Each autumn, the refrain used to be the same: “Time to put on the knobbies!” Winter was on its way.
There’s a good reason they were called “knobbies,” since the old-fashioned winter tires were made up of large blocks of tread – each tread block being four times the size of those on a normal tire.
At the time, these tires with their big tread blocks were the best things for digging through snow, slush and mud. Unsophisticated though it might seem, the thinking was that big pieces of rubber can dig better than smaller ones. This was back when engineering worked by the “eyeball method”: if it looks good, make it.
Of course, these big treads also vibrated and howled on asphalt, and unless you lived in the far north, the roads were cleared within days of a snowfall anyway. So you had to put up with a lot to get snow traction.
More importantly, this type of snow tire had little grip on ice or asphalt – and poor grip on pavement. Another downside was that they wore very quickly, so they were expensive on a per-kilometre basis. Still, they got a lot of people where they had to go with greater safety than a summer tire.
Then along came a little more science: tiremakers started adding sipes, or miniature cuts, into the tread blocks. They did improve a tire’s traction, especially on ice.
Then, in 1988, the Bridgestone Blizzak overtook plain siped tires with its ability to grip ice. Instead of tiny sipes, Blizzaks were a “multi-cell compound” in which the rubber was filled with microscopic air pockets. Cut in cross-section and viewed under a microscope, the rubber looked like one of those mousse-filled candy bars.
The tiny cells gripped ice like a wool sock, but that made the tire a bit squirmy and subject to faster wear.
But the idea of improved grip on ice resonated with consumers and Blizzak stormed the market. Bridgestone stiffened the compound to improve the wear. Other manufacturers took note of its success, and so the race was on.
Other manufacturers worked on developing their own formula for improving ice grip: Toyo added ground walnuts, Goodyear added volcanic sand; both worked.
As technology improved, so did the the rubber, the grooves between the tread blocks got smaller on winter tires. Grip could be achieved by better designed tread blocks, more sipes and the addition of air bubbles in the rubber.
And unlike “knobbies” of old, new winter tires no longer howl like a banshee at Halloween: engineers now make the tread blocks slightly different sizes so the noise is self-cancelling.
By John Mahler
Originally published in the Toronto Star.