Originally Posted by
Caliper
... But, lower pressures on the same sticky tire DO result in more grip because of the larger contact area. More rubber interfacing with the road means that there are more points of mechanical interface where the rubber is physically interlocking with the pavement surface and to make the tire slide, those pieces of rubber must be physically sheared off ....
Just so people don't think I'm agreeing, this is actually an incorrect description of friction. I don't intend to argue about it or explain though.
But it does bring up something interesting about the mechanism of friction: that it works on the molecular level. Interlocking shapes of the molecules themselves as they are pushed into each other, plus potentially the effects of electrical charges. Of course on the much larger scale there are also peaks and valleys in the material itself (and the surface) and that also is part of it, but the interesting thing is that you're actually getting molecular interactions. The harder you push them together, the closer those molecules are and that increases the friction! A larger area spreads that force out, resulting in less force on any part of it, and it sums up the same. Which is why friction is independent of the contact area. Yes, even if you reduced it to the size of a pencil eraser, with the same material and same weight you get the same friction. It will tend to bounce off the surface though (= no friction) and wear out rather quickly
While we're at it, these laws of friction are not really scientific laws. We don't *know* the full physical basis for them, because they are derived entirely by empirical observation. We do know that they work for most (but not all!) materials. So any time we hear someone making absolute statements, contrary to the empirical laws, and claiming some sort of scientific reasoning for it, I tend to take that also with a grain of salt.