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I think there's two types of chain guide mounts, one of them is screwing in screws and another one is using the bottom bracket to hold it in place. Why not just use one type of mount? Is there a reason why they do this? Cuz I'm using the bb one and I don't see any problems. I want to add a chain guide that has a wheel at the bottom of the chainring to hold the chain but all of them are just exclusive for screw in ones.

And also, what's the name for both of them? this is the screw in one

and this is the bb mounted one

  • The ones with holes in the frame are implementations of ISCG03 or ISCG05 (International Standard Chain Guide) standards. For BB-mounted ones, I do not know any name. I guess they are inspired by now rarely seen E-type front derailleurs. And interestingly enough, there are adapters to add ISCG tabs to a bike without them. – Grigory Rechistov Dec 10 '19 at 13:26
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Not all frames have threaded bottom brackets with external cups that can be used to mount a chainguide. Frames with press-fit bottom bracket bearings need to provide threaded holes for bolting on a guide.

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Among reasons given in other answers I can add the following.

  • The ISCG holes allow to install both upper chain guide (above the ring) combined with a bash guard (below the ring) or lower chain guide.

  • Properly installing the bottom bracket-mounted chain guide is hard — it tends to rotate when one tightens the right cup.

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