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I'm using a Gates Belt drive CDX setup with a Shimano Alfine 8 hub on a SOMA Wolverine frame. Over about 6000 km, I've worn out 3 stainless steel rear sprockets; the last one lasted only about 600km. The small protrusions that are supposed to hold the sprocket fixed on the axle look like they have been completely filed away, and the sprocket just spins freely on the axle. Also, for two of the three broken sprockets, there was a small crack in one of the 'arms' that center the sprocket on the axle. The corresponding surfaces on the axles (I also exchanged the hub once along the way) don't seem to have any abnormal wear on them.

I realized now that the chainline on the bike isn't perfect (front is about 45mm, back is 43.7, where Gates say +/- 1mm is ok). The bike is mostly ridden in the city, not always in perfect conditions but not terribly muddy either.

Before I invest in another rear sprocket, any idea about what I can do to prevent this from happening again.

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    Can you improve the chainline with some shims ? a flattened out aluminium soft drink can is about 0.4mm thick and one of those would get you back in the acceptable range.
    – Criggie ♦
    Commented Feb 13, 2023 at 1:56

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It looks like either you didn't have your lock ring tight enough and the splines of this sprocket were able to chatter and wiggle themselves into non- existence, and /or your belt was too tight. If you were able to measure offset that is outside of the spec for your bike's set up, I can imagine that putting a tad of stress on the splines as well.

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  • I agree. The installation sounds like it's not right. It can be very hard to get it on correctly without the tool. I did about 5000 miles on mine before deciding I wanted a slightly lower gear. So I had to get a new sprocket and belt. I could not get the new sprocket on without the tool and in the end had to buy it. Now done another 5000+ miles on the new sprocket.
    – Ifor
    Commented Feb 13, 2023 at 13:29
  • Gates has an app that is great for setting the belt tension. Most of the gates equipped bikes I work on come in too tight. Even new bikes out of the box from the most reputable brands. I recommend the app, and NOT the Krikit finger tool to set the tension correctly.
    – bradly
    Commented Feb 13, 2023 at 18:45
  • Thanks @bradly - what exactly do you mean by "you didn't have your lock ring tight enough"? There seems to be a groove on the axle that the lock ring snaps into, but even with that there was a little bit of lateral play in the sprocket. Is that normal, or is there anything to fixate it better?
    – user1248490
    Commented Feb 13, 2023 at 21:05
  • I see that it is a sure fit. I hadn't noticed that before. I have personally never serviced a sure fit set up, but there is a special tool required to install it. There is a retainer on the hub that the three fingers have to snap over. I imagine the end result should be that the ring doesn't have any play. If the sprocket isn't firmly seated or can move under load, I can imagine that is why your sprockets were destroyed. There could possibly be damage to the sprocket carrier on the hub, and you may want to ask a local pro to do the installation and inspect the health of the mating parts.
    – bradly
    Commented Feb 14, 2023 at 16:12

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