Traction adventures in treacherous conditions
drive SUVs and trucks.
When it comes to traction, tires make the biggest difference. Winter ice tires would have helped many drivers stay on the road or just made the morning commute that much easier and safer. The next most important traction factor is how the power is put down to the road.
Having power to all four wheels will provide excellent traction, and that fact is recognized by automakers in their wide selection of all-wheel drive vehicles. AWD spreads the engine torque out to each tire so that each can provide some of the grip necessary for acceleration and steering control.
AWD vehicles use a transfer case that splits the power to each end of the vehicle, but still allows each axle to turn at different speeds. This can be accomplished several ways. Some use a planetary gearset and clutches in the transfer case. Others use a viscous coupling unit that utilizes silicone fluid between two plates to drive the axles. Others use a Torsen design differential in the transfer case.
There are other methods as well, but the purpose of all these designs is to allow the wheels at each end of the vehicle to turn at different speeds.
Four-wheel drive may seem to offer the same advantages, but it also has a major drawback: this system drives both the front and rear axles at the same speed. That wouldn’t be a problem if we only drove in a straight line on a smooth road. But hit a bump or round a corner and the wheels suddenly turn at different speeds because they are not following the same path of travel. All-wheel drive allows this. Four-wheel drive doesn’t.
In four-wheel drive, the front and rear driveshafts are mechanically locked together to turn at the same speed. This drives the tires at the same speed. Try going around even a gradual curve and one or more of the tires has to lose traction. The tighter the corner, the more the tire has to slip on the road surface.
On dry pavement, this action would wear tires and place a tremendous load on drivetrain components, wearing them out quickly. But on slippery roads, the forced slippage of the tires on corners will typically cause the vehicle to lose steering control. You can turn the steering wheel but the vehicle won’t turn because the tires are not gripping the road at all.
That’s why four-wheel drive vehicles are not the best on slippery roads. Look in the owner’s manual of a fourwheel drive vehicle and it will tell you that four-wheel drive should be used on loose or off-road surfaces only.
Many SUVs and some pickups have an “Auto” mode for their four wheel drive. This mode allows the vehicle to operate in two-wheel drive (either rear or front, depending on the model of vehicle) and the other two wheels are only powered during acceleration or when the drive wheels begin to spin. Auto mode works fine on slippery roads because, as soon as you back off the throttle, the vehicle returns to twowheel drive mode and the tires will get traction again.
Four-wheel drive may rule the roost on off-road adventures in mud and deep snow. But, on icy, slippery roads, all-wheel drive is better any time.
Just remember: All-wheel drive may get you going faster, but stopping isn’t any faster with an AWD vehicle. So install good winter tires and keep your speed down for safe, secure driving. Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of