Re: [Phys-l] car skidding and spinning

[John Denker <jsd@av8n.com>]

On 12/04/2011 02:03 PM, Bob Sciamanda wrote:

> > -- 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.
>
> My aging brain is missing something.
> Why is turning INTO the turn always more dangerous?

Two answers:

I am always fond of qualitative arguments, so here is the
zeroth-order qualitative analysis of the steady state during
a skid.  We consider the case where all four wheels are
skidding.  To a qualitative approximation, the direction
the car is /traveling/ is completely decoupled from the
direction it is /pointing/.

So, the car is /traveling/ in a straight line, no matter
what.  The only question then is which would you prefer:
  a) Facing the direction you are traveling -- which is
   what happens if the rear tires are "better" i.e. if
   the rear tires produce more drag, or
  b) Facing the opposite direction, which is what happens
   if the nominal "front" tires are better.

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

We can obtain a stronger and more interesting result by
re-analyzing things more quantitatively.

The slip/skid angle β is by definition the angle between
the way you are /pointing/ and the direction you are
/traveling/.

Suppose you are initially in a normal steady turn.  Then:
  a) If the front wheels start to give, this decreases β.
  b) If the rear wheels start to give, this increases β,
   making the skid worse.  Positive feedback.

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

There are additional games that can be played, depending
on engine thrust, braking, and steering ... but most drivers
don't know how to play those games, so I'm skipping that
part of the discussion.