Peter Duniho wrote:

"Matt Whiting" wrote in message
...

[...]
That is because you aren't correct. Tire pressure only has a significant
impact on very soft surfaces such as sand, where the extra surface area
helps with flotation. In most snow, it makes little difference.


Actually, we're both incorrect.

My statement was based on a theoretical understanding of friction in which
the friction depends on the force over an area. Since tire pressure
directly determines this, I assumed it had a direct effect on friction.

I don't recall area being a part of the thoeretical equation. My
Physics book says that F=uN, where F is the total force due to friction,
u (mu) is the coefficient of static or dynamic friction as the case may
be, and N is the normal force holding the two surfaces together. Area
isn't part of the equation. Now there are materials reasons that area
does have an impact, that that isn't in the basic theory.


I found at least one reference that says that physicists ignore the area
over which the force is distributed, for the purpose of determining
friction. It did say that's actually an incorrect assumption, but that it's
"close enough" for most purposes.

Actually, every reference I've ever seen ignores area, because it is
only a factor in special circumstances and then it is related to the
materials failing, not to the underlying theory of friction.


I didn't bother to look further to see just how far off this "close enough"
assumption is. The reference didn't go into much detail on that regard.

That is because you are wrong and didn't want to further show that.


Beyond that, the same reference also had a discussion of tires on snow,
oddly enough (I wasn't even looking for that specifically). They claim that
increased tire pressure actually *reduces* friction, because packed snow has
lower friction than unpacked snow, and higher tire pressures result in
greater packing of the snow.

That is a somewhat specious description, but in any event tire pressure
is at best a second or third order effect, it isn't a first order affect.


So, tire pressure has a very significant effect on tire friction when
driving on snow. But it's opposite what would be the case on a solid
surface. So, chalk that point up for the anti-SUV crowd.

Still wrong. Tire pressure is little affect.


I still think it's a silly argument.

Yes, when I'm shown to be wrong, I usually think it was a silly argument
at that point as well. :-)

Matt