Just ordering parts for my build…I notice that the drive pulleys are spec’d as 16-tooth parts and the idlers are 20 tooth. Are they compatible?
Yes, idlers are smooth.
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Thank you for replying!
The site also mentions 20t idler
I am just wondering if a 16t drive pulley is compatible with a 20t idler?
And why are they not both 16t or 20t?
Because. It might have a reason I don’t know, but it works. Don’t question it. 
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it doesn’t matter if the idler is toothed in this case, it contacts the other side of the belt.
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It is a good question. Idlers are used as belt guides, so size doesnt change movement ratio like the pulley size that goes on the motor will change how far the belt moves in one motor rotation… and it turns out tha you can use all smooth idlers and it will still work.
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Smaller idlers are going to make the belt bind though.
16 vs 20 tooth idlers will bind? I wouldnt have suspected that. If smaller than 16, i could imagine the timing belt notches to start to catch. Have you seen this?
The 20t idlers juuuust clear the sideplates. If you have smaller ones, they won’t clear it, will they? Or am I missing something here?
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Oh, you are talking belt alignment and the belt catching on the printed parts. I thought you meant there would be a problem rounding the tighter radius of the idler. I was thinking the belt wont care about the radius of curvature of the idler.
Sorry to confuse the issue. In that case, yes, the belt could be offset too much and catch the printed channel. It is best to use the designed parts.
The 16t drive pulley connects the motor shaft to the belt. The number of teeth here is super important because it balances the torque/speed. This is like the gear ratio on a bike or a car. 16t is ideal and a major design constraint.
The 20t idler is basically a bearing for the belt to ride on around a corner. These ones are smooth (no teeth) and the 20t defines the diameter. The plastic parts are designed for the 20t diameter bearing, which means a 16t bearing won’t push the belt out far enough to clear the plastic. IIRC, Ryan chose 20t because they are higher quality (lower roll resistance) than the cheap 16t bearings. Even if the bearing had teeth and meshed with the belt, it is an idler, and doesn’t affect any gear ratios at all.
So the combination of 16t drive pulley and 20t idler is intentional and correct.
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From memory…
The 16T motor pulley determimes the speed/torque, like Jeff says.
16T idler pulleys have smaller radius, and therefore smaller bearings. I recall that Ryan mentioned having trouble with the smaller bearings before and therefore designed around the more robust bearings present with the 20T idler sizes. He sometimes (incorrectly I believe) associates the smaller bearings with the narrower belt, but Im sure that it comes with the smaller radius of the 16T pulleys.
Steel core belt will break with the pulleys/idlers that we use, and woukd be worse with the smaller pulleys, but because of that, we don’t use those belts. Since the belt goes over the 16T motor pulley, I can’t see it being a problem with smaller idlers, but bearings that wear out faster js a definite problem. Also, the 20T pulleys are in general cheaper, being more common and taking advantage of economy of scale.
That’s my opinion on why we use the mismatch size.