Most UK cars are fitted with summer tyres, and some with all-season tyres. But winter tyres are designed specifically to give you extra grip in cold temperatures and when driving on snow and ice.
The key differences are as follows:
They use a softer rubber compound (usually by including more natural rubber in the mix),
The surface of the tread blocks is covered with little jagged slits – called sipes,
They generally have deeper tread grooves than a conventional summer tyre.
They are good at gripping cold, damp roads, below about 7C.
The key to their improved grip on wet and ice-covered surfaces is the sipes, which provide hundreds of small extra ‘edges’ to grip the road as the tyre rotates. The sipes help, not only because of their edges, but because they enable localised movement of the rubber as the soft compound clings to the road. A larger single, solid tread block, like the ones you see on summer tyres, would stay rigid in such conditions and be unable to maintain grip as effectively.
Winter tyres are also designed to gather a snowy 'in-fill' in the tread grooves and in the sipe slits, to help with grip on loose snow. Think about how you create a snowman by rolling a snowball, bigger and bigger, and hopefully it will help you understand that snow clings to snow, so a covering of snow on the tyre actually aids grip.The extra deep tread grooves also help the tyres to disperse surface water and usually increase resistance to aquaplaning.
What are winter tyres bad at?
At temperatures above 7C they offer significantly poorer grip in dry conditions than the best summer tyres. This can mean a marked increase in braking distances and poorer handling and grip in bends.
Are they just for snow and ice?
No. They are designed for use in all winter conditions – with the tyre manufacturers claiming this means all conditions below 7C.