[sanddrag]

I'm not sure what kind of "upgrade" you are looking for but a lot of teams use McMaster Wedge-Top Incline Conveyor Belting for tread. Is is about the most grippy stuff you can get; you don't have to use a very wide wheel. Many teams put the skyway wheels in a lathe and turn them down just until the tread is flat, and then screw this stuff on. The one problem is that this stuff is so grippy that your robot will not turn unless you have 6 wheels with the center wheel lower or a different kind of wheel on the opposite two.

In 2003 we used the Skyway beadlock wheels with the standard foam-rubber tread that comes on them, turned down until they were flat/smooth. They look really slick, but grip really well. As long as your track width is more than your wheelbase, these turn okay.