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Rear Derailleur broke??

Old 01-09-15, 08:52 AM
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Rear Derailleur broke??

Rear Derailleur broke off when I shifted too low gear?? Specialized 29er hardrock SE, 7 speed 34-11 cassette, Shimano tourney rear derailleur. I converted the bike to electric with a Bafang 750w mid-drive kit which replaces the 42/34/24 chainring with a single 46 tooth sprocket. I did not change the chain. I have been riding it a while without issue (love it) but seldom use low gear. I was going very slow around a bunch of people at an outlook and when I shifted to low; the derailleur just snapped off. I'm thinking the chain length may be marginal with the larger front sprocket??? Would a 7 speed internal rear hub be a better option than a new derailleur?? What would be a good replacement??
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Old 01-09-15, 09:04 AM
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My guess is that your low limit screw was incorrectly adjusted and the downshift threw the derailleur into the spokes. Carefully check the spokes for damage that may mess up your day later on. Replace with similar derailleur and make sure limits are set correctly. But, yes, your chain may also be too short to make the 46 tooth chainring work properly with your low rear cog, and that might have snapped the derailleur as well. Switching to an IGH would be a much more expensive solution.
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Old 01-09-15, 09:14 AM
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The low limit screw does not back off by itself - it is very unlikely that an adjustment was the problem, as you did not change the rear. Either the larger chainwheel (most likely) or a previous fall that bent the derailleur or hanger inward are more likely. If you found it necessary to adjust the OUTER limit because you could no longer reach the small cog then the latter is likely the problem. In addition to replacing the derailleur both the derailleur hanger and the dropout need to be checked for damage or misalignment.
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Old 01-09-15, 09:29 AM
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Extreme chain suck can cause the rear der to break off. Andy.
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Old 01-09-15, 10:26 AM
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Based on my understanding of old Sturmey Archer 3 speeds, a 7 speed internal hub could be problematic with a motorized bicycle because of the torque applied under load.

Even if you ensure that you get a hub/wheel that will handle the torque and work with your bike, you may want to consider whether you really need it or not, since the expense is going to be a lot greater than the $30 or so that you would spend to replace the derailleur and chain.
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Old 01-09-15, 12:19 PM
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The rear derailleur may have broken for either of two likely causes, and some other unknown factor.

First of all, when you added 4 teeth to the largest chainring (from 42 to 46) that would have called (possibly) for a longer chain. If the chain was too short that could have briken the RD off, and possibly caused other damage as well. You can rule this out easily enough by looping the chain around the 34 and 46t spockets, and seeing if there's room to pinch one link together until both sides touch.

The other cause is the RD or chain snagging in the spokes. This is a low limit adjustment issue. CNY is half right when he says the limit adjustments don't change by themselves, because while they don't, they are subject to the position of the hanger. If the hanger gets bent in then all RD adjustments are moved inward with it, which is why a bent hanger may be the primary cause of this type of event. If the RD hit the spokes, you'd see evidence in the form of a bent or scratched spoke.

There are other minor possibilities, but those are almost impossible to reconstruct after the fact.

So you have an option, you can replace the RD an move on. Or you can go with the IGH which involves a larger outlay now, but might make sense. However, If your bike has vertical dropouts, moving to IGH isn't as straightforward because you'll also (most likely) need a chain tensioner.
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Old 01-09-15, 12:55 PM
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Half-right? ME? Surely you jest. I did address the possibility of the hanger bending, though. I felt it important to lead the OP away from the assumption that a simple adjustment was to blame.
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Old 01-09-15, 01:48 PM
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Thanks! Well I did adjust the derailleur because it wasn't shifting smoothly?? Just thought since everything was pretty new the cable/shifter was just settling in. The limits seemed to be fine?? Pretty sure it didn't get in the spokes. Only dropped it once, caught a pedal in some ice-plant. Didn't seem to hurt anything? The inner pressed steel chainwheel side plate on the derailleur is now bent pretty good? Kinda like the sprocket caught it?? Don't have the bike here to check the hanger or chain lenght...
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Old 01-09-15, 03:03 PM
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If we are taking a poll, I'll go with the too short chain. If the length was correct with your prior 42t ring, it's likely to be short with the 46.
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Old 01-09-15, 03:47 PM
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I see bent hangers causing this all the time. You lay your bike down on the drive side, or the bike gets whacked in a self closing door. You don't notice it, and you don't remember it, because it doesn't correlate with the breakage. You ride along unknowingly with a bent hanger. Trouble starts much later, when you shift to low gear. The bent hanger allows the derailleur to travel farther in than before. Bang.
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Old 01-09-15, 07:12 PM
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Thanks guys! Think it's going to shop for a new derailleur and tune... I just don't feel comfortable with this stuff???
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Old 01-31-15, 07:47 PM
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Well I always like to have an end to the story so; Took it in, nothing bent. Replaced derailleur (tourney for Acera) and chain. Chain got messed up in sprocket when derailleur broke with me peddling (and motor pulling). Had to use a chain and a half to get proper length. Just too short a chain after front sprocket went up 4 teeth. Works much better in all gears now. Very smooth.
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Old 01-31-15, 08:22 PM
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Originally Posted by 0ldhippie
Well I always like to have an end to the story so; Took it in, nothing bent. Replaced derailleur (tourney for Acera) and chain. Chain got messed up in sprocket when derailleur broke with me peddling (and motor pulling). Had to use a chain and a half to get proper length. Just too short a chain after front sprocket went up 4 teeth. Works much better in all gears now. Very smooth.
Thank you for the followup

This is a fair warning to everybody, that when you increase the size of either the largest rear sprocket, and/or the large chainring, you have to reconfirm that the chain is long enough. This is especially true if the chain was originally measured by the big/big+1" method, which yields the minimum length chain for that combination, leaving no room for anything bigger.
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