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Re: Friction



Jason Powell wrote:
then asks, "Well, if the area doesn't matter, then how come fat tires on a
race car corner better than skinny tires?"

Being a racer myself, I can tell you that tires don't fit our "area doesn't
matter" discussions....
...and thus not bite
into the track. Tires need to stay soft so some of the rubber actually
tears from the tire to produce best grip.

This kind of discussion brings out some interesting polarities in
thinking among PHYS-L members. Some members seem ready to dump standard
text book explanations at a whim - an experiment doesn't produce the
predicted result, a student asks a question you don't know the answer
too, and suddenly its bye bye text. After all, these "laws" are just
inflated generalizations of what is observed under some specific
circumstances... Other members seem to defend text book explanations
above everything else, including the assumed competence of the person
who does the experiment or asks the question.

I would like to think that I tread some reasonable middle ground between
these positions. The original experiment as described raised some
natural questions about whether the coefficient of friction was the same
for the different wood surface areas considered. The "tire" question
raises a multitude of issues to be dealt with before it can be
considered a counter example to the standard textbook case of linear
friction. Aside from the obvious question, "how does this cornering
advantage manifest itself?" here's a few:

Its starting to sound like one might just as well have countered with
"if area doesn't matter, why does a lot of tape stick better than a
little tape?" Nobody has claimed the law of friction holds for all
contacts between all surfaces, and adhesion effects are an obvious
counter example. It sounds to me a bit like tires, especially race car
tires, are being engineered to have adhesive properties.

I'm also curious about whether larger size means more contact area.
Sure the tire has more surface area, but what fraction of that touches
the ground? The normal force per tire would be N=weight/4. If we
inflate to some pressure P, the tire will flatten to produce the
equibrium with surface area of A=N/P. A larger tire will simply not
flatten as much in order to maintain the same surface contact area.
- Is there a problem with this reasoning?

Another thing that springs to mind, the normal force per tire is
weight/4 if all tires are the same size, but if the back tires are
larger wouldn't that result in tipping the car and having the normal
force on the rear (larger) tires actually reduced? In the case of plain
linear friction, one would actually be increasing the traction on the
front tires (where the steering is, and this would certainly benefit
cornering ability) while decreasing it on the back.

As far as tire width (rather than radius) goes, there must be some high
speed cornering advantages simply due to having a wider wheel base.

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

Doug Craigen "Technology with purpose"
http://www.dctech.com