A friend gave me his old Trek District 1 bike, which has a Gates Carbon belt drivetrain. The bike's about ten years old.
- an old-style (pre-centre-line) CDC drivechain
- a splined (single-speed) free-hub
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.
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.