I was getting my winter tires swapped recently and I wondered whether
I really needed them in the Lower Mainland of BC, where we like to
gloat to the rest of Canada about the mildness of our winters. After
all, it hardly ever snows here. It turns out I hadn’t taken into account
the importance of temperature, not just snow.
Baby, It’s Cold Outside
Rubber
is best known for being rubbery, which allows a tire to stick to the
tiny imperfections in the road's surface. But below the glass transition
point, rubber becomes hard, more like glass. If you’ve ever seen
the Cold Show at Science World, you may have seen how even
pliable rubber tubing can shatter upon impact after immersion in very
cold liquid nitrogen. Based on this idea, hockey pucks are often frozen
so they slide better. To give a good example of how dangerous this
phenomena can be, Richard Feynman found out rubber O rings don’t do
their job when they’re too cold, which contributed to the Challenger space shuttle disaster.
So-called all-season tires do not shatter below about 7 Celsius (45F), but they become somewhat less effective. Canadian Tire experimented with a white-walled tire that would turn blue
when it got too cold—I guess for people who don't believe the weather
reports. Even Vancouver has a monthly average temperature below 7 C,
from November to March. “All-season” must refer to people who fly south
for the winter. Apparently they are now, more honestly, called 3-season
tires. More recent all-weather tires are supposed to be okay for some
snow. If you are looking into tires, then you might consider those. For
now, I will stick with my winter tires without studs (which wreck
roads).
Various types of rubber,
both natural and synthetic, go into making a tire. This about half of a
tire's total weight. Each company has its own secret concoctions
involving many different chemicals. Winter tires are made of rubber that
remains flexible at lower temperatures, providing better traction.
Premium, i.e., more expensive, tires tend to have better rubber.
Tread On Me
The
deeper treads on winter tires can bite into snow at first and then
allow snow to accumulate. I would have thought that was not a good
thing, but snow to snow friction actually helps with traction. Then the
treads can clear themselves
of snow as they roll. The weight of vehicle on the snow creates a thin
layer of water and the smaller cuts in the tire surface called sipes help grip the road and channel water away from the contact surface. All-seasons don't have as many because they tend to cause the tire to warm up more.
Straight and Narrow
In
addition to the surface properties of both the road and the tire,
traction depends on the amount of force the tire exerts on the road. A
narrower tire, increases the weight per unit of area, which I didn’t
think about back when I bought my tires.
No So Fast
Even
with proper tires, you still need to drive carefully and more slowly
under poor weather conditions. Speed affects the amount of friction
between the road and the tire. At slower speeds, when the tire is not
slipping, it is exerting static friction. Anti-lock brakes allow the
wheels to rotate instead of locking so it improves control. When the
tire spins faster, it can slide and this dynamic friction is lower,
which is not a good thing.
What's Driving You?
All
the four-wheel drive vehicles on the road these days might make you
think you don't have to worry about the snow. But all-wheel drive is
helpful only with acceleration (so you’re less likely to get stuck). It
is not helpful with stopping or turning, which depends mostly on the tires.
On All Fours
Speaking
of four wheels, you might think you only need two winter tires, to be
more economical. But this unevenness in friction will make the car more difficult to control. Penny wise, pound foolish.
Wearing Out Their Welcome
Despite
the benefits of the winter tires during the winter, you should take
them off before it gets too warm because they can hinder the car’s
performance in drier, warm weather and the softer rubber will wear out
more quickly.
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