Belts 9mm or 15mm?

Not this season, but, this combination has not served teams well in the past. Lots of teams tried it in 2014 with failures, and my old team had a similar setup in 2011 that had problems as well. 15mm is much much safer for 24T pulleys in a standard drivetrain, even if the 9mm thickness is more convenient for packaging.

Keep in mind that the pulley size alone does not tell you everything you need to know about your belt loading. The ratio of the pulley size to the wheel diameter is a much better value to make a judgment about.

Is there a “golden” ratio to go by or some bounds that teams typically set to prevent the belt from exploding when you least need it to?

I’ve posted this in several threads, but for a full-weight 6WD with direct-driven dropped center wheels, my rule of thumb for 5mm-pitch HTD is “8 pulley teeth per inch of wheel diameter.” Over that, 9mm are going to be fine. Under that, consider switching to 15mm. Obviously, YMMV depending on your specific setup - but I’ve never seen belts snap when following this rule, so I believe it to be pretty safe.

4 of the past 5 years we’ve used 9mm vexpro HTD belts with no issues what so ever. We do a 6 wheel west coast style drop center with the center wheel direct driven. We’ve done it with both 4" colsons and 6" omnis using 30 tooth pulleys.
We seldom have issues with our drive train once it’s tuned and debugged. Our other mechanisms on the other hand…

Did you swap for 15mm in 2016?

Technically yes, but really no, used rhino drive. Has a 15mm belt as part of the kit.

With a HTD belt? GT2 belt? GT3 belt?

You can run different belts on a HTD pulley and get most of the rated strength of the correct belt + pulley combo. The manual is pretty informative.

We ran a 24 tooth GT2 pulley, 9mm wide on our 2014 bot with a 3.5" wheel. The belts lasted about a season before they 1 by 1 failed. We were very careful to tension the belts correctly. If you are going to push the limits on the belt ratings (which that setup is), you should be carefully tensioning everything.

Our team ran 9mm last year for our drivetrain, and we broke like 4 belts. imo 15mm is best for drivetrain

Our experience has been there is a large variation in belt toughness from the various manufacturers. Gates is a name that can be trusted. If your going to live on the edge of spec, get good belts. I tested several 2l v belts for a CVT project. The Gates belts have held up all season and are still going. The others did not make it out of testing.

1073 ventured into using belts in the drivetrain last year, and we had no issues with the belts themselves, but shattered a few pulleys… That was mostly due to the nature of the game.

This year we decided to run 9mm belts with 24t pulleys and 3CIM gearboxes (Hindsight 20/20). It was great until we got into some pushing matches, and there were many belts that had stripped teeth. Thankfully it was only the belts between our two center wheels (we ran 8WD this year) so we changed our wheels and added another pulley which allowed us to run 15mm belts between our two center wheels. We haven’t had any stripping problems since then.

We ran 12mm AT5 belts on 44th lightened pulleys with great success. It was on a very high traction setup (6 CIM tank, 8 cut colsons). We never had any issues with slipping or breakage.

I actually work for BRECOflex, where we got these belts from these year so I can provide a little more insight into our choice. We chose to use the AT5 series over HTD belts because the tooth profile of the AT5 belts provides smoother engagement and greater tooth shear strength. Also because the belts are polyurethane with steel cables running through them, we are able to use thinner belts. While we probably could have gone with belts narrower than 12 mm we wanted to play it safe as this was our first year using belts. I invite everyone to check out our website to see what we have to offer. We have a wide variety of belts available, as well as the ability to make custom belts and pulleys to order. We also offer FRC teams a 40% discount so we hope this encourages teams to experiment in the offseason with our products. Feel free to reach out to me at with any questions or any belt/pulley application related questions. We typically supply large scale industrial applications, but are very interested in supporting FRC teams with quality belt products.

Please, refrain from posting stuff like this without additional info about your drive train (pulley and wheel size, for a start). This sort of unfounded generalization is, to put it simply, bad advice.

Someone reached out to me over PM and he ended up asking an application engineer at Gates. has some good info on mixing and matching belt and pulley tooth profiles. The result of the conversation was that we learned that mixing profiles with 5mm GT2 belts and HTD pulleys isn’t a good idea.

The application engineer actually recommended HTD for FRC applications because it is more tolerant of fixed center to center distances and reversing loads. 971 is going to stick with GT3 because we tension everything properly, but it was really interesting to get Gates’ perspective on the problem.

I learned something, which was awesome.

Please excuse my fellow team member, as he is new to chiefdelphi. More details: Last year was the first year we ran belt in our team’s history. In our first revision we had an 18t pulley with HTD 9mm belts from vex pro (along with 8" AM pneumatic wheels). We would pretty much break a belt every time we attempted to cross a defense. Let’s just say pairing the largest FRC COTS wheel with the smallest FRC COTS pulley was not the brightest idea. We later resolved the problem, but not until after our first district event.

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