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Broken chain which has damaged chainstay

I'd like to improve reliability and have fewer problems with my drivetrain. What more can be done?

I have an electric bike. I only ever use the six smallest sprockets on the 12-speed cassette, even with the motor off/dead battery. The bike is regularly maintained by experienced mechanics at a bike shop. I clean and de-grease my chain, derailleur, and cassette regularly. I replace chains and cassettes at the recommended intervals with equal or better quality parts, and despite this I still occasionally have problems such as the above disaster photo from yesterday.

I have considered switching to a drivetrain with fewer speeds (costly as this means replacing numerous parts), maybe adding a chain catcher, anything else?

I've put 4000 miles on this bike since buying it new 15 months ago, and I aim to do at least 10000 miles before replacing it. I can't have the chain gouging the chainstay and/or getting stuck between the chainring and guard. What would you do, to make sure this never happens again?

(If this is not an equipment issue at all, and is a "you're doing it wrong" problem, I'm happy to receive constructive criticism...)

Comment updates - It's a mid-drive motor, no space for a larger chainring, and not working very hard at time of failure. The chain came off the crank & wedged between it and the guard (second time this has happened). It is a quick link in the photo, but I broke that trying to free the chain, so I could roll the bike back home. It was a KMC DLC 12 chain and I suspect it wasn't the original cause of failure.

Bike shop mechanic has now custom-made this chain guide and replaced the chain, cassette, and derailleur. Also removed more chain links to keep it tighter, with the understanding that I never use the largest 4 cassette sprockets which is fine with me.

Detail of custom-made chain guide above crank

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    Hub or mid drive motor? Was the motor working hard when the chain snapped, and were you? Is there room to fit a bigger chainring without it hitting the chainstay?
    – Chris H
    Commented Aug 4, 2023 at 14:57
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    In the picture I see a quick link at the end of the chain where it came apart. I have always felt that quick links are not as strong as using a chain tool to connect the ends of the chain. Is there an opportunity to make the chain longer by not using a quick link?
    – David D
    Commented Aug 4, 2023 at 15:47
  • @DavidD if this is 12 speed, that seems unlikely - higher sprocket counts are more likely to only be available with quick links, at least from the brands that perform the best
    – Chris H
    Commented Aug 4, 2023 at 17:38
  • Curious - what's the wattage of the motor ?
    – Criggie
    Commented Aug 5, 2023 at 12:01
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    @Criggie Bike manufacturer website says "Bosch Performance CX GEN4, 250W, 85Nm". No aftermarket mods and I was in touring mode on a gentle incline, so motor assist would have been well below maximum at time of failure. Gotta say Turbo mode is amazing but must be really hard on parts, so I use it sparingly. Commented Aug 5, 2023 at 14:39

2 Answers 2

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If the main problem is dropping the chain, a chain catcher or guide should help.

Your chain may also be too long. Or, as you're always in the smaller sprockets, just a bit slack. You should check the length; by putting it on the biggest sprocket it should have next to no slack - the derailleur should be unfolded and the chain pretty straight.

That fancy chainring guard might be part of the problem if the chain can jam between it and the chainring. Basic plastic guards weil just bend if you need to get the chain out of that gap.

If it's breaking the chain, that's probably a matter of high forces.

The mid drive means you and the motor are putting your force through the chain, and using the smaller sprockets probably isn't going to help (higher force, lower pedal and chain speed for the same road speed, compared to a big sprocket).

Last time I broke a chain (on a much more basic bike) it fell off at low effort as I changed gear, but I'm pretty certain that it actually broke at a standing start in a high gear after a sudden stop a couple of hundred metres earlier. The point is the failure can take a little while to work its way out.

Recommended intervals for component replacement are a bit of a worry. You might just be replacing things before you need to, if your riding conditions are benign. On the other hand, in adverse conditions things wear fast. Always running in small sprockets counts as adverse conditions. Drivetrain replacement should be based on wear rather than time or distance. So if you have a favourite one or two out of those six sprockets, it will wear very fast. If your chain skips on a worn sprocket, that can make it drop.

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    Perhaps it's worth getting a chain checker? I regularly use one. The drop in ones are easy to use, and they're inexpensive.
    – Weiwen Ng
    Commented Aug 4, 2023 at 19:30
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    @WeiwenNg it could well be. I've never used one, because a steel rule has always been good enough for me. But now I've got an 11 speed setup I want to check more often so might get one.
    – Chris H
    Commented Aug 4, 2023 at 19:35
  • Chain checker showed very minimal wear after 2k miles(!) so have replaced with another KMC because that's impressive. Bike shop custom made me a chain guide/catcher which I'll add a photo of to the question. Cassette, chain & derailleur replaced and so far so good, it feels pretty solid. Commented Aug 5, 2023 at 7:55
  • My (2x)11 speed has about 2000 miles and wear was minimal when I checked. I should check again soon, and get a spare ready. On 3x9 speed I get about 3000 in all weathers, with a cassette every other chain.
    – Chris H
    Commented Aug 5, 2023 at 8:01
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The reason that seems to me to be most plausible would be a disengaged clutch: heavy cassettes have inertia, and the clutch is not engaged, you can have a sudden release of tension in the upper part of the chain when you stop pedaling. To my experience, a disabled clutch can cause derailment when you stop pedaling (even on smooth tarmac). A plausible scenario based on the picture could be: the clutch is disengaged, you stop pedaling for a second, there's a derailment on the chainring, but when that happens, you resume pedaling. Instead of coming back to the ring, it got stuck between the ring and the guard, you keep pedaling and because the quick-link was by coincidence in the vicinity, it got open (that's the only explanation I've found for some derailments that some friends experienced on flat tarmac).

Other thoughts: the chain is indeed a high-end one, but given quick-links disengaged, a reason may have been that they were not engaged properly, or that the quick-link is "worn" (=if it disengage too easily - possible if you clean the chain very often, they are supposed to be "one use" - not everyone respects that recommendation though). As suggested, in other answers I would check if the length of the chain (if too short, it can be a problem but only in the larger sprockets).

Another thought (with the info provided, it may be hard to have an idea, but don't hesitate to develop in the comments), 12 speed is a bit tricky, and compatibility indications are often incomplete, as there are in fact different "formats". There are "3 teeth profiles" and the matching chains:

  • SRAM "Flattop": larger rollers
  • Shimano Hyperglide+, found on all 12 speed Shimano cassettes.
  • "basic one", evolution of the 11-speed (Hyperglide, without "+"), that you find in other cases: SRAM non flattop, Sunrace,...

In the case of flattop, it's easy, everything needs to be flattop. But in other case, you need to match cassette and chain: it's not uncommon to have a 12 speed non Shimano cassette (and a "non-+" Hyperglide chain) with a Shimano "Hyperglide+" derailleur. That being said, from what I've read, the issue of non matching chain is more "shifting performance" rather than "broken chains", but I don't have personal experience of such mismatch. But without knowing which cassette you have, it may be difficult to clarify that point.

If you only use the smallest sprockets, and consider having less gears, I would recommend to check out the Shimano 'Linkglide' range (yet another tooth profile...), meant to be more durable and developed for e-bikes. If your current cassette has an 11-teeth small sprocket, that would mean replacing chain, derailleur, shifter and cassette, it it's a 10-teeth one, it's more complicated. If you take the 10-speed Deore (RD-M5130), the extra cost should be reasonable (the derailleur+shifter+chain costs less than your current chain)

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  • Failure to properly close master link is a possibility - I think usually the chain should fail immediately, but still good catch
    – Weiwen Ng
    Commented Aug 4, 2023 at 20:00
  • @WeiwenNg I would also think that (unless it has been worn/weakened with multiple openings/closings), but I would classify this answer as "brainstorm" rather than "answer".
    – Rеnаud
    Commented Aug 4, 2023 at 20:02
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    Early on I had the clutch randomly disengage when a piece of vegetation caught on the lever, but I know to check now and that wasn't it. Next upgrade will be to Shimano Linkglide, shop recommended it but none in stock yet. Thanks for the comprehensive answer. Commented Aug 5, 2023 at 7:48

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