It's winter. The nights are drawing in. And on the long dark evenings of the soul, what better to do than have a little chat about rubber.
I presume you're still driving your car around on the roads at this time of year? When the surface is slippery with rain, stirred to slurry by passing wheels and hiding a slick of black ice when the temperature slips down below zero. Yes?
That's exactly why we need to talk rubber. No tittering at the back.
You see, there are winter tyres. And summer tyres. If you get into car esoterica, there are winter wheels and summer wheels (but let's not go there right now). Put simply, summer tyres are hard, winter tyres are soft. Why's that?
Well, rubber is a natural product that reacts to temperature. When made into tyres, it's mixed with compounds to harden it (so that your tread doesn't smear itself all over the road in a few weeks). However, below about 7ºC the rubber is noticeably harder - and needs much less in the way of additives to keep it firm. So winter tyres contain a higher percentage of natural rubber than summer tyres.
Not to mention softer tyres conform to the surface of the road. They grip better - and that's what you need in slippery conditions. So really, winter tyres need to both compensate for the drop in temperature and also for the more difficult driving conditions. They're the law in Germany in 'wintery conditions'. Which basically means that once temperatures start to drop in October, Germans fit winter tyres that they'll run right through the winter until Easter.
There's a good argument that winter tyres are not just for snow and ice. They provide better grip when the weather's dismal, at the onset of rain (at the beginning of a shower the road surface is at it's most dangerous as the initial rain combines with ingrained dust and dirt to make a thin and almost invisible slick of oily mud - although soon diluted and washed away it's not fun if you brake in a corner at that moment). I mean, we usually drive more carefully when snow and ice are visible. But generally dreary weather doesn't provoke much of a response. We're just too used to it. But it can be dangerous.
So, why don't we roll on them all year round? Basically, the softer compound of winter tyres will be too soft as temperatures rise. Remember, your tyres warm up as you drive and without chilly conditions to cool them down they'll be much, much softer (even slightly sticky) in summer. The main effect is excessive wear - the softer your tyres, the quicker they'll lose their tread. You will probably also see an increase in fuel consumption as the stickier tyres require more power to keep them moving. So summer tyres are optimal above 7ºC.
However, here in Britain, the weather is consistently above 7ºC for, what? Three or four months a year? One particularly rubbish summer I left my winter tyres on all year round. Oh dear.
About winter and summer wheels. In case you hadn't notice I drive a Porsche. I love her dearly, even though she can be slightly high maintenance. She comes with winter wheels. And summer wheels. Because the dear folk at Porsche realised that your soft summer tyres are optimal if they are wider with more of the tyre in contact with the road surface. However, the winter efforts, with their softer compound, are narrower (which improves their efficiency at cutting through snow). They also have a different tread pattern to increase grip (and we need as much grip as we can get if we're rear wheel drive and on less than certain ground). So winter tyres have a smaller width. And for different width tyres, you need different width wheels.
There you go. Well you asked. You didn't? Oh well, save those gems for a pub quiz. But the stuff about winter tyres - that could save your life. Or a from stiff fine in Germany. They're very strict, the Germans.
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By Beate Kubitz at 20 Dec 2014, 00:00 AM