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Old 02-07-12, 05:09 PM   #1
big_rider
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Chains Breaking, Karate Monkeys, and Chain Line

Howdy all,

I have an update of sorts to the question I posted awhile back about chain issues with SRAM chains.

I was halfway through a ride yesterday, and snap, my chain broke while I was cranking hard, but not shifting. As stated in the other thread, the chain is breaking due to the side plates bending allowing the pin to pop free on one link. This got me wondering why this continues to happen on this bike, my Karate Monkey specifically. I have never had this happen on any other bike.

So, I measured my chain line from the middle ring of the triple to the middle of the seat tube. I knew it was a little too much, but it measured in at 55-56mm. I am pretty sure this is supposed to be somewhere between 47-50mm. Could this be the cause of my chains snapping? Is there just too much lateral pull on the chain, and eventually one of the outer plates fatigues enough that it just pulls over the pin?

I am thinking that must be the case, but I am not sure. I purposely have a longer bottom bracket on than is necessary to put the front derailer out further for tire clearance on the KM.
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Old 02-07-12, 06:10 PM   #2
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yes the chainline could be adding some extra stress. i find sram chains have less pin holding strength than a shimano. can feel the difference in the chain tool
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Old 02-07-12, 06:27 PM   #3
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I doubt the chainline alone is the issue. Odds are you're not riding the worst possible crossed combinations anyway.

Modern chains break in two steps which can be far apart, so the cause isn't apparent.

Step one is a plate being pushed outboard on the pin with enough force to overcome the rivet head's strength. The most common cause of this is shifting under load. Doing so places tremendous side stress on the plates as the chain makes a hard S-bend between the two sprockets.

Step two happens after the outer plate is hanging halfway off the pin. When there's enough tension the pin will slip off leaving that side supported. The good plate then bends under the load and the pin slips out of the inner link. Note that the second step can happen minutes, hours, days or even weeks after the damage that sets it up. The damaged link holds together until the load peaks.

As Reptilezs said, there's some difference in rivet head strength between some Shimano and some Sram chains. Sram reserves their best riveting strength for their top chains, whereas Shimano uses it throughout most (or all) of their chains.

But the key isn't buying a stronger chain, it's to avoid the kind of aggressive shifting that damages chains.
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Old 02-07-12, 08:09 PM   #4
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Quote:
Originally Posted by FBinNY View Post
Step one is a plate being pushed outboard on the pin with enough force to overcome the rivet head's strength. The most common cause of this is shifting under load. Doing so places tremendous side stress on the plates as the chain makes a hard S-bend between the two sprockets.

Step two happens after the outer plate is hanging halfway off the pin. When there's enough tension the pin will slip off leaving that side supported. The good plate then bends under the load and the pin slips out of the inner link. Note that the second step can happen minutes, hours, days or even weeks after the damage that sets it up. The damaged link holds together until the load peaks.
This explanation totally makes sense, but what bothers me is that this doesn't happen to me on any other bikes, and it hasn't happened on any one that I have owned before. There must be something specific about this setup that is making chains break. I am thinking about the scenario that FBinNY describes in step one, but instead of while I am shifting (because I am diligent about not pedaling hard while shifting), it is happening when I am in a less than ideal gear combination. Which I am usually careful about. On this bike, however, a gear combination that might be ok on another bike is exacerbated by the chainline issue, and the pedaling force in this gear combination stresses the chain like it would if I was pedaling hard while shifting.

What do you think? Is that a plausible explanation? I feel like I am playing Mythbusters with my own bike, but breaking these chains is getting frustrating and expensive!
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Old 02-07-12, 08:23 PM   #5
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It's possible that the side flex is doing the trick, but given that folks have been riding fully crossed for years without breaking chains, I don't think it's too likely. For your outboard chainrings to be the problem, you'd need to be spending lots of time on the inner end of the cassette while putting serious tension into the chain. When you're on the outer end of the cassette the chainrings actually make things better.

Also, I don't know how attuned you are to your chain, but you can hear and feel the effects of riding in poorly aligned combinations. if the situation were bad enough to spread the chain to the point of failure, you'd almost surely know.

As to why it's happening now but never before, look to the variables. The particular chain may be less bulletproof than the ones you used in the past. The cassette may be gated more aggressively than the ones you used in the past.Those are two, if you think hard you might come up with others.
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Old 02-07-12, 08:52 PM   #6
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Have you tried other brands of chain? Makes no sense to keep buying chains that don't work for you.

If you have a couple mm to spare, I'd probably get a shorter bottom bracket as well.
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Old 02-07-12, 09:42 PM   #7
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I suppose now I will try a different chain. I bought a bunch of SRAM chains a while back, but now I think I have broken them all.
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Old 02-07-12, 10:17 PM   #8
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Hmm guy identifies himself as Big _rider , and he keeps braking chains.. on a 29er.
could be less than a smooth pedaling style, too. but I note a statement to the contrary.

perhaps sorting out the centering the chainline Middle ring and the cluster center
is part of it ,

you might just be a hammer head,. just say What a Guy !

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Old 02-08-12, 10:00 AM   #9
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You know there is an issue with chainline, fix that and see if the problem goes away, it should since this is the only bike you have issues with. If your other bikes had the same issue I would blame it on riding/shifting style but it sounds like that is not the case.

Fixing the chainline could be as simple as removing a spacer from the BB or you may need a new BB depending on crankset type.
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