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update at the bottom

I ride a belt drive bike fitted with a Gates CDN front sprocket (on the crank), a CDN belt and a CDX rear sprocket. Around the 2200km/1400mi mark, the CDN belt failed and teeth started snapping off (see pictures below). Since I bought the bike used, I don't know the actual mileage of the previous belt - but it happened after I did 2200km. I then bought a new CDN belt and installed it, and it's just failed now in the same way after another ~1000km/600mi.

This is way below the lifetime I've seen mentioned online, and it's below the lifetime of a chain even. I know that you're supposed to be careful when handling these belts, and avoid twisting or crimping them, I've never done that. I used the Gates app to tension the belt, the recommendation for my type of bike was 35-50 hz - I set it to around 45.

I bought this bike for bad weather commuting, I mostly ride it in rain, but on paved roads. With the second belt, I also did around 300km/200mi around sandy areas and through some mud - but no extreme offroading. I ride fast and accelerate off lights quickly, but I'm no pro.

What could be the reason for these issues? Could it maybe be that the plastic teeth on the front sprocket are worn and damaging the belt teeth? (see picture below). It doesn't seem so though, they fit the belt well. The CDX sprocket seems very durable and the teeth have not been worn at all since I've installed it ~3500km/2200mi ago.

If the CDN belt simply cannot deal with how I use the bike, that's fine, I'll buy a CDX belt - but it would be good to know that it's not an issue with something else in the drivetrain that's ruining belts.

Images for details:

damage to the belt enter image description here

front sprocket enter image description here

rear sprocket enter image description here

rear tooth wear detail enter image description here

enter image description here

update: I've bought and installed a CDX belt, with the same tension as before (45 Hz). I've been riding it in the same conditions since purchase and so far, I've done 2000km/1200mi with no issues. I'll keep reporting on it here as I rack up the kilometers.

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    from the angle the barely-attached teeth are in i have the impression that the damage is caused by the rear sprocket. BTW, without any knowledge whatsoever about the Gates, or other belt drives, i guess cleaning the bike from time to time might still add to the lifetime of the moving parts.
    – Burki
    Nov 18, 2021 at 10:26
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    @Burki i think you are right. I think his tension is too low and the teeth are riding over the rear sprocket
    – Noise
    Nov 18, 2021 at 11:28

2 Answers 2

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I would say there are a few things to check.

Use Gate's visual guide to check the rear sprocket doesn't look worn.

The belt needs to be straight or it will fail early. It is worth using measurements and visual clues to check this. If it isn't a factory set up then this is a real possibility.

Also very important, the belt needs to be tight. Run it at the upper end of the recommended tension scale for a while when it's new as any slipping or riding up on the sprocket splines will damage it quite quickly.

The CDX belt is better than the CDN and is a worthwhile investment. It will work with your existing CentreTrack rings, no problem.

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    When you say it needs to be straight, do you mean the alignment of the front and rear sprockets? That should be a factory setup, this bike is sold as such, so this should be OK. As for the tension, I run it at 45hz, I've added details to the question.
    – user4520
    Nov 18, 2021 at 10:45
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    The damage you are showing is not normal @user4520 so one of these things must be wrong. Even if there is play in the wheel or bottom bracket bearings, it can affect front and rear sprocket alignment. If you are a powerful rider, the belt tension is too low and you should have the CDX belt. I already run my belt over 50Hz on my geared hub to prevent slipping and it doesn't complain about it. Belt still looks like new. The tension recommendations are a starting point: said in the gates book. To me your rear sprocket looks a little worn, it's hard to tell. I'll trust you that you say it's new.
    – Noise
    Nov 18, 2021 at 11:26
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    @user4520 correct, by wiggling things. 3500km is a short distance for these but your system is clearly not running optimally so do check the sprocket tooth shape against the gates picture. Your photo only makes out that this could be a possibility. Before the belt slips, it rides up on the teeth which is I think the cause of the problem you are having, if nothing else is wrong. You are unlikely to damage the hub as the forces involved are very small compared to pedalling forces and internal gear hubs are generally very strong.
    – Noise
    Nov 18, 2021 at 12:45
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    @user4520 the teeth don't look symmetrical any more so the sprocket is worn, and the wear is probably from the same setup issue that has ruined your belt. If it's a symmetrical (not dished) sprocket, you can flip it over and use the unworn side with the new belt. Yes, the adjustable bottom bracket could cause alignment problems but it should be quite easy to line it up properly with the edge of the frame so you'd know if you'd got it wrong. Eyeball alignment from behind the rear sprocket. Use a ruler or straightegde as a visual guide
    – Noise
    Nov 19, 2021 at 9:55
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    @user4520 the fingers act as spring stabilisers, the actual attachment/drive mechanism is the three nubs (just like an old Sturmey Archer), held in place with a spring clip. So it works fine either way round so long as the belt line is good.
    – Noise
    Nov 19, 2021 at 13:13
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through some mud

From the summary here it looks like the CDN belt is not designed to handle dirt (and even sand is under question) so it failed. Only CDX and CDX:EXP are approved for work in dirty conditions.

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