Re: car tire friction & contact area

[John Denker <jsd@AV8N.COM>]

GJ Schade wrote:
> We learn in my high school physics classes that the amount of area in
>  contact between two surfaces does not affect the amount of sliding
> friction between them.

Well, there's quite a bunch of assumptions and approximations
that go into that.

> Invariably, every year a student asks why are wider car tires better
> at handling, etc.

Ugh.  If you ask six people what "good handling" means,
you'll get twelve different answers.  I'll assume for
now that "max mu" is the objective;  if not, please ask
a more-specific question.

> I respond, somewhat vaguely, that the tire is rolling over the road
> with little or hopefully zero slippage, and therefore it is a
> different phenomenon.

True.  Vague but true.

> One student this year, showed me a car enthusiast listserv thread on
> the same topic.  One posting stated that Ff = mu * Fn only applies
> experimentally if mu < 1 and rubber versus road has a mu > 1.

The formula Ff = mu * Fn is dodgy at best, even for mu < 1.

And certainly it is easy to observe things that "look
like" mu >> 1, although at some point you begin to
wonder whether friction is the right term for it.
Imagine sandpaper on sandpaper for starters ... try it.

> Is that really the case?  Does anyone have a high-school level
> explanation for wider car tires that I can pass on to my students?

You have rubber that is soft enough to ooze down
into the texture of the road, but hard enough to
not get immediately torn away by the sideways
streses.  So it winds up being a little like
sandpaper on sandpaper, or like a cog-wheel on
a cog-rack.

Probably 90+% of the car-tires in the world have mu greater
than unity when they are parked on macadam.  Maintaining
that under the dynamic conditions of driving is harder,
but there are high-end sports cars that reportedly
corner at 1.5 Gees or even more.  A percentage of that
involves aerodynamic downforce, so the normal force
exceeds the weight of the car, but I still think the
Fn is less than 1.5 times the weight of the car, so
we're still talking about mu>1.

=========

The Subject: line mentions contact area.  Contact
area scales inversely with pressure.

Again, many possible answers.  For good handling on
snow you either want high pressure or verrrrry low
pressure.  Typical car-tires are an unhappy medium.

If you're worried about hydroplaning, you want high
pressure.

Optimal pressure obviously depends on the softness
or hardness of the rubber, and on the roughness of
the road.