When to change worn-out tires? Manufacturers' opinions diverge

On July 1, 2024, updated UNECE R117.04 standards came into effect in Europe, which increased the requirements for the operational characteristics of vehicle tires, and in particular, the mandatory testing of tires in a worn state for traction on a wet road surface. Now, summer tires must confirm their effectiveness on wet coverage, including with a residual tread depth of 2 mm, to guarantee the minimum required level of traction and at the end of their service life.

Michelin has long recommended that its customers use tires for as long as possible, and Richard Andemur, a sustainable development manager from the company's Swedish branch, says that consumers should use the full potential built into the product.

"The minimum tread depth is not set just like that. We, manufacturers, must develop tires that provide the necessary traction up to a depth of 1.6 mm," he said in an interview with Däcksnack.

At the same time, many other manufacturers recommend changing summer tires long before they wear down to the minimum allowed groove depth, and the main reason is the deterioration of their water-repellent properties and the increased risk of aquaplaning.

"Tire development is always a compromise," Andemur points out. "Physically, it is impossible to create a tire that is the best in all parameters, since they are often conflicting. At the same time, the properties of tires also change during operation, leading to a change in the ratio of their advantages and disadvantages. We believe that the same tire can be equally good throughout its service life, it's just that some qualities deteriorate, while others improve."

According to Michelin, the operation stage accounts for just over 80% of the impact of tires on the environment, and if all car owners in Europe changed tires with a tread depth of 1.6 mm, rather than 3-4 mm, as recommended by many manufacturers, tire consumption in the region could be reduced by 128 million units per year. According to a 2017 Ernst & Young study, such an approach could reduce CO2 emissions by approximately 6.6 million tons per year, including due to the fact that the rolling resistance of worn tires can be 20% lower than that of new ones.

Since tire parameters conflict with each other, it is impossible to simultaneously ensure maximum traction and maximum resource. Traction is created by friction, which automatically leads to tire wear, and Michelin is now actively working on finding new ways to increase the durability of its tires without compromising safety.

"By increasing wear resistance, we can also reduce the initial tread depth, which leads to a reduction in rolling resistance and, consequently, to improved fuel efficiency," says Andemur. "The question is about reasonable management of the advantages of tires and existing risks, in order to ensure the necessary safety and reduce the impact on the environment."

Regarding how reducing the tread depth affects the resistance of tires to aquaplaning, a Michelin representative noted: "This risk increases slightly, but in general, if it's raining heavily, you should slow down anyway. There are no tires that completely prevent aquaplaning, just as there are no cars that are completely protected from collisions. The occurrence of aquaplaning is influenced by a combination of factors, such as speed, contact patch area, vehicle mass, and the ability of tires to drain water. By reducing speed during rain, you reduce the risk of aquaplaning, and tires with a small tread depth will also have many other benefits."

In Nokian Tyres, they have a different opinion, and Emil Sundholm, a product manager at the company's Swedish branch, says that summer tires should be changed when the tread depth becomes less than 4 mm, as "it's a matter of safety". As for the balance between wear resistance and safety, Sundholm notes that "safety is always our top priority. These are two related parameters: the higher the wear resistance, the higher the safety."

The Nokian manager also acknowledged that a tire with a groove depth of 1.6 mm may have advantages compared to a new tire, and, for example, "it is possible to achieve reduced rolling resistance and improved traction on dry asphalt". "However, if we talk about aquaplaning resistance, the difference will be very significant, and here the deciding factor will be the tread depth," he points out. "To displace more water from the contact patch with the road, a larger tread depth is required."

According to him, when the tire tread approaches the set limit in depth, one should first of all consider the increasing risk of aquaplaning, and to extend their service life, it is necessary to monitor the pressure. "Regularly check the pressure so that it is not too low, and also ensure proper storage of tires when they are not in use," he said.

In Hankok, they recommend changing summer tires at a residual tread depth of 3 mm. "The ability to drain water [in worn tires] is significantly reduced, which significantly increases the risk of aquaplaning," noted Christina Silversparre, a communications manager at Hankook in Northern Europe, who, when asked if the company is working to improve the water-repellent properties of tires with a tread depth of less than 3 mm, replied: "We are constantly working to improve the characteristics of our tires, including through the use of new rubber compounds, as well as the development of tread patterns that are manufactured using 3D printing technology and can change as they wear. But ultimately, tread depth is one of the most important factors affecting the efficiency of water drainage and aquaplaning prevention."

"We understand that there needs to be a balance between ensuring safety and protecting the environment," says Silversparre. "While using tires with a tread depth of up to 1.6 mm is a more environmentally friendly approach, for Hankook, safety is always a priority. By developing tires with low rolling resistance and using recycled materials, we reduce the impact on the environment while maintaining high safety standards."