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[Phys-l] car skidding and spinning



On 12/03/2011 06:16 AM, Philip Keller wrote:

The car is held to the circular path by a rope and after a couple of
laps, I have the rope break. It was my intention to show that
without the centripetal force, the car goes in a straight line. But
the first time I ran it, I admit that I was surprised to see the car
move off in a straight line, but also rotate

OK. Let's call that the "inherited" spin.

it was actually a
very realistic view of a car "spining out".

Although it is a nice demonstration of rotational physics, the
"rope" scenario is much too idealized to represent what happens
under realistic driving conditions.

I don't want to seem unduly nitpicky, but there are life-safety
issues involved, so it pays to be careful what we say.

In a real driving situation, it is very likely that either the
front wheels or the rear wheels will "let go" before the others.
A sideways force from one set of wheels without a comparable
force from the other set will produce a torque around the center
of mass. This torque is large enough to very quickly produce a
spin angular momentum that is huge compared to the inherited spin.

The physics of the real situation is very complicated, having
to do with various things including
*) dynamic friction versus quasistatic rolling friction
*) the fore-and-aft position of the car's center of mass
*) front wheel drive versus rear wheel drive, and amount of
engine thrust
*) braking
*) road conditions
*) dissimilarities in the tires
*) good or bad driver technique (!)
*) et cetera

Note the contrast:
-- If the front wheels let go, the car spins to the outside
of the turn ("understeer").
-- If the real wheels let go, the care spins to the inside
of the turn ("oversteer").

The rear-wheel skid is much more dangerous than the front-wheel
skid, so you want to do everything in your power to avoid it.

Serious constructive suggestion #1: There are lots of scenarios
that lead people to replace tires two at a time rather than four
at a time.

You can /sometimes/ avoid this if you are fastidious about
rotating the tires, but sometimes not even then ... for instance
if one of the tires gets damaged.

In such a situation, the new tires should be installed on the rear.
The old tires go on the front, even if this means taking them off
the rear and moving them to the front.

At this point, in the usual situation where the front tires wear
more quickly, you are now locked in to /not/ rotating the tires.
The rear tires will always be significantly better than the front
tires.

To repeat: Put the better tires on the rear. This is counterintuitive
from the "wear" point of view, but it is mandatory from the safety
point of view.

=============================

Serious constructive suggestion #2, for students and everybody else:

This time of year, take a few minutes to /practice/ driving on
snow. Experiment to find out how *your* car behaves. This
includes things like
-- how much provocation it takes to get the car to skid
-- how how it behaves in a skid
-- what you need to do to get out of the skid.

If you haven't ever done this, you reeeeally need to do it.
Even if you have done it every year for N years, it pays to
refresh your memory. If you live in a non-snowy place and
travel to a snowy place, practice as soon as you get there.

Find a nice big empty parking lot with some snow that hasn't
been plowed.
-- Build up a small amount of speed and slam on the brakes.
-- Build up a small amount of speed and then make a sharp turn.

I guarantee you that "antilock brakes" do not solve all the
world's problems.

I guarantee you that what you were told in driver's-ed class is
not the whole story.