When talking about belts, it usually isn't enough just to talk about "9mm vs 15mm". To have a good idea if belts will or will not fail, we will have to know stuff like pulley toothcount, wheel diameter, wheel tread, drivetrain power, and tensioning method. Unlike chain, which can really handle almost anying FRC can throw at it, belts require care to select the correct power transmission components.
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Originally Posted by DonRotolo
I guess your question is, is a 9 mm Gates HTD belt strong enough to replace, say, #25 chain in an FRC drivetrain? My speculation is yes, but I have not done the math. You can also call Gates and ask them, they have a room full of engineers just waiting to talk about cool stuff like robots...
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It depends on who made the belt. Gates GT2 and GT3 belts are able to replace 25 chain in most circumstances at 9mm widths without any strength issues. Unfortunately, the 2014 run of "9mm" Vex Pro belts had some quality issues, and had a tendency to snap even when strength calcs showed them well within their limits. I'd either use 15mm VP belts or hold off on using VP 9mm belts until the issue is addressed. I suspect the problem will be fixed for next year, but can't be sure.
IMO, having done both belt and chain WCDs, belts aren't really worth it. The lead time and serviceability drawbacks outweigh the noise and efficiency gains in my mind. If you're going to try belts, the offseason is the right time, but all things being equal, I'd reccomend using #25 chain instead.