MadSci Network: Physics

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

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 
been asked before.  Anyway, back to the answer.  In addition to the answers 
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 

     "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.

The answer to your question about big fat bold tires on race cars (I'm 
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 
not about friction, it's about STIFFNESS.  Here's the deal:  Your choices 
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 

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.

Yours in Mad Science,


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