From: mrc@Ikkoku-Kan.Panda.COM (Mark Crispin)
Newsgroups: pnw.general
Subject: Re: Weather on I-5?
Date: 17 Dec 92 04:38:17 GMT
Article-I.D.: Ikkoku-K.MS-C.724567097.662824084.mrc
Organization: University of Washington
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Laura Halliday suggests backing off on the power to recover from a skid in a
rear drive car.

Not good, unless your skid is in an American Piece of Junk (APoJ) that tends
towards understeering.  Backing off on the power in an oversteering skid will
cause more weight transfer to the front of the car, which is exactly what you
don't want.

First, some definitions:

The ``contact patch'' of a wheel is that area of the tire which is in physical
contact with the road surface.

``Adhesion'' is the tendency for the contact patch to adhere to the surface of
the road through friction.  That is, adhesion occurs when the contact patch
does not move in relation to the road surface it contacts.  When you have
adhesion, motion occurs only when additional contact patch is being laid down
at the leading edge of the contact patch, and is taken up at the trailing
edge, in the direction the tire is rotating.

The ``limit of adhesion'' is that point where the forces operating on the
vehicle equal the friction that maintains adhesion.  Increasing these forces
beyond the limit of adhesion will cause the contact patch of one or more
wheels to lose adhesion.  Typically, this is on the wheel(s) which have the
least weight and/or the most slippery surfaces.

``Understeer'' is the condition which occurs when a vehicle, brought beyond
the limit of adhesion, loses adhesion on the front tires.  The vehicle loses
steering control and tends to head in the direction pointed to by the rear
wheels.  APoJ's tend towards understeer, since it is less frightening to
inexperienced drivers; and, if the brakes are hit in a panic, may recover
because of the sudden transfer of weight to the front tires.  Then again, a
car in an understeer may lose it totally.

``Oversteer'' is the condition which occurs when a vehicle, brought beyond the
limit of adhesion, loses adhesion on the rear tires.  The back end breaks
loose, and the car turns in the direction pointed to by the front wheels.  The
overall result is a very fast turn with relatively little steering input, a
characteristic much beloved by sports car enthusiasts and the terror of most
other drivers.  Ralph Nader's primary reason for (unfairly) damning the
Corvair was that it was an oversteering car at a time when only rare foreign
imports oversteered.

Icy conditions are quite suitable for oversteering.  The general scenario is
that the driver applies too much braking, causing weight to transfer to the
front wheels.  The rear wheels, suddenly without much weight, lose adhesion
and the car responds only to the front wheels.  The typical reaction of an
inexperienced driver is to overcompensate in his steering and hit the brakes;
this takes even more weight off the back end, and the car swings around in an
even more violent oversteering skid in the opposite direction from the
original skid.  If the driver is lucky, he ends up eventually coming to a stop
after having spun around, with no more damage than to his or her ego.  If
unlucky, the car gets bent.

The right thing to do in an oversteering situation is to apply enough steering
to straighten the car out, KEEP OFF THE BRAKES, and wait until you are pointed
in the direction you want to go, with your rear wheels behind your front
wheels.  Then, PUNCH the throttle -- this will cause a sudden transfer of
weight to the rear wheels, which will dig in and your car will happily go
where you want it to go.

If you are not pointed in the direction you want to go, the fastest way to get
there is to let off the throttle.  The less power you apply to the rear
wheels, the faster your turn will be.  Try to keep your steering wheel steady;
in a skid your throttle is what you use to control your turn.

Whatever you do, keep off the brakes.  If your car is in motion, the only hope
you have of regaining adhesion is if your wheels are also in motion.  You will
regain adhesion when the motion of your wheels match that of the car.  If you
lock your wheels -- which you will do if you hit the brakes in a skid, you
have lost all hope of regaining adhesion.
Newsgroups: pnw.general