Thanks for all the advice. We will be running a 6 Cim drive train (maybe shifting if VEX gets them in stock) with 8WD and it would be 9mm HTD belts from VEX with their 42 tooth versa pulleys. All 4 wheels would be powered on either side. Also, when I said it makes the CAD pretty, all I meant was that it would make my life easier my not having to change all of my mounting plates

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What about having one 15mm belt on the inside part but then 2 9 mm belts on the outside? We might try that to save some space but still have the advantages of 15mm wide pulley on the 4 dropped drive wheels.