Belts 9mm or 15mm?

So after four years of gear drives we are looking at maybe switching it up and moving to belts (assuming the game calls for standard 6 wheel drive.)

So in your experience what is the better choice 9 or 15mm belts?

Or maybe a better question are 9mm adequate for typical FRC loads?

Unlike 25 vs 35 chain the trade off isn’t weight but size.

There are a few threads that mention this choice but none that really answered the question.


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!

We have been using 15mm belts in our drivetrains recently, and everybody likes the belts over the chains. I’d really prefer to go to 9mm belts on whatever we build next though to save space. 987 has been using them for years and they seem to do just dandy, even with big wheels and small pulleys. Seems like if you get the belt tension correct they will survive easily on a 6wd setup with the center wheel direct driven by the transmission. Other configurations might be different.

Not all belts are equal, FWIW. If you’re looking at HTD profile (what VexPro and AndyMark sell) I would recommend 15mm for drivetrain. If you’re running GT2/GT3 belts and pulleys, 9mm could be sufficient.

I believe 971 ran 9mm GT2 in 2013 and 2014, but switched to 15mm in 2015.

9mm can definitely work, but 15mm gives you a lot more headroom.

What would 829 even be anymore? :confused:

“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.

4607 typically uses 9mm when we use belts. In 2016 we used them with no problems. However, there was no reduction; just strictly transfer of power (1:1 ratio). We purchased extra belts and even though we sheered a few drive shafts, our belts were never a problem. And we played in some pretty tough matches throughout the regular FRC season and in the offseason (looking at you 2052 and 5172).

Maybe this year if we have the room (we won’t) we will try out the 15mm belts and pulleys we bought so long ago…

Good luck!

We managed to break a 15mm belt by the end of the season, and I know we’re not the only ones. I would be very skeptical of 9mm belts in a FIRST robot drivetrain.

I wasn’t going off of the load ratings, but mostly personal experience. 9mm pulleys and belts have failed me at 18T and 24T before. 9mm / 24T can work, with active tensioning and very careful management, but this eliminates the big advantage of belts for me - lack of maintenance. Just go 15, and set it and forget it, is what I’d suggest.

I think your rule of thumb is pretty fair.

Anyone saying “X size belt is fine” without naming the pulley size they were using (and ideally the wheel size too, but I guess four inch is implicit for this) isn’t giving the full story.

We ran 15mm belts from AndyMark in our KOP drive train last year. We’ve had no issues after 40+ official and practice matches and at least 50 hours of driver practice and demos.

I am a little curious - why are you going away from chain?

We haven’t had a drive chain on a robot since 2012.

461 has used 15 mm HTD belts every year since 2013 except 2014 where we direct drove. All 6 inch wheels with 42t pulleys. We’ve never had a belt issue in the drive once.

D’oh! Completely missed the part about gear drives.

WARNING…a programmer / electrical person is attempting to pretend to talk about mechanical stuff…but this is our experience over the past 14 years.

We have done all of this…

25/35 chain
9/15mm belt
Never done gears like you.

So far, we like 9mm belt, but ONLY if its a Bando brand and specifically “Bando Synchro-Link HT belt”. DO NOT USE 9mm VexPro belts on the drive system. They break all the time, but the Bando Synchro-Link HT type 9mm, knock-on-wood, we have never broken a drive train belt with this brand and type. The major drawback with 9mm, is you better have a good way to tension the belt. It’s not as forgiving as a 15mm. Kinda reminds me of chain. 25 chain seems to want more tension than 35. 9mm and 15mm seemed to behave the same way with us, 9mm will start pulley skipping, like 25 chain does with loose tension.

Example Part Numbers:
900-5M-9 = list price sample $8.68 (weird price example…LOL)

Good resource:

We like the 9mm over 15mm because we can narrow the wheel wells up and have alot more space inside the robot generally is why we favor the 9mm pulley setup, because putting a double 15mm pulley on the center transmission output shaft on both side of your robot, eats up alot of real estate in the middle of your bot.

Of course 15mm is recommended by most you will probably find, like talking with mechanical super geeks in Indiana, no names mentioned… By design, 15mm was included in Rhino tracks, we did not have any breakage problems with this single 15mm Gates belt supplied but it did stretch over time, by the time we got to world champs, we had to change this belt out with a new one because the tension was so loose it was pulley skipping all the time and there was no way to adjust the belt tension in last years Rhino track setup.

I am not sure if the Bando brand 9mm stretched over time, because generally we used tension blocks on the drive system when we have used 9mm on the drive in the past and the pit crew always made sure the tension was correct on the drive belts, so in other words, if it was stretching, I would not have known because they keep the belts tight.

We like belts for sure when possible everywhere.

I have quite literally never seen one of these break (in fact, I’ve only ever seen a HTD belt from any source break once, in almost a decade of FRC). Are you sure you’re using them within specs?

One thing to watch out for is that belts can break if they are crimped (this was the cause of the one failure I have seen, and is the cause of most of the ones I’ve heard about).

The first time trying 9mm belts, VexPro broke all the time, replaced with Bando, same application, same tension, never broke again rest of season, “back in 2012”.

Never crimped the belts or altered them, straight off the shelf.

I would not recommend using 9mm belts on a drivetrain. We tried it last year to save space on our robot. It was the biggest flaw of our robot. Maybe it was because of missallignment or tensioning but we went through several sets of belts at one regional. Eventually we switched to chain and that was much better. Just save yourself the headache and use 15mm belts or chain.

We have been using 15mm belts for the last couple of years on the drivetrain, and they have been working great. As long as the alignment is correct, no belts should be broken. We have had to tension them a couple of times through the season, but nothing too worrying.

We tried using 9mm belts on a test drivetrain that we built. It weighed only around 50lbs, and after only 30 minutes of use, we ripped one of the belts. We replaced it and haven’t ripped one yet. I would go with the 15mm to be safe, but you have to take the extra width into account when designing the robot.

Can people that list belt sizes please list the pulley and wheel size? There is an direct correlation between pulley size and maximum torque, and maximum torque and wheel size, and maximum torque and force on the belt.

My 24T pulley worked great the entire season. It never failed once and I would recommend everyone use this pulley.