I can Speak to a 9mm Belt in Tube Drive, Having just completed a Offseason project with the Team that used exactly that.
We ran a WCP 2 CIM DS Gearbox, Speeds of 16.5 ft/s and 5.5 ft/s, in a 6 wheel drive with a single Belt from the Center to the Front Wheel. We used 24T 9mm HTD Pulleys from VexPro, and they have worked perfectly for our needs. We milled out a bearing hole, and used exact CC plus 5 thousandths to get the right tension.
More Pictures and Information can be found in this thread.
http://www.chiefdelphi.com/forums/sh...d.php?t=139599
I can say without a doubt that this method is highly effective. Thus far, we have run into no problems, other than a few hiccups in assembly. In multiple pushing matches with our 2014 bot (A very pushy robot), we stalled both bots, and did not have any breaking or ratcheting on the belts.
We would make changes if we were to do this in a competition, such as a method to access the belts without disassembling the frame, and maybe a tensioning method, as we have not run the robot for long enough to accurately judge long term wear on the belt.
I am more than happy to answer any other questions that people have.