Friday, November 6, 2009

John Mahler Opinion on the use of Winter Tires

John Mahler is a veteran automotive journalist with the Toronto Star's

We asked his opinion on Bathurst High School's Multi Function Activity Vehicle (MFAV) and the use of all season ribbed tires on the front wheels with winter tires on the back. When we wrote to him about the tires, the Department of Transportation had just changed the all season tires on the front to another brand after we complained about the use of all season on the front tires.

This is what Johh Mahler said:

The changing of the front tires on the mini-bus has not improved the vehicle from a safety piont of view. They have changed from one brand of all-seasons to another brand. Both are considered "ribbed tires" with the new Goodyear having slightly bigger rain grooves.

Concerning grip: it is conventional wisdom in the tire industry that all-season tires, winter tires and summer tires have the same grip (within a percent or two) at 7 degrees above zero. After that as temperatures drop the summer tire looses grip very quickly, the all season tire looses grip slowly and the winter tire's grip increases relative to the other two.

So as temperatures drop, the mini-bus in question will have more and more grip in the rear and less and less in the front. This makes the handling of the vehicle very unpredictable. That means the driver may find the vehicle does do react as expected when brakes are applied or the steering wheel turned. Unpredictable vehicle reaction in an emergency situation can lead to a crash.

John also gave us this quote from an article he wrote on the use of winter tires several years ago:

Copy of an item in my column from a few years ago:

You need four good winter tires. This has been the generally accepted practice in the industry for a number of years. Most reputable tire shops will refuse to install just two winter tires for fear of liability if the car is involved in an accident. So I would be suspect of dealing with the shop that recommended installing just two tires.

The reasoning is simple: you want all four corners of the car to work together in an emergency. That means having the same handling characteristics, so that you can brake, steer, accelerate and have equal lateral grip.

Were you to install just two tires on the rear drive wheels, you would have traction to go, but the front tires would lock up sooner than the rears under braking, so you would lose all steering control. If you install the two tires on the front only, you would not have much traction and under braking the rear tires would lock up first and the car would slew sideways and spin. General cornering would make the car understeer (go wide) if the winter tires were on the rear and oversteer (tail comes out) if the winter tires were on the front. Neither scenario makes for safe driving.