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For context this bike is fitted with a Continental CDS drive belt, but the belt broken and as far as I can tell Continental stopped producing these belts so I can't get a replacement and they're also not great since the original belt broke pretty fast.

Due to this I'm considering switching it over to a Gates belt system and would like to validate if this is feasible and what parts I'd need for that.

Based on the bike information, I believe the relevant parts for the belt system are:

  • the chainstay length is 462mm
  • front sprocket is 39T on a 130m 5 arm pedal
  • rear sprocket is 20T on a Shimano Alfine S700 8-Speed, RapidFire Plus
  • the original gear ratio is 1.95 and Shimano seems to recommend 2 - 2.25 for that hub

Using the Gates' drive calculator the following setup look like it would be closest to the 462 chainstay length: 55T front, 26T back, 125T belt => gear ratio of 2.115 and chainstay length of 461.96.

Do you think that setup would make sense for this bike? I'm assuming that I can just get the 5 bolt sprocket and fit it on the existing pedal or do you see any issue with that? And to confirm that I'm looking at the right rear sprocket, are the Surefit ones the correct ones to fit on Shimano Alfine hubs?

Also do you think it's worth switching to another belt system for this bike or should I consider a chain drive instead?

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Confusingly, they have a couple different calculators, and the one you linked to is the oversimplified one. It gives what appears to be a hypothetical center to center chainstay length needed for a given set of components, but it doesn't say anything about needed adjustment range, for being able to adjust out slack (and for getting enough slack to remove the wheel on a backwards-opening dropout). For your current purpose use the "Carbon Drive Calculator" spreadsheet linked to here.

A chainstay length number is not enough to know whether a particular combination of Gates parts will work. You need to know the minimum and maximum adjustment range. For an eccentric BB as on the bike in question, use a meterstick or tape measure, rig up a way of measuring accurately to the BB center and dropout center, and rotate the eccentric around to get a minimum and maximum chainstay measurement. You'll then have the data you need to know whether the spreadsheet output is a hit.

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The reason to be particular about this is that Gates parts are expensive and a hassle to order twice, and when looking at published specs it can be hard to trust what a singular chainstay length number is actually measuring on a bike with an eccentric (or sliders, or horizontal dropouts etc). The 462mm could be the midpoint in the adjustment range or it could be the maximum.

Whether the gearing chosen makes sense (probably does) or whether to switch to chain are personal choices.

The "Surefit 3-lobe" rear cogs are the correct ones for all Alfine hubs.

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