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broken dirty chain

Was pedalling up steep hill on gravel road. No gear change. Just seemed to go mid push. Was very steep and out of saddle....had to push the remainder of way! 🙈

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    It is a bit dirty – Swifty Jul 22 '18 at 19:19
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    Were you really Touring without a chain breaker? Consider it a lesson learnt and think about what other tools you might not be carrying that can prevent a minor mechanical turning into an epic. – mattnz Jul 22 '18 at 20:23
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    I would definitely advise carrying a multi-tool with a chain breaker and a couple spare quick links. I always carry this for mountain biking and I'm surprised at how often I use one or the other. Gave out two quick links on a recent trip to other riders who snapped chains. Typically I blame poor shifting under load, especially on 2x or 3x drivetrains. I've had the worst luck with KMC 10-speed chains and have switched to SRAM or Shimano (depending on the brand of cassette I'm running). – Benzo Jul 23 '18 at 0:55
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    @Benzo I've had the worst luck with KMC 10-speed chains and have switched to SRAM or Shimano FWIW I've never had a KMC fail, but I have had two Shimano failures over the years. – Andrew Henle Jul 23 '18 at 11:40
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    Lots of responses! Great. Didn't say we didn't have a chain breaker(!)... was expressing its my first break. Easy fix as we had the links. It was minus 2 degrees therefore too cold on a rainy mountain pass so thought best to stay warm and not fuck about in wind and rain. Defo too lubbed and dirty in hindsight. Lots of good advice and food for thought here for my inexperienced self. Thanks heaps! Chain wear gauage....now i like that! – Sam Parry Jul 23 '18 at 18:55
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Well, only really one proximate cause for this: chain was weakened through wear and fatigue (it looks a bit old), you applied enough force to break it.

Likely that there is no single event that broke or weakened the chain.

Chain failure happens. Think of it a a rite of passage.

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    Another cause is a poorly-executed joint, when installing the chain. – Daniel R Hicks Jul 22 '18 at 20:31
  • @DanielRHicks Installing cable? Do you mean the pin? – Argenti Apparatus Jul 22 '18 at 20:32
  • Oops -- yep, a poorly-executed joining of the chain ends. Though looking at that picture I notice that the pin is still in one side, unlikely where a link has been "broken" and rejoined. – Daniel R Hicks Jul 22 '18 at 20:36
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The chain could have been damaged earlier, during a poorly-executed shift. Or the chain and/or sprocket may have simply been worn out -- a worn chain/sprocket combo causes "chain suck" which can break the chain.

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Possible causes:

  • Worn chain: Check your chain wear regularly using a chain wear gauge. If the 1.0 side falls through it’s high time for a new chain. For me with 10-speed chains the 0.75 side usually falls through after about 2000km. chain wear gauge

  • Bad installation. You should never re-use a normal chain pin. Only use a “master link”/“missing link” or a special closing pin.

  • Previous damage. Maybe you previously had a bad shift or bent the chain when uninstalling the back wheel etc. Try to be careful, chains are strong in the direction of their links but weak when twisted or loaded sideways.

I’ve never had a broken chain despite bicycling tens of thousands of kilometers, often through mud and with luggage under high power. Therefore I wouldn’t carry around a chain tool despite what others have suggested.

  • "You should never re-use a normal chain pin." that's a bit of a strong statement. For 3/32" chains plenty of people reuse pins just fine. – whatsisname Jul 23 '18 at 20:30
  • 3/32" chains are normal derailleur chains, right? Bad idea to re-use a pin there, that’s probably the main reason why broken chains happen. I think pretty much all chains come with a master link or closing pin, so why would you even re-use a pin, unless in a pinch? I think the only way of safely re-using a pin is if you have a Rohloff Revolver chain tool which can actually close the pins. – Michael Jul 24 '18 at 8:09
  • Back in the fixie/single speed craze, plenty of people reused pins in their chains with no problems. And indeed, in a pinch can happen often, and a chain tool can be the difference between walking and riding – whatsisname Aug 4 '18 at 2:25
  • @whatsisname: Maybe that’s the reason why plenty of people also have/had chain failures? Maybe there is a causal relationship here … – Michael Aug 4 '18 at 6:09
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I personally haven't seen a metal failure in a chain plate.

Clean it up and have a look under a microscope at the plate to see what the fail edge looks like. (Your cellphone will take a photo through the microscope nicely)

It is hard for me to imagine a failure mode other than a manufacturing defect (crack) in the plate. The failure modes I am familiar with result in popping the plate off the pin. (e.g piece of gravel between chain and sprocket)

  • Failure in the plates can happen. I replaced a 10 speed ultegra chain a few years back due to this. I was doing a quick check over bike before a cycling trip and found one cracked plate - further inspection after changing it showed 2 more plates were also cracked. I attributed it to wear and tear rather than a manufacturing defect as the chain had done 2000mi by that point and i'm pretty sure I would have noticed them before if they had always been there. – Andy P Jul 23 '18 at 10:56
  • @AndyP I replaced a 10 speed ultegra chain a few years back due to this. That wouldn't have been a 6701 chain, would it? I seem to remember Shimano might have made a bad batch of those - I definitely had one fail on my, maybe 5 years ago. I had another similar chain failure on a 10-speed Shimano chain, but I'm not certain that was a 6701. – Andrew Henle Jul 23 '18 at 11:43
  • @AndyP: 2000 miles? That’s 3200km. I (65kg rider, but with relatively high power) have never had a chain last this long, even in the summer. Maybe it was completely worn? – Michael Jul 23 '18 at 12:09
  • @AndrewHenle it was indeed a 6701. However it was part of a twin pack and its twin gave no issues. – Andy P Jul 23 '18 at 12:46
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    @Michael At the time I was 58kg with ~220W FTP and very much a high cadence style, so chain gets a very easy life. – Andy P Jul 23 '18 at 12:46

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