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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 1: Rear-Wheel Drive Vehicle
In order to compare the differences between various types of tires, several members of 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 rear-wheel drive vehicles, we used the 2006 BMW E90 325i sedan equipped with new, full tread depth 205/55R16-sized tires mounted on 16x7.5" wheels.
Before we began evaluating acceleration, stopping and cornering capabilities, we plowed, packed and 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.
Mixed vs. Matched
While it was a common practice to install just a pair of winter / snow tires on the rear axle of a rear-wheel drive vehicle decades ago, the advances in winter tire technology and traction that have occurred since have made that practice unacceptable leading Tire Rack to establish a policy of only selling winter / snow tires in sets of four.
We've previously experienced the reduction in dry and wet road handling when rear-wheel drive vehicles equipped with all-season tires on the front axle and winter / snow tires on the rear began to spin out (oversteer) at the vehicle's cornering limit. Now we wanted to explore the differences in braking and handling capabilities of rear-wheel drive vehicles on a snow-packed road.
The braking comparison from 30 mph showed us that the car equipped with winter / snow tires all the way around again stopped in a distance of about 58 feet, while the car equipped with all-season tires on the front and winter / snow tires on the rear took an additional one and one-half car lengths to stop, or about 20 more feet. In essence, braking with all-season tires on the front only resulted in a stopping distance a little shorter than a vehicle with all-season tires all of the way around. Our conclusion was that the vehicle didn't fully benefit from the pair of winter / snow tires on the rear axle.
When we attempted to drive through the cornering comparison at 25 mph, only the winter tire equipped car was able to complete the corner, while the car with all-season tires on the front axle slid off the road in the tracks of the car equipped with four all-season tires. Again the vehicle didn't significantly benefit from the pair of winter / snow tires on the rear axle.
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 expect to encounter deep or frequent slush, snow or ice.
Tires are often the difference between wintertime gripping and white knuckle slipping, and only matched sets of four will do!