### Re: If friction forces don't vary with surf. area, why are race car tires wide?

Date: Mon Oct 12 01:43:02 1998
Posted By: Luis A. Porrello, Grad student, Civil Engineering - Transportation,
Area of science: Physics
ID: 907124994.Ph
Message:
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Greetings Daniel

You have asked a very interesting question and one that can be very tricky
to answer.  So I'll do my best.  There are two approaches to answering
your question.  One is the Physics approach, the other is the Automotive
Engineering approach.

First, on the Physics side, the friction and surface area argument could be
one of semantics.  I refer you to the answers given to Markus Engelhardt
and James Reid, users who asked the same question as yourself, by our
scientists.  To find their questions go to our search engine
(circumnavigator) and type in their names as a the keyword.  You can also
use this search engine in the future to find if your specific question has
given to the aforementioned users, I'm going to give you another physics
answer, one which I found in a physics book (I'm an engineer, not a
physicist):

"Intuitively, you might have expected the frictional force increases
with increasing contact area.  On a microscopic scale, your intutition is
correct:  the greater the area of actual contact, the greater the friction.
But microscopically, only a small fraction of the area you measure
macroscopically is actually in contact with the other surface.  As the
normal force between the surfaces increases, the surface irregularities are
crushed together and the actual contact area increases, so that the
frictional force increases."

So is that a confusing wording game or what!  Aha!, do not worry my friend,
because I got the answer you are looking for!  Keep reading, I'm almost
done confusing you.

disappointed you picked Indy car as an example since I am a fan of Formula
1 racing instead), we must look outside of the realm of Physics and
friction, and look instead into Automotive Engineering.  Guess what, it is
as a Formula 1 (or Indy if you really must) race car mechanic are skinny
tires or fat tires.  You put on the skinny tires and let the driver loose
in the circuit.  As he/she accelerates thru the first turn, the car spins a
hundred times and smacks the wall.  Your driver and team owner are pretty
upset at you.  The car simply did not have the "grip" necessary to remain
on the turn at high speed.  Here's the trick!  "Grip" is NOT necessarily
"Friction".  Grip is really how well a tire can remain in contact with the
road surface.  It does have to do a little with the friction between tire
and road surface.  But it has to do a lot more with the stiffness of the
tire and how much it deforms under the forces of acceleration.  As your
driver accelerated through the turn the skinny tires deformed excessively
and therefore were not able to remain in contact with the surface.  If you
had chosen the wider tires, however, the tire would of deformed less under
pressure because of its greater stiffness and therefore you, your driver
and the guy who is paying for the car would be on the road to winner's
circle.  So, as the car commercial says, "Widder IS Better".

Oh, but wait a minute, what about our scientist friends in thermodynamics?
Do you see the can of worms you have opened?  There is another reason for
the wider tires.  Tires heat up very fast.  This is good, for a while.  But
you do not want them to overheat easily.  That would cost you precious time
in the pits changing tires!  So you put on a wider tire to increase the
surface area by which the tire transfers some of it's heat into the
pavement.

So my friend, the answer to your question is not so simple.  But if you
want a one sentence answer (you're are thinking why I didn't just give you
that in the first place), the reason is this:  Fat tires are not about
friction.  Increase width, increase stiffness, decrease deformation,
increase grip. That's the ticket!  You want the formulas?  I didn't think
so. He,he.  Thanks for your interest.  Take care of yourself.

Luis

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