Friction on Rubber surface Area

Theoretically surface area doesn't matter...but of course we all know that's bollocks not necessarilly true in all practical case.

I suspect with a material like rubber there are many complex variables.
I'd believe experiment over calculation in all but the simplest cases....
I'm willing to be wrong on this though.

Del

No. Surface area usually does matter in the case of rubber, but there are no first principles that apply universally - thus no equations. There are some "fudge" factors that people use to modify the simplified equation you used, but most people use graphs based on experimental data.

If area didn't matter, Formula 1 cars could run on bicycle tires. The tire, however, is not on a perfectly smooth surface, and is deformable. As a result, it shapes itself to the "grain" of the road and gets extra friction. The larger the contact patch, the more friction it gets.

Of course, compound and other factors are also involved, but this is a perfect illustration of why area sometimes matters.