[Cory]

for purchased wheels with lots of traction, I like the skyway pneumatic knobbies. I believe they're 6.75" in diameter and they have a tread that's about 2" wide. They give tons of grip, but watchout for not being able to turn due to gearing.

You can make your own fairly easily by taking the KOP wheels, mounting them up on a lathe, and turning the thread down until it's flat. You can then glue/bolt/otherwise attach your favorite traction material to it.