What is Tyre Tread Wear Index?
Tyre life is almost the only parameter that manufacturers do not publicly declare. However, many tyres have a Tread Wear Index.
Wear resistance is the most challenging property of a tyre, but at the same time, it is very easy to be checked. You install tyres of one of the same model to several vehicles and run them on the track till the rubber is completely worn out. Tyre manufacturers know this value for each of their models, but they are not eager to share it with car owners. Tread Wear depends on many factors, such as driving habits, type of asphalt, vehicle condition as well as tyre pressure maintenance. Therefore, there is always a driver who wears out tyres faster than it should be according to the declared mileage, and then he initiates an anti-advertising campaign in the press or in court, which affects the manufacturer's image.
The Americans found the solution. Their National Highway Traffic Safety Administration (NHTSA), equivalent of our Department of Transportation, together with the Society for Testing Materials (ASTM), developed a method to measure Tread Wear in relative values, regardless of operating and environmental conditions. In this method, a new tyre is compared to a reference one made by the ASTM.
How Tread Wear Index is calculated
The method looks simple. Two identical vehicles with test tyres mounted to one vehicle and reference tyres on the other run along the established route. From time to time, each tyre is remounted from one position to another on the same vehicle and from one vehicle to another. The tyre pressure and wheel alignment (camber) are constantly monitored and kept during testing.
During test, the tread depth of both sets of tyres is continuously measured. Test miles equal to 10,300 km (6400 miles). Based on the data obtained, the total life of a tested tyre and a reference tyre is calculated. The resource of the reference tyre is taken as 100 nominal units, and the resource of the new tyre model is determined, respectively, as greater or less, adding a proportional number of points to 100 or subtracting.
This index is called Tread wear, and if the reference tyre is always 100, then the minimum value for a tested model may be 60, and the maximum value is 620.
Tyre Manufacturers’ attitudes to Tread Wear Index
Tyre designers consider a Tread Wear Index to be questionable value because it is very often influenced by actual operating and environmental conditions during testing. But at the moment in the world there is only such a method to calculate this parameter. However, not all countries require a Tread Wear Index to be specified.
However, in America, there is a Uniform Tyre Quality Class (UTQG), which obliges manufacturers to indicate a Tread Wear Index on tyres.
It is beneficial for a responsible manufacturer to indicate a Tread Wear Index to support its positive image in the market. Based on this index, a customer can compare the mileage of different tyre models and make a purchase decision.
Therefore, tyre manufacturers turn to NHTSA to test tyres and apply the appropriate value to the sidewall. Tread Wear Index of most summer tyres is within 280-500 range, that is, they are 2.8-5 times more resistant to wear than some reference samples from ASTM.
This means that according to the results of tests carried out by UTQG Tread wear evolution (USA), the Tyre Tread Wear meets the American road safety requirements.
Let us remind you once again that the higher the index, the greater the tyre mileage. Based on DOT calculations, a reference tyre with Tread Wear index of 100 nominal units runs 48,000 km at the test site. In real conditions, tyres have less mileage, but the relationship remains the same.