Recommended Timing Pulley Size

Hey, so I’ve been CADing a test bed for my team, and we’re trying out rear-mounted gearboxes. I want to put on a belly pan (so we don’t have to constantly screw around with polycarb shields on the bottom), but our normal setup (36t 9mm pulleys) means that we have belts and pulleys dipping below the bottom of the frame, resulting in a really convoluted pan that seems worse than using polycarb shields. What is the minimum ‘safe’ size for pulleys on a drive frame with the gearbox in the back (9mm or 15mm)?

I highly recommend using 15mm belts and pulleys for a drivetrain application if you’re going to use a pulley size under 36t. 9mm belts are already somewhat at risk of breaking with 36t pulleys and going under that pulley size will just increase that risk significantly. Something like 24t 15mm pulleys will take up more drivetrain real estate but its much better to have that than having to replace broken belts in the middle of competition.

If you’re familiar with 254’s 2017 design binder, you might want to look into running your belts with the setup that they have (although they are running chain). That way, if one belt is compromised, the gearbox is still able to power the other set of wheels. It’ll take up a lot of space though as far as most transmissions go, but how comfortable you are with that is up to you.

Depends on your wheel size. With the reasonable FRC assumption of similar speeds and similar motor power, which means similar forces on the edge of the wheel, a larger wheel means a larger lever arm and more force on the belts.

I think SPang’s rule of thumb assumes 4" wheels, which is pretty standard but not ubiquitous.

Here is a post I made asking about this same thing. I was using 30T pulleys 15mm belts on 4" wheels. I had heard that 24T was a “safe” choice for this application, so that thought that 30 would be super safe. People seemed to think it would do well enough.

I am by no means an expert, but I have dabbled in timing belts for a few years now for various design and prototype applications. This experience is not based around drive trains, instead it is mostly low torque applications in 3d printing.

As Chak said, it depends on wheel size. This should be what you primarily design around; take into account belt strength, diameter of pulley, material of pulley, and number of teeth in mesh.

On diameter of pulley / number of teeth in mesh: A good rule of thumb is 6 teeth minimum in mesh with the belt per pulley. The diameter of the pulley, the tooth profile & pitch are all closely coupled, be careful if you intend to take one of these parameters to an extreme (i.e if you go with a small OD you will need a finer pitch (in order to keep an appropriate number of teeth in mesh, not to mention the availability of the part) and therefor have less “bite” on the belt due to tooth profile). You be best served by copying an existing design’s parameters.

THIS is an excellent calculator for timing pulleys

**In general design and layout of synchronous systems:

  • Pitch, pitch diameter, and pulley OD are intimately related
  • Bigger pitch belts (and gears) can handle larger loads - at the cost of smaller ODs
  • Rule of thumb: every pulley should have at least 6 belt teeth in mesh.

For 9mm belts, I’ve heard that 9 teeth per inch of wheel diameter is safe. For 15mm people get away with as small as 18t pulleys in some cases. Personally if I ever ran a belt drive I would use 15mm wide belts and 24mm pulleys for maximum safety. Drivetrain and wiring are two areas to never compromise in.

Here we go again. I swear, there’s more superstition with belt design than anything else in FRC. Everyone has an antidote about what they’ve heard works, etc. is the definitive source of information on if your belt will survive for your application. If you want to do this right, take the time to read the manual and look at the safety factors for your application. Gates also seems to have some pretty good application engineers who have been helpful when I’ve contacted them in the past.

I’m happy to answer questions if you have them. I’ve spent many hours pouring through that manual.

According to that manual on page 45, if I have 150in-lbs of torque at the wheel (approximate maximum torque for a single wheel of a dropped-center WCD with 4" wheels and a 1.0 static CoF) at 10 rpm, and am using 9mm HTD belts, I must use a 72 tooth pulley to have a FoS of 0.97.
If I use 15mm belts and the according 1.89 “width multiplier”, then a pulley with 44 teeth on it can work with a factor of safety 1.05.
The length correction factor I got was close to 1, so I left it at 1. Higher RPMs only reduce the rated load of the belt even further.

Clearly the experience in FRC differs greatly from what this manual recommends. Do you have any explanations for this massive discrepancy? Is there a way to account for it that I haven’t seen other than applying an arbitrary correction factor?

My understanding of the discrepancy is that the manufacturer ratings are geared towards someone building something to last “indefinitely”, whereas FRC usually only needs to last for somewhere under 100 hours, though that may vary. In general I would think that institutional knowledge and recommendations for belt design are the way to go.


The “8 teeth per inch wheel diameter” rule of thumb is what I have always followed, without ever seeing a failure resulting from it - however, this has only ever been with gearboxes direct-driving the center wheel. Obviously, the loads will be somewhat higher with the gearboxes driving the rear wheels.

We have decided on 30T pulleys with 15mm belts, for now.

That sounds about right. There’s a pretty easy explanation. HTD sucks if you are looking for high power density, which is the metric you are using here. It’s old technology. The only reason to maybe use it is low backlash or because you are trying to use technology old enough that it isn’t encumbered by patents and therefore licensing costs. Neither of those should be the case in a high performing drivetrain. We run GT3. That 44 tooth pulley, 15mm wide with GT3 is twice as strong. (~2.2 times). GT3 is a game changer compared to HTD, which was a game changer.

(If you want to be really scared, look at the old english timing belt specs. MXL/XL/etc. Imagine designing a drivetrain with one of them.)

The institutional knowledge will only be applicable to similar setups to the setup of team that used it. Tread style, wheel drop, CG height and layout, same impact loading, same belt tensioning, same driven wheel, belt setup, driver driving style etc. The only real way to know what’s going to work is to try it and break it if you want to go that route.

We’ve stripped out and broken a lot of belts over the years. My goal is to have a maintenance and failure free drivetrain. I’ve seen belts which outright yielded (indicative of over-tensioning), belts which lost teeth (indicative of under-tensioning), and belts which have both failures (under-specified for the application). I’ve seen a lot of belts.

If you want institutional knowledge, there are plenty of threads that you can search for with lots of antidotes in them. I think there are even spreadsheets floating around in them with all the data collected up. Having spent a fair amount of time looking through them, all I can say is that what some teams successfully run would get destroyed in our robots. My takeaway is that it’s not a good idea to trust what has worked for other teams, unless I believe that they have the same usage patterns as us.

We run a safety factor around 1, maybe slightly more (calculated using the force required to slip tires), and have finally stopped breaking belts. That’s with GT3 belts. Our only antidote here is that we are making the assumption that our tires will slip with a COF of ~1 - 1.2 ideally, and the mass will be equally distributed on the left and right sides. We are letting the infinite life rating of the belts take up any errors in our assumptions. Maybe there’s something to this whole safety factor thing after all. Do you really want to cut it that close in your drivetrain? I don’t. I want mine to work for years.

You are welcome to continue collecting antidotes. I think I’ve made my point (though I’ll be happy to discuss it if people want to).

HTD isn’t really ideal for our application. I’m not really sure why it became the “FRC Standard” - I’ve heard things ranging from patent issues (GT2-GT3 is still patented?) to HTD potentially being better with reversing loads? In any case, I’ve looked at these load ratings before and I remember 15mm / 27 tooth being right on the edge of acceptable for a 4 CIM drivetrain. Looking again, I’m not seeing the same numbers, though, which confuses me.

One thing to consider is that the Gates load ratings are very conservative - they are assuming a continuously loaded belt running for several thousand hours, not an intermittently used belt for tens of hours.

I wouldn’t do 9mm HTD in FRC without a fairly large pulley. I have done 15mm / HTD / 24T on 4" wheels with 4 and 6 CIM drives. I have done 15mm / HTD / 27T on 6" wheels with 4 CIM drives. Both setups have used Colsons, AndyMark HiGrips, or treaded traction wheels without problems. The “weaker” setup (6" wheels) even used only a 4WD, so all the load was on one belt. None of these hinted at a problem at any point. That’s really all I can say on the issue - what’s worked for me in the past, and what I’ll likely continue to do in the future.

If a fast vendor starts stocking COTS FRC compatible GT2 / GT3 stuff, I see no reason not to switch to that. The fast ship time of Vex components is the biggest thing driving the design choices I make at this point.

Can you point me to a reliable source for both the pulleys and belts? Thanks in advance.

SDP-SI or BBMan. I’ve ordered from SDP-SI and if it’s in stock it’s awesome. Out of stock GLHF. I’ve heard good things about bbman but most of the things I order for have gone through SDP-SI because they already have my CC info and I’m lazy.

That still doesn’t explain why teams run HTD belts with the “8 teeth/inch pulley” rules and not shear belts. Most teams run HTD belts in their drivetrains if they are running belts at all and get away with factors of safety well below 1, excepting the KOP chassis which has a FOS of just about 1.
I like GT2/GT3 but I don’t find the load ratings in the guide reliable if they’re not working for GT2, GT3, *and *HTD. That makes the fact that 971 uses GT3 successfully with the load ratings specified in the manual essentially another anecdote, that being the case. My best guess would be that teams are failing belts for reasons different from just shearing the belt.
I would run GT2/3, but why spend the extra money and risk not being able to replace it at competition when I can just run the readily-available HTD with some thumb rules? AFAIK the patent is why Vex and AM use HTD. They can source pulleys and belts from non-Gates distributors. I don’t know how WCP sells GT2 hardware unless it’s from Gates.

Hey guys,

I know the prevailing convention in FRC is to use HTD neoprene/fiberglass reinforced belts, but I would like to offer an alternative that is possibly better, and field tested by 1923 last season. This past year we used BRECOflex AT5 belts and pulleys on our drivetrain and found that we had absolutely zero issues. No broken belts, no skipped teeth, no noticeable wear. (Full Disclosure: I work for BRECOflex). We found that the belts were much stronger than the Gates HTD belts, the BRECOflex ones are steel reinforced polyurethane. Our exact setup was 12 mm wide and 44 tooth pulleys. (We could have safely used down to 15 tooth pulleys with no problem, but the students chose the larger ones for ease of modification). If anyone is interested you can call or email us and I would be more than happy to chat about if our belts are right for you. I know the standard belts are generally acceptable, but for anyone looking to push the boundaries that last little bit, it might be worth looking into.

We get a lot of belts and pulleys from Browse by Category - ROYAL SUPPLY . They are a Gates distributer. SDP-SI will have some sizes that aren’t available from Royal Supply, but SDP-SI can run out. Whenever we design with SDP-SI using a non-standard size, we immediately do a risk buy for the entire season. The standard sizes and part numbers are in the timing belt. Once you have the standard sizes out of the Gates manual, you can start searching around pretty reliably and find alternative vendors.