Dear Tom and Ray:
Since early childhood, I have been told by many a cigar-smoking,
wrench-toting, grease-covered mechanic that a normal rear differential
delivers power to only one rear wheel. I was further told that this
phenomenon was so complicated and required such advanced knowledge of
physics that the normal layman could never fathom it.
I have always found this story somewhat hard to swallow, but have
finally found it in writing. In discussing four-wheel-drive vehicles, a
fellow columnist of yours in another paper states: "The vast majority of
the time, most of these vehicles are operating in two-wheel drive, which
means that only one wheel out of four has "motive traction" --
transmitting power to the road .... Even when it's driven in four-wheel
drive, usually only one wheel per axle has motive traction."
Now my question. I'm planning a trip in my '84 Nissan pickup and expect
to encounter hilly, wet, slippery roads. I have three tires with
not-so-good tread and one with very good tread. I would, of course, like
to have my best tire mounted on the wheel that provides my "motive
traction." Can you tell me which wheel it is? -- Jim
Ray: Wow, Jim. This is an impressive example of turning one incorrect ![[bluecar.gif]](/common/images/bluecar.gif)
piece of information into a completely bogus theory and then into a
full-blown ill-advised plan of action. I'm impressed!
Tom: Our "fellow columnist" is wrong, Jim. On a standard,
two-wheel-drive car, most of the time, traction is going to BOTH wheels.
Ray: Right. It's only when one wheel is slipping that more power is
sent, believe it or not, to the slipping wheel (I know that sounds
wacko, but unless the car has a "limited slip differential," that's the
way the differential works).
Tom: Why does the differential work this way? Well, as you say, Jim,
this phenomenon is so complicated and requires such an advanced
knowledge of physics that no Italian mechanic could ever explain it
without using his hands.
Ray: A typical four-wheel-drive vehicle works the same way, except there
are two sets of driven wheels. So in four-wheel drive, most of the time,
all four wheels are being powered. It's only if one or more wheels start
to slip that the power distribution changes.
Tom: So what does that mean for your Nissan pickup, Jim? It means you
should forget about this cockamamie "motive traction" idea and go out
and buy yourself three more good tires.
Ray: Not only will good tires improve your chances of not getting stuck
on wet, slippery roads, but, more important, four good tires will help
you TURN and STOP on wet, slippery roads -- and most drivers consider
that an advantage
Why do unmitigated cheapskates like Tom continue to buy nothing but old clunkers? Find out by ordering Tom and Ray's guide "How to Buy a Great Used Car: Secrets Only Your Mechanic Knows." To order, send  (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.
© 1997 by Tom and Ray Magliozzi and Doug
Berman Distributed by King Features Syndicate, Inc.
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