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The science of winter tyres: how they work
Stuff We've Done
04 December 2012 11:00
Do you use winter tyres? After several cold snaps in recent years, Brits have slowly woken up to snow tyres which can keep going where normal rubber gives up. The UK still has very few winter tyres in use - just 3% of tyres sold in 2011 were designed for cold weather use - but trade bodies say the uptake is increasing.
Should you bother buying winter tyres? How much do they cost and, crucially, do they actually work? CAR has been on a winter tyre masterclass to bring you all the facts and figures you need to know about cold-weather rubber.
Winter tyres: what's in a name?
First things first: ‘winter tyres’, as we’re accustomed to calling them, is actually a misleading title. These aren’t massive chunky mud-plugging boots with knobbly, noisy treadblocks and spiked studs protruding like a rolled-up hedgehog.
More accurately, they're cold-weather tyres, designed to work in lower temperatures, on wet and dry roads, as well as giving better performance on snow and ice – which, given the inconsistent gritting on our nation’s road network, is just as well. We need a do-it-all tyre.
So, how do cold-weather and winter tyres work?
The science bit is easily explained. Winter/cold-weather tyres contain more natural rubber than regular tyres, and are constructed of a softer compound. This allows them to stay supple as temperatures drop below 7 degrees C, maintaining higher grips levels on the road, in conditions where a normal tyre becomes hard and less keyed-in to the asphalt.
If you see a winter tyre up close, you'll spot that the tread is different to a more conventional summer tyre. The contact patch of a winter tyre is more rugged: they're covered in thousands of ‘sipes’ – tiny channels cut into the rubber which help displace water at a faster rate. On snow, it’s these little crevices that bite into the soft stuff, giving purchase and grip. A regular tyre’s channels quickly become clogged with compacted snow and effectively become a racing slick, hence the Bambi-on-frozen-lake handling.
On the down-side, the wobbly treadblocks mean cold-weather tyred cars are less responsive in milder conditions, exhibiting more lateral roll, and understeer when temperatures rise above 7deg C.
Why should I bother considering winter tyres?
Cold-weather rubber is becoming more relevant to more drivers. Why? Two reasons. Firstly, because our winters are getting more and more unpredictable. Sometimes they’re mild, and sometimes the country grinds to a halt under a couple of inches of the white stuff. You might quite like the idea of a few impromptu days off work, but if you want your supermarket shelves stocked and doctors on call, then hedging your bets in case of an inhospitable cold snap is undeniably sensible.
Secondly, you can now get yourself winter rubber in a massive range of sizes, from supermini boots right up to gargantuan 20-inch wheel compatible items like you’d find on the back of a Porsche Boxster. It's potentially a much simpler way to keep mobile without investing in a more expensive 4x4.
Why might I not want winter tyres?
There's the obvious financial outlay to swallow: a set of four winter boots for a Fiesta will set you back in the region of £350. Shoe a BMW 5-series for winter and you're looking at an £800 bill, and the only way is up. A set of four winter tyres and 20-inch 'RS Spyder' alloys for CAR's long-termer Porsche Panamera GTS comes in at a cool £4060, including fitting.
Winter peace of mind doesn't come cheap, and yes, you do have to get four tyres: settling for just a pair for the driven wheels will do more harm than good, as this unbalances the whole vehicle. Once you've sorted your winter rubber (usually on another set of wheels altogether, meaning yet more outlay) you need to find somewhere to store your regular summer tyres, too.
Of course, don't underestimate the wonderfully infuriating British weather. If Michael Fish gets his isobars mixed up, a mild winter would actually unleash little (if any) benefit from your expensive new tyres. The saving grace is that your summer tyres won't be wearing in winter and your cold-weather rubber won't wear in summer; after that initial outlay, you'll end up buying replacement tyres less often.>> Click here to read CAR's review of winter tyres