Tread depth indicators: Revolutionizing tire safety and performance
Appropriate tread depth in tires is extremely important, especially when driving on wet surfaces and for winter tires, as well as on snow. Manufacturers include a TWI indicator that shows the tire's level of wear. Recently, a new solution involves the appearance of digits on the tread.
These types of markings, as seen in the photos, were first introduced by Nokian. Continental and Goodyear are also using them, so they can be found on tires from various manufacturers. The mysterious numbers are nothing more than an indication of tread groove depth in millimetres. Disappearing numbers indicate a worn-out tread.
The smallest visible number is 4. Such a value suggests that a tread thickness of 4 mm (0.16 inches) no longer provides optimal performance for either summer or winter tires. When only the "four" remains, the tire is still usable, but it suggests the need to consider a replacement. When this value also disappears, immediate tire replacement is necessary.
Why is tread depth so important?
The primary task of tread grooves is to channel water away from the tire and ensure grip in wet conditions. Thanks to the grooves, water is collected into these spaces, preventing loss of grip. Without sufficient grooves, water can't be effectively pushed out by the tire, leading to hydroplaning.
The more tread grooves in contact with the surface, the more space is available for water collection, improving traction on slippery surfaces. Tread grooves are more effective when they are deeper, wider, and more numerous on the contact surface between the tire and the road. This is why winter tires usually have a diagonal pattern, known as a herringbone, which allows for more "reservoirs" on the surface compared to longitudinal grooves.
Tire manufacturers determine the nominal width and number of grooves in the tread, so we have no control over choosing a specific tire model. Tread groove depth, however, is closely related to tire wear. It's the only aspect we can influence as users. Early tire replacement will improve performance on wet surfaces, while delaying replacement will significantly worsen these parameters.
To understand the significance of the number and capacity of tread grooves, it's worth looking at some motorsports where tread cutting is used in wet conditions. When it starts to rain, teams cut the tires to adapt them to the new conditions, at the expense of dry grip. Cutting increases the number and width of grooves and their water capacity on the contact surface with the road.
At what tread depth is a tire no longer safe?
According to the rule described above, we're talking about a depth of 4 mm (0.16 inches). Below this value, tires perform significantly worse on wet surfaces. It is worth noting, however, that according to the UN ECE-R117 regulation, whose amendment comes into effect on July 1, 2024, tire manufacturers must ensure good properties up to the tread wear limit, which is 1.6 mm (0.06 inches).
