Differentials, explained.

Dear Car Talk

Dear Car Talk | Oct 01, 1995

Dear Tom and Ray:

I bought a new Mercury Sable wagon back in 1991. Several times, I've noticed that only one of the front wheels has power transmitted to it for traction purposes. This has resulted in one wheel spinning on snow covered surfaces while the other wheel doesn't help at all. The dealer's representatives claim that all their cars are manufactured like this. Is that true?

TOM: Yes. That's exactly how it's supposed to work, James. It's the ugly, dark side of the otherwise miraculous device called the "differential."

RAY: When the first cars were being developed--right after they solved the square wheel dilemma--they noticed that cars were fine as long as they were going straight, but they had a hard time turning left and right.

TOM: And they figured out that when a car is turning, the wheels must rotate at different speeds. Imagine that you're driving in a tight circle. The outside wheels are going to travel further, and therefore faster, than the inside wheels, right? That's not a problem on a stage coach, because all of the wheels are all free to spin independently. But when there's power delivered to a set of wheels (the front wheels in your case) they need to be able to turn at different speeds, or you're going to "drag" the outside wheel along the pavement.

RAY: To allow for these different speeds on turns, the differential was invented. The differential is a brilliantly designed box of gears that sits between each pair of driven (powered) wheels. What it does is send power to the wheel that's the freest to turn, and take power from the wheel that's less free to turn. So if the outside wheel speeds up by five revolutions per minute, the differential makes the inside wheel slow down by five revolutions per minute. So instead of having two wheels turning at 100 revolutions per minute (like you would if you were travelling in a straight line), you have the outside (freer) wheel turning at 105, and the inside (less free) wheel turning at 95. And, miraculously, it works!

TOM: But if you take the differential's action to its extreme, here's what happens. If one of the wheels is stuck on snow or ice, it's VERY free to turn--infinitely more free to turn than the other wheel-- so the differential sends it all of the power. And then what's left for the wheel that's NOT on ice? Zilcho! And that's why you get stuck.

RAY: So what do you do about it? Well, for years, a lot of rear wheel drive cars have had an option called "limited slip differential," which transfers the power to the other wheel when one wheel is spinning--to prevent the exact problem we just described.

TOM: An even better solution is something new called "traction control," which prevents the driven wheels from spinning in the first place by actually applying the brakes to a freely spinning wheel. And traction control is available on an increasing number of both front and rear wheel drive cars.

RAY: But for front drive cars built before traction control was widely available (which is only recently), you're out of luck, Jimbo. But it's really not so bad. So you get stuck on ice or snow every once in a great while. Just think of it as the price of being able to turn left and right the rest of the time!

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