Dear Tom and Ray:

It's this neighbor of mine, see. The guy always makes wild statements
after we put a can of beer in his hands. This time he was after the snow
chain manufacturers, claiming they make a bundle, unnecessarily, because a
regular car is only driven by one wheel, not two. So, he says, one chain
would be enough. Is this guy right? And if he is, is four wheel drive
really two wheel drive? We finally got him to shut up, but he'll be back,
so please help us.

Albert

RAY: Hey Albert, have you ever heard the expression "a little knowledge is
a dangerous thing?" Well, we'll add a corollary to that: A little
knowledge and a can of beer is an extremely dangerous thing.

TOM: Under normal circumstances, a two-wheel-drive car is a
two-wheel-drive car. That is, power is distributed more-or-less equally to
the rear wheels.

RAY: In between the wheels is something called a "differential," which
adjusts the amount of power sent to each wheel when you're turning left or
right, so you don't, for instance, drag the outside wheel around a curve.

TOM: The differential is a miraculous device, but it does have one
unfortunate quirk. If one wheel is spinning on ice or snow, all of the
power goes to that spinning wheel (I said the differential is miraculous, I
didn't say it was smart). That's why, when your car is stuck on ice or
snow, you may see just one wheel spinning. And that's probably the source
of your friend's "little bit of knowledge."

RAY: So why don't you just put chains on that one wheel? Well, first of
all, unless your passenger was the Amazing Kreskin, you'd never know which
wheel was going to get stuck. But more importantly, even if you did guess
correctly, then the other wheel--the one without the chains--would spin!
And then the wheel with the chains will get, what? Nada! So you'll still
be stuck.

TOM: And what's true for two-wheel-drives is also true for
four-wheel-drives. Under normal circumstances, you have four wheel drive.
But if one front and one rear wheel happen to be stuck on ice at the same
time, you wouldn't go anywhere.

RAY: So you need chains on both wheels of a two-wheel-drive car. Giving a
traction advantage to just one rear wheel would actually make the situation
worse....just like I'm sure your friend will do when he tries to weasel out
of this argument by saying "well, what I meant to say was...."