Traction Questions

[glennword]

So I've heard a lot of people talk about increasing traction/friction between their robot's wheels and the carpet by stacking extra wheels on top of each other or using wheels wider than 1". Both of these methods sum up to increasing the amount of surface area in contact with the carpet. This is confusing to me. IIRC, I learned in physics that the force of friction is dependent upon the coefficient of friction and the normal force, and surface area has nothing to do with it. Given, that equation only holds true for hard surfaces, not "squishy", yielding surfaces like carpet and roughtop tread. Round wheels on FRC carpet will "sink" into the carpet slightly. The only effect I see of adding more or wider wheels is to spread the weight of the robot out more evenly, and reduce/relieve the "pressure points" in the carpet. This might have a positive effect on wear characteristics, but that's beside the point. So, with that in mind, I pose these questions:

-Is there a difference between traction and friction?

-If so, what is the difference?

-Does adding wheels/width increase traction, friction, or both?

Thanks!