[Oblarg]

Originally Posted by Mark Sheridan

Well sounds like a butter fly drive set up would be perfect for 3.5" wheels. The extra .25" clearance over the pulley OD will help keep the pulley and belt safe and since the wheels are capture in a module, you could rise the frame a bit to give it a good gap over the the carpet.

You still don't want the belt the scrape the ground even if its not common. the 3" wheel has no margin for that. Do you need a smaller wheel for more gear reduction?

3.5'' wheels aren't compatible with the dead-axle Colson hubs from WCP. 4'' could work, but are bigger than I'd like (smaller module = less weight) and I'd have to change our planned gearing.

That said, 4'' might end up being the way to go. Our initial design had us making our own ~2.8'' wheels our of delrin and plaction tread with custom pulleys out of pulley stock, and while we're going to be building that initially I'm trying to move later iterations towards as many COTS parts as possible (I'd have designed for Colsons originally, but was not aware of the COTS hubs for them and thought we'd have to turn our own).

As it stands, it seems that with 4'' Colsons, everything except the side-plates of the modules (which we can make ourselves to high precision) will be COTS.