Should I be using 4WD in cold, rainy conditions?

Dear Car Talk

Dear Car Talk | May 01, 2001

Dear Tom and Ray:

Yesterday, I was driving my 1999 Ford Expedition on the highway in Vermont. There was a cold rain when I started out, but it was above freezing. I was driving at about 60 mph when, without warning, I must have hit some ice and I lost all contact with the road (the thermometer read 32 after the incident). The car went sideways, then spun completely backward, then a tire hit something and the car spun another 360 degrees the other way. I finally slid to the guardrail and stopped in the right lane, facing the oncoming traffic. Incredibly, I hit nothing, and no one was hurt. Since then, the scene has been constantly replaying in my mind. I am a careful driver, without a single accident in my 24 years on the road. But I am trying to identify my mistakes from yesterday. I know mistakes No. 1 and 2: Don't drive in a storm, and if you have to, go slower than 60 mph. I wonder whether there was also a mistake concerning my use of the four-wheel drive. At the time of the incident, I was in four-wheel-drive mode. During cold rain, is that the right mode to be in? -- Alan

TOM: Well, going 60 mph on potentially icy roads was a major mistake, Alan. For some reason, people who drive SUVs suddenly think the laws don't apply to them anymore -- the laws of the road, the laws of common sense and even the laws of physics.

RAY: But you might have made the situation worse if you were in the wrong 4WD mode. There are several 4WD choices on your Expedition. There's 4WD LOW, 4WD HIGH and 4WD AUTO.

TOM: We know you weren't in "low," because you wouldn't have been able to go 60 mph in that mode. But if you were in 4WD HIGH, then you might, in fact, have triggered the loss of control.

RAY: Traditional 4WD systems do not have center differentials. And in 4WD HIGH, your Expedition operates just like an old-style, traditional 4WD, with both the front and rear axles (actually driveshafts) forced to turn at the same speed.

TOM: That's fine for getting out of a snow bank. But if you're driving on the road in that mode and you take even the slightest curve, like one you'd encounter on a highway -- especially at 60 mph -- the front and rear wheels no longer want to turn at the same speed. And the inevitable loss of traction between some of the wheels and the road can lead to strange handling and loss of control.

RAY: Particularly when you factor in ... ICE!

TOM: And that's the second factor. Even if you were in 4WD AUTO on the highway -- which is the mode you should have been in for on-road driving -- you still shouldn't be driving that fast in icy conditions. When it's wet out and anywhere NEAR freezing, you have to anticipate that ice might be present on the road. And if you happen to hit ice at 60 mph and then try to steer the truck even a little bit -- which shifts the weight of the vehicle -- you're very likely to induce a spin.

RAY: So lesson No. 1 is: Be sure you know what kind of 4WD system you have. If you have a traditional 4WD without a center differential, then you should only use 4WD on slippery terrain, like snow, mud or sand, and only at low speeds.

TOM: If you have a center differential, or the electronic equivalent (which is what 4WD AUTO is on the Expedition), then you can safely be in 4WD anytime.

RAY: Lessons No. 2 and 3 you've already figured out: Don't drive in a winter storm if you don't absolutely have to. SUVs are not magic carpets. They're still subject to the laws of physics, especially on ice. And if you do have to drive in a winter storm, go very slowly and carefully.

TOM: And if you want to add another lesson to that, don't trust the accuracy of that little "outside temperature" reading on your dashboard. Assume that at any reading under 38 or 40, you could encounter ice, and drive accordingly.

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