[Jared Russell]

To make things more complicated, things like wheel width and diameter have been shown to make a difference in the actual effective "coefficient of friction" you will see when running on carpet (proof that the Coulomb model of friction isn't a great model for tread-on-carpet interactions).

Another consideration is tread wear. Brand new roughtop treads on small diameter, wide wheels really dig into the carpet, but after a couple of competitions, you will probably want to change your tread.

You can buy a selection of tread materials from McMaster-Carr if you do a search for "incline conveyor belting". I would recommend getting a few samples of various materials and you can trade off between traction on carpet, traction on any other field surfaces, and wear. Note that McMaster only sells this belting in 3" width (minimum), so you will need to use a vertical band saw (or similar) to cut them down to wheel size.