Winter TyresNovember 25, 2011
Green car guide carried out various tests to compare the performance of vehicles with normal tyres against vehicles with winter tyres – and the results were amazing.
What will provide the best traction up an icy hill? We test a four-wheel drive car with standard tyres versus a two-wheel drive car with winter tyres?
Answering questions such as this was the aim of an event organised by Michelin and Porsche at the Porsche Driving Experience at Silverstone, with its simulated icy surfaces and the latest range of Porsche products.
Various tests were conducted to compare the performance of vehicles with normal tyres against vehicles with winter tyres – and the results were amazing.
Porsche 911 with normal tyres on ice hill
The first test was a standing start on the ‘ice hill’ (a very low friction surface covered with water) in a rear-engined, rear-wheel drive Porsche 911 with normal tyres. This car has the advantage of lots of weight over the driven rear wheels, and as proven by Porsche in rallies on snow over many years, this gives excellent traction. However, normal levels of acceleration from standstill on the ice with the standard tyres simply resulted in lots of wheelspin and very little forward progress. If great care was taken with the accelerator, then some forward motion could be gained.
However if any change of direction was then attempted, the car would certainly not go where you wanted it to. The 911’s traction control system was then switched off, which resulted in virtually no progress whatsoever, and if there was any movement, it was a complete lottery in terms of where you’d end up. So this test represented what most motorists experienced during the snow of the last two years – lots of slipping and sliding, and it being highly unlikely that you’d end up at your required destination in one piece.
Winter tyres tested on ice hill
The second part of the test was exactly the same – on the same hill, again in a 911 – but with winter tyres. This time upon acceleration, incredibly, the 911 started making progress up the same ‘icy’ slope without any problems. Even more amazing, the car also responded to steering inputs. This all seemed too good to be true, so we attempted more enthusiastic acceleration at the same time as cornering. This did result in the car sliding, but with a gentle lift-off, the 911 instantly regained its line and we lived to tell the tale.
So test one proved that a car on standard tyres is pretty useless on a hill of sheet ice, but the same car with winter tyres has completely transformed abilities.
Porsche panamera tested with normal and winter tyres
Test number two was in a Panamera – front-engine, rear-wheel drive – in a skid. The car entered an area of simulated ice and a skid was induced by a ‘kick plate’ – a piece of metal under the car’s rear wheels that shifts one foot either left or right to throw the rear end of the car across the surface.
Again, the first part of the test was in a car on standard tyres. It’s likely that you can predict the outcome – the rear end of the almost two-tonne Porsche was flung across the ice and despite the best efforts of opposite lock the car had very little grip. Braking was the next issue, when it seemed as though the car would never stop. When throwing braking into the mix, it was obvious that if this hadn’t been a test but it had actually been a real-life incident on a public road, a number of other vehicles are likely to have been involved in what would have been a big accident.
The test was repeated again with winter tyres. The rear end of the car was thrown across the slippery service but this time the car was much more controllable. It was easy to catch it and then, even more incredibly, under braking the car pulled up in a vastly shorter distance than the car on standard tyres. Winter tyres are actually likely to halve your braking distance – at least. We conducted our own test two winters ago in the blizzards and the result was that one of our press cars with standard tyres took eight – yes eight – times the distance to stop when compared to a car with a standard fitment on/off-road combination tyre (not even a winter tyre!).
Porsche Cayenne on the ice hill
The final test was perhaps the most interesting. This involved a four-wheel drive Porsche Cayenne on standard tyres versus a rear-wheel drive mid-engined Porsche Boxster on winter tyres on the ice hill. Most people would imagine that a four-wheel drive car would always have the edge over a two-wheel drive car on ice. However this test proved otherwise. The 4×4 Cayenne’s wheels were merrily slipping and sliding in an effort to gain traction up the hill. It did manage to get moving, but on the standard tyres it was a real effort. In comparison, the two-wheel drive Boxster had no problems setting off up the hill on the ‘ice’, and it certainly would have beaten the 4×4 in a side-by-side race.
The vehicles were also taken around Porsche’s handling circuit to see if the winter tyres had an impact on the cars’ driving characteristics. Overall, under normal driving conditions, most drivers wouldn’t notice a difference. However if you’re driving more progressively then you will notice that winter tyres won’t give you as much of a sharp turn-in on corners; this difference was most marked in the Cayenne, and much less obvious in the Panamera.
Winter tyres myths and facts
One common misunderstanding about winter tyres is that people think that they are just for use in snow and ice . In fact they perform better on normal roads than standard tyres whenever the temperature drops below 7°C. So Michelin would prefer that they are called ‘cold weather tyres’ rather than winter tyres.
So winter – or cold weather – tyres obviously work, but how do they work? There are two main differences between normal tyres and winter versions, the first being the rubber compound. Winter tyres have a softer compound, with more natural rubber and silica, which doesn’t harden in cold temperatures. Standard tyres, with more synthetic rubber, will harden at temperatures below 7°C and this means that the tread hasn’t got the flexibility to grip in to the road surface as effectively. This in turn means that the traction, braking and cornering performance all deteriorates.
The second main difference is the tread pattern . Yes, winter tyres are likely to have a more aggressive overall tread pattern, but it’s actually the large number of ‘sipes’ on winter tyres that make the difference – these are the very thin grooves in the tyre tread, which act as ‘claws’ in snow and on slippery road surfaces to improve traction and mobility.
It’s essential to fit four winter tyres , rather than just two to the driven wheels. If you fit winter tyres to the front of a front-wheel drive car, but leave the standard tyres on the rear, then the front of the car will have grip but the rear end is likely to slip around like a mad thing when cornering or braking. If you fit winter tyres to the rear of a rear-wheel drive car and leave standard tyres on the front, you’ll lose the ability to steer on slippery surfaces.
So what do you need to do to switch to winter tyres? One option is to swap your normal tyres for winter tyres and have them fitted on the same wheels. However a better plan is buy a spare set of wheels and have those fitted with winter tyres, and swap during November, and swap back around March. Buying a second set of wheels is certainly an extra cost, and this could be fairly expensive depending on the vehicle and the wheels. Winter tyres with a higher profile and narrower width perform better in snow than low profile tyres on fat alloy wheels, and so dropping a wheel size for winter tyres makes sense to allow for a higher tyre profile – as long as the smaller wheel still fits over the brakes.
Although there will be the one-off expense of an extra set of wheels, you won’t be wearing down the tread on the summer tyres when you’re running on winter ones, and winter tyres are likely to last longer than standard tyres at cold temperatures, so it may help to save money.
Do winter tyres increase fuel consumption?
But what about the green credentials? Surely winter tyres must increase fuel consumption? Michelin claims that this is not the case, as their winter tyres are still low rolling resistance. Also, winter tyres are generally narrower than standard tyres, which makes them more aerodynamic, therefore improving fuel economy.
However even if you did lose a couple of miles per gallon, that seems a small price to pay to be able to drive around safely in the snow and ice without crashing.
So how do you get hold of winter tyres, and what’s the availability like? It’s illegal to drive on snow and ice on non-winter tyres in Germany, and so winter tyres represent 48% of the market there. There’s also legislation covering winter tyres in other European countries. There’s no legislation in the UK at the moment so winter tyres represented just 0.4% of the market last year, with patchy availability, but this year availability should be better, and increasing numbers of manufacturers are offering winter tyre swap programmes, including storage options for the wheels and tyres you don’t need.
Michelin offers different cold weather tyres for a variety of vehicles ranging from family vehicles to performance cars to 4x4s to vans, and as this test showed, having the right tyres for your particular vehicle in winter really can make the difference between slipping and sliding, or arriving alive.
See our own road tests of cars with different tyres on snow: