Given the last couple of harsh winters, and the ever present hysteria about how bad this winter’s going to be, more people are wondering about fitting winter tyres to their cars.
So, what are winter tyres? They’re tyres (duh) with a very different tread pattern, made from a different compound of rubber, so as to optimise grip in cold, wet, icy or snowy conditions.
All tyres have an optimum working temperature. You can see a perfect illustration of this in a wet Formula One race: what happens when the track dries out? The wet tyres used overheat and degrade very quickly – the water keeps the tyres cool. But the dry tyres used need to reach a much higher temperature to grip effectively: how many drivers spin off on cold tyres after a pit stop?
Road car tyres are nowhere near as specialised as this, but similar principles apply. Winter tyres use a compound of rubber which is designed to grip at lower temperatures – typically 8°C and below. Your standard road tyre can begin to lose grip at this point.
This is down in part to a material parameter called the glass transition temperature (stay with me). A very simple view is that below this temperature, a material crystallises and becomes hard and often brittle; above it, it’s soft and pliable. The best way to see this in action is put a rubber band in a freezer. At room temperature, the rubber is stretchy: when it’s frozen, the rubber is brittle and snaps. The rubber has gone below its glass transition temperature.
That’s the sort of thing that’s happening with your tyres. The standard tyre just can’t get warm enough (i.e. soft enough) to grip properly.
[Note: There is much, much more to tyre rubber compounds than this: the ingredients used affect not just the rubber’s operating temperature range but the grip levels (for example, sports tyres generally grip more than your average tyre), the rolling resistance (how hard it is to roll the tyre along the road – many “eco” tyres are low rolling resistance to reduce fuel economy, as the easier it is for the car to roll the less fuel it needs to use to move), and durability (how long it takes for the tyre to wear out).]
Now we come to the tread pattern. The tread on standard road tyres is designed to move water away from where the tyre meets the road to allow the rubber to contact it: it does this via a pattern of specific grooves in the surface – known as the tread pattern. If it fails – if the water isn’t driven away – you lose traction, otherwise known as aquaplaning. Again, you can see this during a wet Formula One race: if a driver inadvertently drives over a patch of standing water, the sudden loss of traction can mean he loses control of the car.
Winter tyre tread patterns look very different to their summer equivalents. The grooves are more frequent and a very different shape, some almost like wiggly lines cut into the tyre; the tread blocks (the areas of rubber separated by each groove) are much smaller and much more densely packed.
Side-by-side comparison of standard & winter tyres
This is because winter tyres are usually designed to cope with snow as well as cold and rain. These tiny grooves, the wiggly lines, are called “sipes”, and these are designed not only to move water away more efficiently, but also to bite into the snow to give traction. The larger grooves of your summer tyres just fill with compacted snow, meaning they don’t grip: the sipes, on the other hand, remain unfilled so the tyre can grip.
What does all this mean?
If you drive an average car and fit winter tyres if it gets cold, you should see a definite improvement in cold and wet weather grip – even if it doesn’t snow.
I definitely have. I have a small front wheel drive car, with relatively skinny tyres, and a whacking great weight of a diesel engine over the front axle. It has no electronic traction aids (not even ABS – it’s that old). In cold, wet weather, on greasy roads, on its usual “eco” low-rolling resistance tyres, it understeers – safely and predictably, but also considerably.
On winter tyres, even on soaked roads, it understeers so much less I turn into corners a bit early until I’ve got used to having more grip. Puddles of water along the side of the road which previously would’ve pushed the front end all over the place … don’t. Even under heavy braking, it’s much more stable and sure-footed.
Obviously, this isn’t a licence to be a hooligan, but it gives me much greater confidence that should snow fall, I can keep control of the car. In fact, when it snowed last year, my car with winter tyres outperformed all those with flash 4x4s who couldn’t get any grip on their summer, performance-oriented tyres.
So, do I think winter tyres are worth it? Yes, I do.