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A friend gave me his old Trek District 1 bike, which has a Gates Carbon belt drivetrain. The bike's about ten years old.

The equipment

  • an old-style (pre-centre-line) CDC drivechain
  • a splined (single-speed) free-hub

The problem

The drivetrain is extremely unreliable and annoying.

However much it's tightened, the rear sprocket (CDC 22T) eventually unwinds the ring that tightens it on the splined free-hub. The nubs on the sprocket that engage with the splines are worn, but whether this is the ultimate cause or effect of the loosening I can't say (the sprocket definitely needs to be replaced, anyway).

Below you can see the step-shape that all the wide nubs on the sprocket have been reduced to.

Chewed-up Gates CDC sprocket

No matter how carefully aligned, the belt squeaks and rubs on something or the other (this is why my friend lost patience with it, and I've spent a lot of time messing about with it too).

What parts should I replace to try to rescue this bike? Do I need to replace belt, sprocket and chainwheel, or can I start with just the first two?

And, which of the new versions should I choose?

The replacement parts are pretty expensive, so I'd like to avoid wasting a lot of money on a futile repair attempt.

  • Can you explain what you mean by the sprocket unwinding itself? How does the sprocket attach to the hub, there are several types - surefit, splines/bolts etc. A picture would likely help a lot. – Argenti Apparatus Jun 1 '19 at 19:25
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    Please add some photos of the rear cog and the bike's hub. I wonder if its some kind of backwards flip-flip hub, or is it geared with an IGH ? – Criggie Jun 2 '19 at 1:22
  • Can you include a picture of the sprocket interface on the hub? – Argenti Apparatus Jun 9 '19 at 16:32
  • @ArgentiApparatus It's just a standard hub, I slipped some Shimano sprockets over it to check and they match, so I'm rather inclined to give up the idea of a hugely expensive belt system and perhaps replace it with a chain drive. – Daniele Procida Jun 9 '19 at 16:35
  • FYI I'd ask about replacing the belt with a chain in this question and delete the other question. Also, provide details of hub and crank as no-one will know what components are on an 'old' Trek District. – Argenti Apparatus Jun 9 '19 at 16:48
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Hard to give a good definitive answer to this. It's impossible to predict of replacing one or both sprockets and the belt will solve the rubbling and squeaking problem, although it's possible that upgrading to the newer type Gates belt would help (presumably they have changed the design because it was better).

If the rear sprocket retaining ring comes loose, something is probably worn badly, probably by riding when the ring came loose the first time.

The retaining ring really needs to be tightened to the proper torque specification with a torque wrench, which is may be way more than you are estimating.

If the sprocket spines are damaged the hub splines may be damaged too. I'd evaluate the hub before deciding how to proceed.

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  • The newer belts and sprockets have a centre-track guiding system, unlike the old ones that relied on perfect alignement. The hub splines (steel) are fine, only the sprocket (aluminium, though perhaps it's actually butter) are chewed up. – Daniele Procida Jun 9 '19 at 16:32

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