Think driving an all-wheel-drive vehicle means a slick road is nothing to worry about? Think again.
That feeling of invincibility can be pretty satisfying, but it can also get you into trouble. There’s always a little voice in the back of your head saying, “Go ahead, you’ve got this…” You figure that if you’re driving a four-wheel-drive or all-wheel-drive vehicle, a slippery road shouldn’t be much of a problem. Well, let’s see about that.
Let’s start with something as basic as braking. You’re having a little too much fun accelerating in a big 4WD SUV when, suddenly, the traffic light ahead turns red. The pavement is wet and slick.
Your brakes may be excellent, but there’s one important detail: every other vehicle on the road is braking with four wheels, too, not just two. And your SUV is likely heavier than a comparable two-wheel-drive model, which means it can take longer to stop. Before you know it, you could be getting dangerously close to the car ahead.
And if that happens, the vehicle won’t be the one to blame. The driver will.
That means you.
Now let’s try turning.
With old-school rear-wheel-drive cars, the basic rule was fairly intuitive: if the car starts to slide on a slippery road, ease off the throttle immediately. Even your instincts can tell you that.
Front-wheel drive changed the equation. In some situations, you actually want to maintain the throttle rather than suddenly lift off it.
So what happens with four-wheel drive?
That’s where things get complicated, because not all 4WD systems work the same way.
Take a traditional part-time 4WD system with a mechanically locked connection between the front and rear axles. This is the type of setup commonly associated with trucks and off-road SUVs.
On a slippery road, driving one of these vehicles smoothly through a corner takes some skill. When a vehicle turns, each wheel needs to travel a different distance because the wheels follow different-radius paths. But when the axles are mechanically locked together, the drivetrain doesn't allow the wheels to differentiate their speeds.
That’s exactly what you want in mud, sand or deep snow. On a slick paved road, however, it can become a problem. One of the tires may have to slip to compensate for the difference in wheel speed.
Once a tire starts sliding, it loses much of its ability to handle lateral forces. And the remaining tires may not be able to keep the vehicle on its intended path.
The result can feel less like controlled driving and more like an impromptu rodeo—and not everyone is going to walk away from it with their dignity intact.
Things are different when the drivetrain normally sends most of its torque to the rear wheels while allowing some torque to reach the front wheels.
Unlike a conventional rear-wheel-drive vehicle, an AWD system can respond to a loss of traction by shifting more torque toward the front axle. That can help prevent the rear tires from breaking loose during hard acceleration.
But there’s an important catch: the exact behavior depends heavily on the vehicle’s drivetrain design and calibration.
AWD systems can behave very differently from one model to another.
Some vehicles use a conventional center differential, while others rely on electronically controlled clutches to distribute torque between the axles. Some maintain a small amount of torque transfer to the rear wheels almost all the time. Others operate primarily as front-wheel drive until the system detects a loss of traction.
In certain systems, the electronics can send a substantial amount of torque to the rear wheels almost instantly when the front tires begin to spin. In the wrong situation, that sudden change can contribute to a rear-axle slide.
So what’s the general rule?
In most cases, an AWD vehicle that begins to slide should be brought back under control with smooth, modest throttle input rather than abrupt movements. Steering inputs should also be gentle and deliberate, with the driver carefully steering in the direction of the slide as needed.
But there’s a big difference between knowing the general rule and understanding exactly how your own vehicle behaves.
If you really want to learn the limits and quirks of your particular AWD or 4WD system, there’s no substitute for practicing in a controlled environment with a qualified instructor.
The most important thing to understand is that four-wheel drive is not a cure-all for slippery roads.
In inexperienced hands, it can even create an additional hazard by giving the driver a false sense of security. It’s easy to think, “The vehicle will take care of it.”
Unfortunately, it might not.