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Today's new cars, vans and light trucks are originally fitted with either summer or all-season tires as they leave the factory. Summer tires are designed to enhance traction in dry and wet warm conditions, but were never intended to encounter winter's cold, slush, snow or ice. And while all-season tires are intended to provide traction in a wide variety of weather conditions, we've found they can behave like a Jack-of-all-trades, master of none.
Many of today's vehicles are also equipped with anti-lock brake, traction control and dynamic stability systems that help them utilize more from their tire's potential. However none of these driver's aids actually generates traction. They are only capable of trying to limit the vehicle's acceleration, braking and cornering capabilities to the traction provided by the tires. And we've learned sometimes that's not enough.
Wintertime presents the most challenging driving conditions drivers typically ever face. In addition to the reduced hours of daylight and colder temperatures, drivers must combat winter's wet, slush, snow and ice-covered roads that conspire to reduce traction.
So what's the difference between wintertime gripping and white-knuckle slipping? Often it's simply the tires; and we've found that satisfying wintertime grip typically comes from tires developed to provide their best traction when road conditions are at their worst.
Part 2: All-Wheel Drive Vehicle
In order to compare the differences between various types of tires, several members of the Tire Rack team conducted tests in winter driving conditions. Since summer tires aren't designed to ever encounter wintry conditions, our test focused only on Original Equipment all-season tires and aftermarket winter / snow tires. To represent all-wheel drive vehicles, we used 2006 Porsche Cayennes equipped with new, full tread depth 235/65R17-sized tires mounted on 17x7.5" wheels.
Before we began evaluating acceleration, stopping and cornering capabilities, we groomed the snow to provide as consistent a surface as possible to minimize the variables associated with driving in snow. We then tested each pair of vehicles simultaneously and re-groomed the track repeatedly throughout the test.
All-Season vs. Winter / Snow
The acceleration comparison measured the tires' ability to provide traction when accelerating as quickly as possible in a straight line with the vehicle's Porsche Traction Management and Anti-Slip Regulation operating. With the two Cayennes sitting at the line, we did a countdown to start both drivers and compared the time it took for their vehicles to accelerate 200 feet. Including the drivers' reaction time, the winter tire-equipped car crossed the finish line in just over 6 seconds, while the all-season tire-equipped car completed its run in 8 seconds. While we don't recommend trying to accelerate as quickly as possible in snow when driving on the street, these results show how much more traction winter / snow tires can provide when accelerating from a stop. As much as we expected the winter / snow tires to accelerate better than the all-season tires on snow, we found it interesting that the all-wheel drive Porsche Cayenne equipped with all-season tires was only able to accelerate about as quickly as the rear-wheel drive BMW on the dedicated winter / snow tires used in Part 1 of this test. While acceleration helps you get going, it was now time to find out how well the all-wheel drive Cayennes could stop and corner.
The braking comparison measured the tires' ability to provide traction during an ABS-assisted panic stop in a straight line. We drove the two Cayennes side-by-side at a speed of 30 mph, gave both drivers a braking signal at the prescribed mark and compared the distances it took them to come to a complete stop. The winter tire-equipped Cayenne stopped in an average distance of about 61 feet, while the all-season tire-equipped Cayenne took 102 feet (an additional 41 feet or about two and one-half car lengths). A 41-foot difference in stopping distance during a panic stop at 30 mph on a snow-packed road is more than enough to determine whether it's a near miss or an accident!
Additionally, while the all-wheel drive Cayenne offered noticeably faster acceleration than the rear-wheel drive sedan, the winter tire-equipped BMW's 59-foot stopping distance and all-season tire-equipped 89-foot stopping distance showed that all-wheel drive didn't really offer a measurable advantage when it came to stopping.
The cornering comparison measured the tires' ability to provide traction during a 90-degree left-hand corner. We drove the two cars nose-to-tail beginning at 15 mph and increased the speeds on successive runs. When we attempted to drive through the corner at 25 mph, only the winter tire-equipped Cayenne was able to complete it, while the all-season tire-equipped Cayenne slid off the road. Even though the all-season equipped Cayenne included Porsche Stability Management, the PSM system could not overcome the laws of physics when the tires' traction limit was exceeded.
While all-season tires may provide enough wintertime traction for drivers in areas of the country that only receive occasional light snow, Tire Rack feels there isn't a viable alternative to dedicated winter / snow tires if drivers of all-wheel drive vehicles expect to encounter deep or frequent slush, snow or ice.
Tires are often the difference between wintertime gripping and white-knuckle slipping.