February 3, 2012
Many people still think in terms of "snow" tires. But the more accurate current description is "winter" tires. The difference is, the more modern "winter" tire (characterized by a "mountain/snowflake" symbol on the sidewall) will outperform a "snow" tire in all but the very deepest snow. But the more important comparison is between winter and all-season tires.
It is easy to question the added expense of a separate set of tires for the winter months when you already have a perfectly good set of all-season tires.
What you might not realize, however, is that the all-season tire is really a three-season tire. In other words – a compromise tire.
All season tires are not as capable as a pure summer tire for spring through fall driving, but they are better in winter. That said, they are nowhere near as capable in the cold months as any of the new generation of winter tires.
The biggest difference between winter and all-season tires is that the grip available from an all-season tire starts to deteriorate as the temperature reaches about 7 degrees C and continues to lessen as the thermometer drops.
The tread of an all-season tire, has been developed and compounded to deal with an extremely wide range of temperatures, from 10 degrees C to more than 110 (the treads really can get that hot!).
At the lower end of that scale it become hard and loses its ability to flex and provide traction.
A pure winter tire, however, is compounded to remain flexible over a wide temperature range as well – but a much lower one. Winter tires remain flexible well into the minus-double-digit range, continuing to flex and mould themselves to the road surface.
Tens of thousands of little cuts or sipes open, close and flex with the tread, supplying grip on pure ice tat no all-season tires is capable of. I’ve participated in numerous tests on pure ice with winter tires compared back-to-back with top quality all-season tires and the difference is remarkable.
Thanks to the wonders of science in general and chemistry in particular the winter tire also remains effective well into summer temperatures – although it will wear more quickly at those levels.
I have had the opportunity to test Goodyear’s new generation of winter tires, designed for SUV and light trucks, on ice in sub-zero temperatures. This was another case where the difference in grip between the new Ultra Grip Ice winter tires and high-end all-season tires was clearly evident.
During that same test session, I was also able to compare the two at ambient temperatures in the 20-degree C range on a wet surface. Even at this warmer temperature the new UltraGrip proved superior to all-seasons.
But what I still wanted to find out was how the current generation of winter tires acted on cold, dry pavement – the conditions many of us face more commonly through the winter months. Granted we get snow-storms, and occasionally drive on ice – but the more common situation is driving daily on cleared, dry but cold roads.
To conduct this test I compared a set of Goodyear Ultra Grip Ice WRTwinter tires to a set of Goodyear Wrangler RT/S – a top quality all-season tire that is standard equipment on a wide range of new SUVs and light trucks, in this case a four-wheel-drive Ford Ranger XLT.
The UltraGrip was the same tire I had tested previously on ice and in warm wet conditions. Goodyear’s internal testing and ratings show that it is superior in the wet, offers much better snow and ice traction, is also slightly quieter than the Wrangler RT/S and the two tires have equal dry traction.
But those dry traction ratings are obtained in controlled conditions at above-freezing temperatures. What about real-world, sub-zero conditions?
I set out to find an answer to that popular question – are winter tires really necessary?
First-hand experience on test tracks in several countries with winter tires from Bridgestone, Goodyear, GT Radial, Michelin, Vredestien and Yokohama had proven their superiority on snow and ice. What I wanted to find out was whether tires compounded for winter conditions offered the same superior grip on dry, cold pavement.
The best way to do this is through emergency braking – braking so hard the ABS system kicks in and maintains maximum retardation until the vehicle comes to a complete stop. ABS removes any driver error and the need for controlled or threshold braking. Simply stomp on the brake and hold it there until the vehicle stops.
The test equipment comes from RaceLogic in Great Britain. It’s the same type of precise, GPS-based instrumentation used in testing for AJAC’s Canadian Car Of The Year program.
The test vehicle was a Ford Ranger equipped with four-wheel ABS. Tire size was 255/70x16, set at the factory recommended pressures. The conditions were -4 degrees C and overcast.
Ambient conditions were overcast and –4 degrees C. The paved road was dry and level and had the whiteness resulting from applications of salt, but there had not been any for several days and it was clean enough that vehicles did not raise any dust or other sign of having passed. Perfect conditions – a well travelled, cold winter highway!
I selected a level portion of the road and when no traffic was present conducted a trio of braking tests on each set of tires, averaging the results the RaceLogic box displayed.
The first lesson is that cold, less than perfect pavement does not offer anywhere near the degree of grip it does at higher temperatures and when washed clean. Although the equipment showed a peak of 0.9, G which occurred at the initial instant of application before the ABS was activated, the average was only 0.6G – on both sets of tires. Experience has shown that would have been higher in warmer conditions.
The shortest shopping distance was 62.8 metres and the longest 64.4 – well outside the 43-54-metre stopping distance experienced previously on warmer paved surfaces and similar tires.
LESSON ONE – Cold pavement does not supply the same degree of traction as the same surface when warmer. You need more distance to come to a stop in an emergency situation. Remember to leave extra distance when driving in winter – even on dry pavement.
LESSON TWO – Even thought the two tires, from the same manufacturer, have identical dry traction ratings, the winter tire stopped an average of 1.6 metres shorter on the cold surface. If the conditions had been even colder, I suspect that advantage would be even greater.
That may not sound like much of a difference, but it’s a third of a typical car-length. That could be enough to prevent a collision – or even to save your live.
LESSON THREE – While it is accepted that winter tires provide noticeably more grip on snow and ice than all-season tires, they also enable a shorter stopping distance in very cold conditions.
In addition, the more tightly-packed tread of the Ultra Grip Ice provided a noticeably quieter ride than the more open-lug Wrangler.
In summary, based on this test, winter tires provide superior grip not only on snow and ice, but also on cold, dry pavement.
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