Belt types

I understanded I am using mm and this wcp is bugged in mm so I should make it in inches

I did a little digging, and it does look like the WCP belt calculator is indeed bugged in mm. You can do your calculations in inches, or use one of many other belt calculators online. They should all be based on the same calculation, which is explained here. Careful observation reveals a very small approximation in determining the actual covered arc length of the pulley, but the error is tiny and results in a formula that is way more computation-friendly.

Not sure who to @ for this, but I have some other info about this calculator's code.

So, it looks like there are a couple issues. First, the tooth counts given in step 4 seem to be multiplied by 25.4 erroneously. Easiest fix might be to copy the system used with editableInputWithUnits to add a blacklist to unit conversions for those three textboxes. The current line for that reads if(!adjustUnit && formData.calc_unit == "mm" && typeof inputToUpdate.attr("readonly") != "undefined" && typeof obj == 'number'){.

The second issues occurs inside when computing desired_belt inside of calculations(formData,adjustUnit). calculate() reads the form exactly as the user entered, with the desired center in the selected units, and passes it on with no modification. The desired_belt line, however, assumes that the value is in inches and multiplies it by 25.4 (not inchToMM). I’m not sure what the best way to fix this issue is, as there doesn’t seem to be a clear organisation as to which values are in form units and which are in inches. The cleanest solution overall might just be to only swap the value of the pitch variable (and user distance inputs) when the selected unit is changed, and then do no other conversions.

I didn’t notice anything else while poking at this, but I was only playing around with some simple hacks. There may have been something going on with changing the units not changing all the fields, but that was probably related to the first issue above.

There are some slight differences between a lot of the online belt center-center calculators. It’s usually no more than a few thousandths of an inch, though.

I’ve written my own belt calculator that uses the SPD-SI formulas.


I think the person to ping is @R.C


Please email me at rc @ Thanks!

Sometimes designers will subtract an amount from the calculated c-c distance for a few reasons.

For example, to get less friction in the system, or to compensate for possible assembly tolerance issues when belting between different parts that are imperfectly connected.

I’ve seen values for this number between .01" and .05", but I think people are just arbitrarily making up numbers every time they do it.

If you’re confident you can get an exact distance when the mechanism is assembled, use a value of zero for maximum strength and minimum backlash.

We use 9mm wide 5mm pitch GT3s on the drive train with correct pulleys. We have found them a lot more reliable the the HTDs which we snapped or ratcheted regularly. We have also found the Gates brand a lot more reliable than the generic HTD belts. 9 mm is a little on the light side for drive trains. The 15 mm gives a lot more safety margin.

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Isn’t gt2 less suited to high loads due to the smaller pitch? Or are you assuming pitch size being equal (i.e. 3mm htd compared to gt2, as opposed to the 5mm commonly used in FRC)

You would only compare belts with the same pitch and width. GT2 and HTD or HTC or XL etc is belt and tooth shape then you have usually a -2 or -3, -5 -8 which is the pitch and then there is a width and belt construction material + reinforcement like steel, glass fiber , kevlar etc etc. and obviously you want to compare apples with apples. The reinforcement usually helps keep the belt from stretching as they mostly are rubber based in some form And then of course there is a ton of belts in US standard size and pitch like 1/2 in or 1/8 in etc


Ah okay – that makes a lot of sense, I was mostly just confused because in FRC 3mm GT2 and 5mm HTD are the norm so I thought you might be comparing the two. Thanks for clarifying!

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And also to make sure. Testing is done under the same standards as there are a lot of things that determine the end result. So for example a belt can be reinforced and you can make it thicker but the teeth cant be reinforced or made thicker unless you change the pitch. So the weak link are the teeth but then the bigger the pulleys the more teeth are engaged and if the pulleys are unequal in size then there are even more teeth engaged on the big pulley and less on the small soo you got to adjust test results for that. etc etc. Tests always are only meaningful if they are performed under exactly equal conditions

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Tbh, my biggest complaints about both HTD and GT2 3mm/5mm belts is that if you want them in a size that isn’t a Vex/AM standard, there’s basically no way to get them quickly. They don’t appear to be that common outside of FRC uses, and the vendors that do carry them tend to be quite slow with shipping.

Also, not sure if the belt’s we’ve used are out of spec, or if the WCP Belt Calculator is just wrong, but we’ve had an awful time with belts being way more loose then they aught to be, which has resulted in some rather hap-hazard last-minute tensioning solutions. Kinda defeats the purpose of using belts to try to reduce backlash in a system. :confused:

We get our HTD belts from VBeltGuys for super cheap and typically get them within a few days. They have pretty much every size we’ve ever wanted.


Strongly recommend B&B Manufacturing. They’re based out of Chicago so they’re pretty close to you and they have exceptional customer service. They have a large stock outside of what FRC vendors carry.

If you want to stock up on other sizes but can deal with a longer lead time V-Belt Guys seem to be the cheapest belts out there. It was pretty slow to get our order in 2020 but that’s likely because Spectrum posted about them and they got swamped with all the FRC teams ordering belts at once. We ordered this season and got belts pretty quick.

As for your tensioning problems, I strongly recommend the path length constraint method in Solidworks. WCP has a great video series on belts that I would strongly recommend including how to design in tensioners. The only time this method has failed me has been manufacturing tolerance issues and stack up. We’ve also seen 3d printed pulleys come out undersized which allowed a loose belt.

If you’re desperate Stock Drive Products is another option, but i don’t find their price / shipping times to be better than the others, they do have a good selection though.

WCP Belts Videos
Part 1
Part 2
Part 3

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Unfortunately I’m using Autodesk Inventor, and the belt design tool presets don’t support HTD/GT2 sizes. I will take a look at those WCP videos though.

Pretty sure inventor will do arc length dimensions. Not quite as good as path length, but you can do a formula for it. For a 2 pulley system your line length between the two pulleys can be defined by ((belt length) - (arc length of wrap on pulley 1) - (arc length of wrap on pulley 2)) / 2. This formula gets a bit tougher when you add idler pulleys but can be worked out using references to other dimensions and the same principle. In general, path length dimensions are one reason I’m a huge Solidworks fan.

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B&B was quite slow the last time I used them for GT2 3mm 72T (?) belts. YMMV. They’ve been quicker in the past but maybe the oddball size threw them off and they didn’t bother to list that on their site.
Better to design around 5mm HTD when possible, or standard size (multiple of 5 or 10 teeth) belts.

Royal Belting Supply is also a good source. I think most of the big suppliers rely on Gates to keep stock. That is why when one is out, they all are.

Question about If I find a 5m-09 belt (like this one) in the size I need that doesn’t seem to be labeled HTD (or anything else), can I assume it is HTD? Looks like it is to me based on looking at other belts, but not sure. Thanks!

5M is another variant of HTD-type belts. I had to look that up myself last week.

I find that Misumi often has a decent comparison of sizes if I’m not sure if two types are compatible…