Why don't people use GT3 pulleys and belts?

Original post here

I’ve been doing some calcs. A 28T to 60T GT3 3mm pitch belt is mechanically capable of giving 46.5 lbs of forward thrust using a 90mm wheel. And that’s while staying in spec.

the 28T HTD pulley can handle 7.0 inlbs of torque. The GT3 pulley can handle 17.9 inlbs. These are all ballpark numbers but it’s a general trend, over twice the load capacity…

So that brings up my question. As the DIY community has so many people pushing the boundaries of what is possible and putting our components past their limits, why does nobody use GT3 belts?

That 46.5 lbs of thrust is the max while in spec… We obviously are exceeding the load capacity of our belts hence why people tend to shred them so often or have issues with skipping. So if we push GT3 to the limit we could surely get much better force out of it.

This may not apply to acceleration since we are usually motor limited, but i think it’s important for braking. If you can’t push your brakes to the max without skipping belt then you’re not at peak performance.

And in addition to better wear resistance and load rating, they also cite quieter operation.

EDIT: After further research, I have come to these conclusions.


Here are some direct comparisons followed by interpretations of the data.

General Lifetime:

Durability This is the general lifetime testing of the different belts. Each test was conducted with the same pitch, pulley sizes, speeds, and torque. It concluded upon belt replacement was necessary, or until enough data was collected (no need to go further, in the case of GT). The baseline for these tests was the HTD performance, set at 100%. These tests were conducted at speeds and pulley sizes that result in accelerated belt wear, which will be explained further down this post. GT 5M has nearly 190% the lifespan of HTD 5M.

Tooth Jump:

Tooth%20Jump These results are not particularly favorable for GT 5mm pitch, at least upon first glance. HTD and GT seem to be very similar in the 5mm pitch, however it is important to see that this test was only tested up to 2300 RPM, while we typically run our motors around 6000 rpm. This results in much worse tooth skip, as it is far beyond the specifications of the tooth profiles. This higher rpm with higher torques is where GT shines greatest, and as you can see GT and HTD profiles begin to diverge as installation tension decreases. We run our tension somewhere in the 2-3 lb range. Some with idlers as low as 0 or 1 lb of tension.

Operating Belt Tension:

Belt%20Tension Here is the exact maximum belt specification for the tension on the taut side. For explanation’s sake, I’m going to divide numbers by 2, so the belt width is equal to 12.7mm, which is close to our use case. HTD 5M is rated at 51 lbs of tension. GT is is rated at 80 lbs. Please note that this is independant of ratcheting or pulley size, this is only the maximum allowable tension on the belt.

Minimum Pulley Size Per RPM Unit: Pulley%20Size

This is fairly straightforward. A 22 tooth HTD 5M pulley is rated to operate at 1160 RPM. This is over 5 times less than the 6000 rpm we operate at. A 22 tooth GT 5M pulley is rated to operate at 2000 RPM. Again, we are far out of spec, but by much less. This is 3 times less than the 6000 rpm we operate at. Unfortunately, both will experience accelerated wear, but HTD will suffer much worse.

Exact RPM And Torque Ratings:

HTD%20torque GT%20torque

These graphs show the operating RPM and torque ratings. HTD 5M is rated to 75 in lbs of torque at 6000 RPM. GT 5M is rated to 250 in lbs of torque. This means that GT 5M can run at 3 times the torque of HTD 5M. While we are not typically exceeding these, it still means that GT will suffer much less wear over time. Please note that this data is unrelated to pulley size, that data will come next and will show further disparity between HTD and GT.

Exact Torque Ratings Per Pulley Tooth Count:

HTD%20exact%20torque%20and%20lifetime GT%20exact%20torque%20and%20lifetime

I’m going to use the math for 5000 RPM, as it is the closest common value to 6000 rpm between the two graphs. HTD 5M is rated to 11.6 in lbs of torque with an 18T pulley. GT 5M is rated to 25.2 in lbs of torque with the same rpm and pulley.

What is more interesting is the low RPM values. At 100 rpm, HTD is rated to 25.7 in lbs of torqe, while GT is acceptable up to 58.5 in lbs with an 18T pulley. Our intense startup torques far exceed HTD specifications.

However, HTD is already in the “accelerated wear” category all the way up to 28T pulleys at 5000 rpm. HTD suffers from excessive wear from an 18T pulley at any rpm of 1160 and up. GT is acceptable wear at 5000 rpm at any pulley size 18T and up. GT is functional from 200 rpm to 14000 rpm with an 18T pulley. It only suffers from excessive wear from 100 rpm and lower, which we are rarely in when riding. For a visualization, this is when the motor makes a full turn every .6 seconds, which is a very slow creep for us.

We will be exceeding the torque ratings of these, so GT will also suffer accelerated wear, but not nearly to the extent of HTD since it is already going to wear excessively at any torque.

Other Details, Unrelated to Performance:

Ignore this if all you care about is raw performance, only the above information will be relevant to you.

Sound The sound levels are not easily interpreted from this. Decibels are determined on a logarithmic scale, not linear. So 100 dBA is much more than 2x louder than 50 dBA. GT is significantly quieter than HTD.

Idler%20Size We use backside idlers. GT and HTD perform the same here, but the data shows that we are seriously exceeding the spec here lol. Reversed bend radius very negatively affects belt life (given that all other wear factors are already in spec. We use idlers so other factors like belt slippage don’t occur, which is objectively a greater concern for wear).

Totally random thing I found about belt movement that I thought was kinda neat: Interesting%20detail

I found this very interesting. In a tuned pulley setup where they are in line and not twisted of offset at all, the belt will always ride to one side. If you have a precision CNC’d motor mount setup that ensures your pulleys are properly lined up, then you will still suffer from belts moving sideways. It is dependant on the direction the fibers internal to the belt are twisted. S twist is the opposite of Z twist. So if you’re using two of the same belts, they will both move in the same direction, ie: one will move away from your truck and the other towards it. Just thought this was interesting to read about.

TLDR: All Gt profiles are far superior to HTD, particularly in lifespan, torque ratings, and rpm. If you want a quick interpretation of the data, here is a comparison of many profiles: In%20Conclusion


What are the numbers for HTD5? If anything you should compare HTD5 vs GT3.

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GT3 is the tooth profile, the 3 isn’t the pitch. GT3 comes in 5m as well.

Also, my mistake on the calcs. Those were all for 6mm wide belt, so doubling the numbers for 12mm wide would be accurate.

For 18T pulleys with 15mm wide belt, HTD is rated to 27.7 inlbs of torque at 1600 rpm, while a GT3 belt would be capable of 49.1 in lbs at the same RPM. These are pretty significant numbers… All for 15mm wide belt 5m pitch.

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I have had this same question for a while


GT3 belts seem to be 50% more expensive but that’s pretty insignificant considering it’s still less than 20 bucks and you crazies are strapping over 1K in electronics and hardware onto a board :rofl:


In your first sentence you said 3mm - so everything that comes after I read as HTD3. That’s why I said the numbers should be for 5mm as that’s what we use.


Either way, the numbers are pretty similar, but ofc there are variations along the rating curves.

What is that supposed to mean? If you cite numbers they need to be for the proper belt pitch and as basically everyone is using 5m pitch you should cite the 5m numbers.

Also the cost difference is not solely the belt but you need different pulleys as well. I am not aware of any seller that has either motor or wheel pulleys with GT3 tooth profile.

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The exact numbers don’t matter, it’s the ratio that’s important.

GT3 in all pitch lengths generally has 2x the load rating for the RPM’s we use.

And yes, that’s the whole point of my question. Why aren’t these being made? They’re objectively better.

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You have to imagine a noob reading your numbers and using them from now on when he talks about what his belt can deliver. If I had not said that those numbers are only for 3mm pitch he would be using wrong numbers. Also the numbers do matter: if a HTD belt is good enough there is no reason to go for a GT3 even if it’s twice as good.

And nobody is using them because as you said yourself: they are more expensive when you have to find the right pulleys as well. Also no idea about trademarks and patents and whatnot. Are sellers even allowed to make GT3 Profile pulleys themselves?

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You’re allowed to sell products that include the GT3 profile, but I’m not sure about selling individual pulleys.

Many companies sell pulleys that they say are “GT3 compatible” and name them something else, they just have slight variations but still maintain compatibility. The name is allowed to be used in marketing, aka you can market a product as GT3 compatible.

I imagine as soon as you have slight variations in your profile your numbers are all over the place and nobody can tell you if there would indeed be even a difference to HTD. Also I expect tooling for a square tooth profile like GT3 to be much more more expensive than the round HTD. So pulleys will be more expensive as well. It all adds up and since the HTD belts have shown to work well enough nobody bothers using GT3. With 3D printed pulleys this might not make a difference so go ahead and test them side by side for science.

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@TowerCrisis Do you have a link where we could find GT3 pulleys with 5 mm pitch? I looked on aliexpress but was not able to find any, I am just curious to see the cost. If I did not already have so many HTD pulleys and belts I would probably be tempted to try.

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IDK where you read that but GT3 is not a square profile. They are round toothed, deep groove belts made by Gates to be the successor to GT2, which was the successor to HTD.

I can’t manage to find any, but it’s quite possible to 3d print them.

GT2 and GT3 are both backwards compatible with HTD pulleys. So using an HTD wheel pulleys would be fine. It’s the motor pulley that you would really want to match to get peak performance.

Gates says that 5M GT2/3 is not recommended for HTD, citing that htd pulleys may not be rated for the additional loads of a GT2/3 belt. But for our use this really doesn’t matter, the wheel pulleys isn’t the bottleneck in this situation. So long as you use a GT2 pulley then you’ll get the benefits.

Also, an important distinction to make: GT2 is the same profile as GT3, the only difference being in the tensile strength of the belts. You won’t find any results for GT3 pulleys as there are none, they’re all named GT2 pulleys and are inclusive of GT3.

GT2 pulleys can be bought from B&B manufacturing, or sdp-si.


Screenshot_20181013-103901_Dropbox That’s why - HTD is much rounder (the 20mm at the bottom is HTD and is basically circular) while GT3 are “squarish”

I am also wondering if all your calculations are based on Gates marketing material. They will obviously say that their own brand is better than HTD and the information should be taken with a grain of salt. Are there independent comparisons out there?

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HTD and GT2 are both made by Gates. They don’t have marketing material, they have test results giving specific load ratings for these profiles that’s meant for people designing industrial machines.

And also gt2 not being perfectly round does not affect cost at all. Both HTD and GT2 pulley stock are hobbed using specialty cutters designed specifically for one profile. The difference in curvature is just a matter of having a slightly different grind on the cutting edge.

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Though wheel pulleys would be a lot harder to make / get (right form) unless 3d printed. I guess

Only 2 valid reasons to use htd instead that I know of: idler make gt3 somewhat less important, and availability is annoying

Let’s the community forward! If someone does a build with gt3, 3mm pitch and idlers to achieve aggressive gearing (4:1 or something) and vesc 6, then they’d be able to use higher Kv motors which are more powerful, and build a dual 5060 setup with as much power and torque as a dual 6374 setup.


I would love to see GT3’s hit the market,

If they could handle loads of torque then we would all be less interested in gear drives.

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