Quote:
Originally Posted by Chris is me
15mm is what I would feel safest with in a drivetrain, particularly with the smaller pulleys typically used in west coast drives. 9mm is very marginal for most standard drivetrains using 24T or 27T pulleys, and can definitely fail. Once you go above 30T (for example, with a dead axle setup bolted to the wheels directly), you can probably get away with 9mm. It's a little extra space, but the result is a zero maintenance, highly reliable drivetrain. Go for it!
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"Marginal" is the crucial term here. 9mm belts can be fine, or not, depending on your drive. 15mm are basically always safe, however, unless you're doing something really weird (exceptionally big wheels w/ exceptionally small pulleys). That said, there's certainly nothing
wrong with using 9mm belts - you just have to be careful.
Also keep in mind that the stated belt specs from Gates are not necessarily accurate - those are calculated for lifetimes waaaaaaaay beyond what we expect in FRC, and we actually have a fair bit more headroom than they indicate.
If I were to give a rule of thumb for 9mm HTD belts based on my personal experience, for standard FRC drive parameters (traction-limited with a wheel COF around 1), I'd feel comfortable with 9mm belts up to, say, 42t pulleys with 5'' wheels (that yields a tooth-per-wheel-diameter ratio of ~8 teeth/in, meaning for a 3'' wheel you'd want no smaller than a 24t pulley, etc). Keep in mind that the pulley diameter/wheel diameter ratio is indeed the operative quantity here, since that determines the force on the belt.