Ray corrects a few misconceptions

By Ray Magliozzi

Dear Car Talk:

I purchased a 2004 Nissan Altima with front-wheel drive last winter. At one point, my right front tire was stuck on ice but my left front tire was on dry asphalt. I tried to get off the ice, but the right tire (the one on ice) kept spinning, while the left tire, on dry pavement, did nothing. I was under the impression that if one tire slipped, the traction would be switched to the non-slipping tire (left). When I called the dealer, he stated that the car is manufactured so that the right tire gets the power, and that next time I should consider parking so that my left tire is on ice and the right tire is on dry asphalt. I asked him if he was serious, and he said yes. What is the real story about front-wheel-drive cars — specifically, the 2004 Nissan Altima — with regard to which wheel gets the power on ice? — Marc


Dear Marc:

Wow, you guys are a wellspring of misinformation!

All cars have something called a differential. The differential is a magical box of gears (sorry, that’s the best I can do without using diagrams and my hands) that send power to the wheels while — at the same time — allowing the left and right wheels to turn at different speeds.

Why is that necessary? Because when you’re going straight, your left and right wheels rotate at the same speed, and all is right with the world. But when you turn left or right, the inside wheel has to turn slower because it’s traveling a shorter distance than the outside wheel travels (draw a picture of where a car’s wheels go when it turns, and you’ll see that the outside arc is longer than the inside arc). If the inside wheel couldn’t turn slower, you’d drag it along the pavement. Not a good way to turn.

The only downside of the differential is that because of the way it works, there are two properties that are always true. One is that the combined speed of the two wheels always stays the same. So the outside wheel speeds up by the exact amount that the inside wheel slows down.

And, more importantly for your purposes, the differential always sends the exact same amount of torque (twisting power) to both wheels.

Why is that a problem? Well, if one wheel is on ice and it requires practically zero torque to make it spin (which is what happens on ice), then the other wheel — the one on dry pavement — also gets zero torque. So the wheel on ice spins freely, and the car doesn’t move.

That’s been addressed over the years in some cars with a technology called “limited slip differential,” which detects when one wheel is slipping and sends torque to the other wheel. But that wasn’t available on your car.

These days, most cars have computer-controlled dynamic stability control, traction control and ABS brakes, which address the same problem.

But to answer your question, under normal circumstances, both wheels get power in your 2004 Altima — as they do in pretty much every car. And whoever answered the phone at your dealership had his head up his differential.

* * *

Got a question about cars? Write to Car Talk in care of this newspaper, or email by visiting the Car Talk website at www.cartalk.com.


Leave a Reply

Your email address will not be published.