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Re: car tire friction & contact area



Rick Tarara wrote:
As you will see from the responses, this can get pretty complicated
with nobody really knowing exactly the answers.

Yup.

1) Here the width is more about heat dissipation than friction
because the mu for a rubber, liquid rubber, and asphalt interface is
really low! In fact, watch what happens when they spin up the
tires--they get higher and more narrow!

It's a mystery to me why they don't put pressure regulators
on the things. Heat dissipation is surely a problem, and
without a regulator the pressure will be less-than-ideal
early and/or more-than-ideal late.

2) The 'burn out' intentionally melts some tire material over the
area from which the car starts. Turns out that mu between 'rubber'
and 'rubber' is higher than between asphalt and rubber.

Yup.

===========

Also note that it is a multi-variable problem. For starters
we have
-- width of contact
-- length of contact
-- pressure

You are free to vary any two of them at the expense of the
third.

I imagine that for any given contact area, other thing
being equal (?!) you'd rather have more width and less
length, so as to have less sidewall flex. But as usual
other things are not equal, and if the tire gets too
wide the air chamber takes on a grossly non-round cross
section,
http://www.imac.ca/technofocus/tirenomenclature.htm
and it takes a grunch of engineering to make it hold that
shape and not bulge out.

===========

I doubt that tires are made wide on account of radiative
cooling. There's so much air flowing over them that I
doubt radiation plays a significant role.

My car has wide tires, and it surely has nothing to do
with cooling, by radiation or conduction or otherwise.
The tires are rated for 170mph, and I imagine at 170mph
they would get pretty toasty, but at the speeds I usually
drive heating is just not an issue. I bought them because
test reports indicated excellent handling under a wide
variety of conditions ... not because of the speed rating.