm no longer provides optimal performance for summer or winter tyres. When only the "4" remains, the tyre is still usable, but it suggests that replacement should be considered. When this value also disappears, immediate replacement of the tyre is necessary.

Tread grooves have a primary task - draining water from under the tyre and ensuring grip in wet conditions. Thanks to the grooves, water is collected in these spaces, preventing grip loss. The tyre could not effectively expel the water without sufficient grooves, leading to hydroplaning.

The more tread grooves that make contact with the road surface, the larger the space for water collection, thereby increasing grip on slippery surfaces. The tread groove is more effective the deeper and wider it is, and the more numerous it is on the tyre's contact surface. That's why winter tyres usually have a diagonal pattern, known as the herringbone, which allows for more "reservoirs" to be formed on the contact surface than longitudinal grooves.

Tyre manufacturers specify the nominal width and number of grooves in the tread, so we have no influence on choosing a specific tyre model. Tread groove depth, however, is closely related to tyre wear. This is the only aspect we can control as users. Replacing the tyre earlier improves its performance on wet surfaces, while delaying replacement significantly worsens these parameters.

To understand the importance of the number and capacity of tread grooves, it is worth looking at some motorsports where tyres are cut for wet surfaces. When it starts to rain, teams cut the tyres to adapt them to the new conditions at the expense of grip on dry surfaces. Cutting increases the number and width of grooves and their water capacity on the contact surface.

According to the rule described above, we are talking about a depth of 4 mm. Below this value, tyres work significantly worse on wet surfaces. However, it is worth noting that according to regulation UN ECE-R117, which was revised and took effect on 1st July 2024, tyre manufacturers are required to ensure good properties up to the tread wear limit of 1.6 mm.