[meaubry]

Please keep in mind that what is written into fomulae is often NOT directly transferrable to robot drive systems in a way that can be accurately and completely calculated.

Yes, you can make general assumptions regarding friction and the effects of one material vs. another, etc. - but, some arguements relative to traction, best drive system configuration, best wheel type and material, treads vs wheels, and other drive system decisions - are best left to experimenting and lessons learned in the real world.

Therefore, overly concerning yourself about static friction will only address one element of the problem. Robots are usually not designed for static friction.

Something I learned a long time ago relative to contact area and friction. This applies more to mechanisms that are designed to slide, not grip.
If the opposing materials are too smooth (maximizing the contact area) they will react opposite of what you would expect and want them to do. They have more difficulty sliding over each other. Sliding is accomplished easier when the contact surface is a little rough, giving up a little contact surface is productive in some cases.

As to my own experiences in drive systems relative to this question, I would have to say that our robots with more contact area produced better traction against the carpet. When comparing the robots using wheel chair wheel (smooth) vs. treaded pnumatic tires - the treaded pnuematic tires won hands down. The differences could be attributed to a combination of both different material and more surface contact as the pnuematic tire actually increases in contact area as they are pushed against due to the forces subjected to.

Will you get to a point where increasing contact area no longer makes a significant difference? Yes and No - it all depends on what you are attempting to do with it. Andy explained it well in his response. But at the same time, would the Beatty Beast have been such an immovable object without all of those file cards??

Just my thoughts - good topic,
Mike Aubry