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All-season tires aren't ideal in any condition

A 15

A 15" Firestone Wilderness AT tire had a tread separation in laboratory testing. Photo Credit: AP (2001)

This is where the rubber really meets the road.

Your old tires are so worn that they're practically transparent. But do you get summer tires and then swap them out for winter tires later on? Do you buy all-season tires and not worry about it? Or maybe a set of summer tires and then all-season tires for the winter because it doesn't snow all that much where you are?

It's really too bad that one tire can't do it all, because the simple fact is that the average all-season tire is a compromise that isn't ideal in any condition.

Bill Vandewater knows. As one of Bridgestone's tire engineers, he talks about tread compound, gripping edges, siping and aspect ratio around the water cooler at work like the rest of us talk about last night's game or the local news.

Keeping it simple, the basic thing to know, according to Vandewater, is that summer and winter tires are specifically built for those seasons, temperatures and driving surfaces. All-season tires, on the other hand, are a compromise between both since the properties of a good summer tire and a good winter tire contradict each other in nearly every aspect, from tread design and softness of the compound (which used to be made of rubber but is now a blend of synthetic ingredients) to sidewall construction.

For summer

Since all-season tires are so popular, the summer tire has evolved into the role of a high-performance tire built for speed and agility.

"A summer tire definitely differs in the tread," says Vandewater. "Big, blocky and larger is more stable than a tire with a small pattern."
And in a high-performance tire that needs to quickly and precisely react to what the driver is doing with the steering wheel and the brakes, being stable is everything. That also means a relatively shallow tread depth that reduces the tire's tendency to squirm around and deflect when pushed closer to its limits.

A summer tire also has a short, stiff sidewall to aid steering response. That means they'll generally have a 30- or 40-series sidewall height, which is a number that indicates the percentage of the tire's overall width. The lower the percentage (30 versus 40), the shorter the sidewall for a given width.

"Sizing is a big issue since there are no 60 or 70 series summer tires," says Vandewater.

As well, outside shoulders have larger tread blocks for cornering, while the inner area is built to channel water away to keep the tire from floating on top of the water, which is a condition know as aquaplaning.

For the winter

In sharp contrast to the summer tire, the winter is all about paddling slush and snow out of the way.

A different mission means completely different qualities. A winter tire has deeper grooves and more of them, which means more gripping edges to help get over slippery surfaces. They're usually narrower, too.

"Wider tires have a tendency to float while narrower tires dig into the snow," says Vandewater. By contrast, a wide summer tire puts more tread on the pavement, which is a good thing.

"Tread is designed to work in a temperature range. Winter tires are designed to work in cold temperatures."

A winter tire is soft to stay pliable in the cold, which helps it grip. A summer tire hardens up in the cold and actually works against itself, unable to conform to surface irregularities.

"Running winter tires all year around . . . (they're) relatively soft, which makes them unstable."

They also don't last. Driving snow tires in the summer is a sure way to kill them quickly says Vandewater, since they're so soft. They'll wear down in no time flat, pardon the pun, rendering them ineffective in deep snow.

"Your fuel economy also takes a hit."

The deep grooves, about 30-50 percent deeper than that of a summer tire, might help you get through the snow, but they also take away from steering sharpness.

As soon as the snow is gone, change to your set of summer tires.
Increasing the traction for a snow tire are sipes, which are tiny, nearly invisible slices in the tread that add more gripping edges. By contrast, a summer tire has no sipes, which helps to stabilize the tread to make it more responsive and better able to take the heat from high-performance use.

All-season dilemma

Since summer tires are generally reserved for high-performance vehicles, your only choice might be all-season tires for summer driving. And while they're a compromise between winter and summer tires, that doesn't mean they're bad.

They have a moderate tread depth, somewhere between summer and winter tires, more sipes and gripping edges than a summer tire and less than a winter tire. That makes an all-season less grippy in both extremes.

The compound is designed to work in a broad temperature range, again a compromise.

Driving a vehicle that came with summer tires from the factory in the winter means a change, but to what? Vandewater suggests winter tires if they're available to fit your vehicle. Low-profile winter tires are harder to come by, but "we do make 40-series winter tires."

If not, it might be possible to buy different (smaller) wheels and winter-specific tires with taller sidewalls, which will also help spare your shiny summer wheels the grief of soaking in road salt.

Either way, you'll need to back off aggressive driving in recognition of the decreased traction.

"All-season is a compromise but the most practical tire for most people."

Jeff Melnychuk is Wheelbase Communications' managing editor. He can be reached on the Web at: 

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