Due to a number of cold and snowy winters, the UK population now understands winter tyres both exist, and aren’t just for snow and ice - they also offer a real benefit when the conditions are cold and wet. This is why tyre manufacturers recommend that for the highest level of safety during year round motoring, you should switch between dedicated summer and winter tyres.
What you might not be so sure of is how the performance degrades as the tyre wears. The UK has no particular laws surrounding winter tyre use, but in parts of Europe where winter tyres use is a legal requirement such as Germany, a winter tyre is no longer deemed a “legal” winter tyre below 4mm of tread depth, with some countries have the legal limit at 6mm!
To find out how snow performance degrades as the tyre wears, Continental invited us to test its new Continental WinterContact TS860 at full tread depth (around 9mm), 4mm, and the legal UK limit of 1.6mm.
Testing inside the Arctic Circle, and driving new Audi A3s with 205/55 R16 tyres fitted, a group of testers ran forty or so snow traction and braking runs on identical cars, in identical conditions fitted with the tyres at the various tread depths.
The results plotted an interesting graph. The drop off wasn’t linear as you’d perhaps expect, from 8mm to 4mm tread depth, the reduction in braking performance per mm wear is around 2%, from 4mm to 2mm it doubles, to over 4% per mm. This means that when stopping from just 30 mph on snow, a tyre at 4mm would take an extra 14 meters over a new tyre to stop you, and at tyre at 1.6mm a massive 26 meters!
Wet braking, which we didn't experience first hand due to conditions, is even more apparent, but not exclusive to winter tyres. Like the snow, from 8mm to 4mm you lose approximately 2% per mm of tread, but at some point after 4mm the degradation doubles to over 8% per mm, largely due to the micro aquaplaning influence!
Winter tyres rely heavily on the tread pattern during snow and ice performance. A winter tyre needs good tread depth and a blocky pattern to pack snow into the tread, and numerous sipes with sufficient length to bend and provide plenty of edges to cut through the snow and slush to the ice below.
As a tyre wears, the volume of tread void to pack snow into lessens, and the number of edges decreases, resulting in a winter tyre at 1.6mm with a largely similar performance to a summer tyre.
If you want any meaningful snow and ice performance from your winter tyre, make sure you don't run them to 1.6mm. If you finish a winter season with less than 4mm of tread depth, consider running them in the summer to the 1.6mm limit, so as not to waste any tread life.
In 2016 we'll be testing the dry performance of a winter tyre as it wears.