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Winter Driving Guide

Does all-wheel drive actually help in winter driving?

Published October 25, 2012
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The automotive world abounds with myths and misconceptions.

There can be much controversy over technology and driving tactics made by experts, manufacturers and the driving public in general. Everyone seems to have an opinion or suggestion particularly about driving.

There is one in particular that is quite relevant this time of year. This particular misunderstanding is actually one expounded by the manufacturers and it entails all wheel drive (AWD) and four wheel drive (4WD). It is not uncommon to see TV commercials in which the manufacturers are linking safety with AWD.

The last time I wrote that AWD was not a safety feature I had numerous readers telling me that I had no idea what I was writing about. According to many of them, AWD was God’s gift to winter driving.

As winter sets in, I think this is an excellent time to discuss it. Usually motorists with AWD or 4WD become overconfident in limited traction conditions leading to a visit to the ditch or the rear bumper of another vehicle. Each winter there is a disproportionate number of AWD vehicles sitting in snowy ditches.

Safety is all about how much grip you are getting from your tires. The more traction your tires develop, the shorter your stopping distances and the superior your steering inputs will be.

To begin with, tire grip is only a function of the interface of your tire tread and the road surface. What determines the amount of grip is the rubber compound of the tread and how well it reacts to cold temperatures, the construction and age of the tire, the depth of the tread, air pressure and the size and shape of the actual contact patch.

A rolling tire will give the driver only as much traction as its above-stated characteristics dictate. Putting engine power through that tire will not make it deliver more traction. In other words, a given tire on a skid pad will only develop “x” amount of grip. If you try to power that tire by putting engine torque to it, that tire will not make any more grip than if it was freewheeling.

Having said that, AWD or 4WD will not help a vehicle turn (steer) with more traction. For example, if we had a vehicle in which AWD could be turned off so the vehicle also drove with only 2WD, that vehicle will generate the same amount of lateral grip on the skid pad whether it was in AWD mode or 2WD mode.

Bottom line, AWD or 4WD will not enhance the active safety of steering, such as collision avoidance or cornering grip.

Now if we look at another active safety feature of vehicles — braking — AWD does not help shorten braking distances. It actually can make braking distances longer due to the added mass of the AWD system.

So what benefit is there to having AWD on a vehicle? It allows the vehicle to accelerate better in slippery conditions. It is basically a performance feature, not a safety feature.

AWD will distribute the torque to the tires with the most traction and to all four tires if required. This translates into less tire slip and better acceleration. This is also a double-edged sword.

Many readers have told me because there is less chance of the tires slipping (only when power is applied) that this in itself is a safety feature.

Not necessarily so. That slip of a tire under power can be a good thing. It tells the driver there is very little traction. If the tires do not slip as can happen with AWD, it can mask how slippery the road surface really is and contribute to an overconfident driver.

There is no place better to show this concept than at our winter driving schools. When we discuss tire grip and AWD in the class, I’ll get my share of strange looks from the students as this is contrary to what they have been told. It’s not until they are driving on the ice and realize they cannot negotiate the skid pad, slalom, collision avoidance or emergency stopping any better than front wheel or rear wheel drive vehicles. Then I get the “I see what you mean” look from the drivers.

If you drive a vehicle with AWD or 4WD drive keep these rules in mind.

1. You may be able to get going much better than 2WD vehicles but you cannot stop or steer better. AWD only helps acceleration.

2. If you drive a vehicle with AWD or 4WD you should have winter tires on all four wheels. It’s the stopping and steering that will save your life and AWD does not enhance these aspects of active safety.

3. Slow down as there may be less grip than you are experiencing while accelerating.

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