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The Globe and Mail

This is the worst winter driving idea ever

Winter tires are best when you’re driving through snow, or in temperatures below 7 degrees Celsius.

I'm trying to find documentation about the effects of having three snow tires and one all-season tire on the car, this being the result of a mismatched spare tire. Any idea as to where I should look? – Claude

Documentation? In all my years of tire-testing, that is a pairing I have not run across.

We have used a pickup with shaved (bald) tires on the rear to teach how to deal with oversteer during skid schools, but that was with two treadless tires on one axle, not one. I can only imagine how terrible the situation would be with three winters and one all-season. Whether the pairing was on the front or rear wheels, the mismatch would be monumental and dangerous.

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The problem is a difference in grip from side to side. In an emergency-type manoeuvre – say, trying to steer around an object or person and back into your lane, the vehicle would respond differently in each direction.

For example, if the tire with less grip was on the right front, the vehicle would turn to the right okay, but when asked to turn to the left, with all that weight transferred onto the right front tire, there would be an almost immediate loss of grip and the car would plow or understeer directly ahead.

If the mismatched and lesser tire was on the right rear, the loss of grip would also occur in a left-hand turn, but in this case, it would be the rear that would lose grip and slide out first. You would go off the road backward.

Then there is the issue of whether the vehicle was front-, rear- or all-wheel-drive. In the case of a mismatch of grip on the drive wheels – whether front or rear, the tire with less grip would spin excessively, placing undue stress on the differential, leading to extensive and expensive repairs.

If the vehicle has a now-common electronic traction control system that employs the brakes on one wheel to limit spinning, the system would be stressed, especially the brakes on that one wheel, probably overheating and leading to failure.

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