Re: Automobile tires and friction

["John S. Denker" <jsd@MONMOUTH.COM>]

Michael Bowen wrote:
...
> if the
> coefficient of friction is roughly the same regardless of the contact area
> between two surfaces, wide automobile tires are commonly considered to have
> "better traction" than narrow ones.

Good question.

> Is the superior traction of wide tires a truth or a misconception?

Yes and no.

Note that other things being equal, a wider tire corresponds to lower
pressure.  Obvious facts include:

1) On a road with standing water, you will get more traction with a
_higher_ pressure.  Other Things Being Equal (OTBE) this favors narrower
tires.  The rule is that the speed at which you will hydroplane scales
like sqrt(pressure).

2) On a road with a small amount of snow, you will do better with a
higher pressure, so you can punch through the snow to the underlying
road.

3) Off-road, on e.g. sand, you will do better with a lower pressure.
OTBE this favors wider tires.  This is not primarily a traction issue;
it's just that you don't want to sink in.

4) On a thick layer of snow, you will do better with the lowest possible
pressure, so you can float on top of it.  At some point caterpillar
treads are what you want.

5) In cases where you want a high pressure, at some point traction is
limited by the yield strength of the rubber.  A hard tire gives poor
traction.  A soft tire gives good traction up to the point where you
start erasing the tire.  Spreading the force out over a larger area
helps prevent erasing.  This is obviously outside the elastic regime and
obviously nonlinear, so one would not expect the usual linear models of
friction (coefficient of friction etc.) to apply.

6) Last but not least..... For any given tire, over-inflating it or
under-inflating it will do bad things.  On my car, the tire-tread is
designed to pump water out from underneath the tire.  Over-inflating it
would distort the tread and probably impair not improve the
anti-hydroplaning performance.  If you want to investigate the effect of
width (or pressure) you need to change tires, not just monkey with the
air-content.

7) There are presumably other factors that didn't immediately occur to
me.