I'm confused on the whole traction thing. I've been shown the math on why surface area shouldn't influence traction, but here's why I disagree:

I built a kitbot with 4 andymark kit wheels, all powered from two BB gearboxes. It pushed a decent amount of stuff in our shop. Then I pulled the wheels off, and doubled up the andymarks. Now I had two wheels per shaft, effectively doubling the surface area on the ground. This bot pushed more, and could shove me across the ground when I was fully resisting. The other setup, with single wheels, could not shove me.

I have yet to take physics (that's next year for me), but I'm still basically familiar with the issue... Anyone care to hep me out here?