The profile depth of a tyre is very important. When it rains or the road is wet, the depth of the profile determines how much contact you have with the road. And therefore how much grip you have. The less tread the tyre has, the longer the braking distance. Moreover, there is a chance of aquaplaning. The less tread, the harder it is for the tyre to evacuate the water. In racing, it’s not for nothing that the mud (completely slippery tyres) are immediately replaced by tread tyres with a pit stop if it starts raining unexpectedly.
Contact with the road
You can make a print of the contact surface of a tyre on the road surface. Like a fingerprint. We did this for tyres with different tread depths, at different speeds and with 3 mm of water on the road surface. Two things then stand out. The faster a car drives, the less road contact it makes. But also: the less tread the tyre has, the less contact there is. The combination, low profile with high speed, is life-threatening. The contact is then virtually gone.
If there is a lot of water on the road, aquaplaning can occur. The tyre then loses contact with the road and becomes uncontrollable. If the tread depth decreases, aquaplaning is more likely to occur. We have measured the speed at which aquaplaning occurs on tyres with different tread depth on a road with 6 mm of water. The diagram clearly shows what used tyres do.
If the tyres under a car have a different tread depth, there is a chance that one wheel is more likely to suffer from aquaplaning than the other tyres. The car will then break out or slip uncontrollably. We therefore recommend not fitting tyres with a tread depth difference of more than 3 mm.
The braking distance on a wet road surface is always longer than on a dry road surface. The length of the braking distance on a wet road surface is also strongly determined by the tyre’s tread depth. The more tread depth, the shorter the braking distance.
Minimum profile depth
For passenger car tyres (both summer and winter tyres) the minimum tread depth is 1.6 mm. This is regulated by law. For road safety reasons, we recommend replacing summer tyres when a tread depth of 2 mm has been reached. This will also prevent you from trespassing unnoticed. However, winter tyres begin to lose their winter characteristics at 4 mm. We therefore recommend a minimum tread depth of 4 mm for winter tyres in extreme winter conditions. A minimum tread depth of 1.6 mm also applies to commercial vehicle tyres.
Measuring the tread depth
The tread depth is measured in the main grooves of the tyre. The main grooves are located in the middle 3/4 of the width of the tyre tread. The Tread Wear Indicator (TWI) shows whether the tyre has reached the legal minimum tread depth of 1.6 mm and is therefore worn out. The TWI is then equal to the tread. With winter tyres, the Winter Wear Indicator indicates whether the tread depth of 4 mm has been reached.