AWD doesn’t make you invincible in icy weather
Television news seems to take perverse delight in the first major winter storm of the year by showing the effects on traffic. Along with shots of vehicles struggling up icy hills or being dug out of snow banks, there is invariably footage of some crumpled SUV or crossover in the ditch or on its roof.
Is this simply a matter of motorists forgetting to drive to the weather conditions, or, especially for owners of these SUVs and crossovers, a misplaced confidence in the traction abilities of their vehicles?
“Sport utility vehicles offer a marked improvement in tractive effort and handling in snow,” says Chrysler’s Raynard E. Durham, vehicle line executive of SUV vehicles, “but what people often forget is stopping distance. In fact, it is very much compromised in the snow.
“Even with active chassis systems that keep the vehicle in a straight line when stopping in the snow, it still takes a much longer length of road compared to dry and wet conditions to bring the vehicle to a stop.”
No matter the type of drivetrain, it takes all vehicles longer to stop on snow-covered roads, which makes winter tires a must. Testing by the Ontario Ministry of Transportation of vehicle-stopping distances from 50 km/h (on asphalt roads covered with three to five centimetres of compact snow) showed a fourwheel-drive SUV coming to a halt in 35.7 metres when fitted with winter tires. With all-season tires on, the stopping distance increased to 42.8 metres. That can be the difference between a close call and a trip to the autobody shop — or hospital.
“AWD systems work very well as long as: a) the tires have grip, and b) the vehicle is travelling at a velocity that does not overcome the grip,” says Hayato Mori, manager of product planning at Honda Canada.
“Once one or both rules are broken, what you end up with is a very heavy vehicle ... that starts to follow Newton’s Law of Motion. AWD is not a substitute for snow tires, and the object in motion will want to keep moving unless an equal or greater force acts upon it. All of those SUVs in ditches: (Their drivers) likely broke one or both rules. An SUV going too fast in slippery conditions without snow tires is just waiting to slide off the road.”
“You can have the best AWD system in the world,” says Adrian Squires, technical trainer at Volkswagen Group Canada, “but you still have to drive within your skill level and use winter tires.”
Squires says people must realize that all-season tires are really only three-season tires.
“A good winter tire has a softer rubber compound that you don’t get in all-season tires.
“They would wear out too quickly,” he says.
So, having a sport ute, crossover or one of any number of all-wheeldrive cars (the Subaru Legacy, for example) won’t cloak you in invincibility when the snow flies. But what, exactly, are the advantages over front- or rear-wheel-drive vehicles?
“Four-wheel-drive and allwheel-drive vehicles always offer improved traction in unfavourable weather conditions over two-wheel-drive vehicles,” says Durham.
Vehicle Stability Assist (VSA) systems, a combination of traction control and braking systems, help any vehicle maintain direction and stability by slowing down one or more wheels via the brakes. But it only works when the vehicle is in motion, says Honda’s Mori.
“Traction control simply slows down power delivery so you can slowly roll forward rather than slipping the wheels when starting. These systems on front-wheeldrive cars with snow tires will get you through most slippery conditions,” he says. “All-wheel drive adds another level of traction by sending power to the axle with the most grip.”
While the benefits of having traction at all four wheels are obvious, there are a couple of specific disadvantages, namely the added cost at purchase and the penalty at the pumps.
“You’ll notice that fuel consumption can go up when you have all four wheels reacting,” notes Squires.