# Rear Derailleur sheared off



## [email protected] (Apr 9, 2008)

My friend was riding about ~13 mph and had the rear derailleur shear off and lodge into her wheel. She skidded to a stop and managed to keep it upright. What do you think caused this? The LBS is considering reusing the chain and RD. I feel neither can be trusted again. Thanks for any insight or advice.


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## tihsepa (Nov 27, 2008)

If the LBS did something to cause this the frame is damaged.


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## wim (Feb 28, 2005)

*Some thoughts.*

One way for this to happen is that the freehub/cassette assembly becomes attached to the wheel (like a fixed cog on a track bike) for some reason. _The instant the rider stops pedaling_, the rotating (i.e., no longer freewheeling) sprocket pulls hard at the the lower run of the chain, wrapping the derailleur around the pivot bolt and shearing it off.

As to what causes a derailleur bike to become "fixed" is anyone's guess. Generally, it's something wedging itself into the space between the largest cog and the spokes/hub flange.

/w


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## looigi (Nov 24, 2010)

Likely the low limit screw was set improperly allowing the RD cage to catch in the spokes.


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## Kerry Irons (Feb 25, 2002)

*Trusting*



[email protected] said:


> My friend was riding about ~13 mph and had the rear derailleur shear off and lodge into her wheel. She skidded to a stop and managed to keep it upright. What do you think caused this? The LBS is considering reusing the chain and RD. I feel neither can be trusted again.


The derailleur and chain need close visual inspection. You can tell if either one is damaged. They could both be trash or they could both be fine but a good hard look will tell the tale. I would be surprised if this were a JRA story - something came loose or was improperly adjusted and she attempted a shift which caused the chain to wedge or the derailleur to catch in the spokes. If the spokes were invovled, you should be able to see scratches on some of them. I would want to be sure that the replaceable derailleur hanger was properly installed and that there was no other frame damage.


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## bigbill (Feb 15, 2005)

The rear derailleur is easy to check out. If the chain was wadded up and twisted, it might have a bent sideplate. Nine speed chains are cheap, no need to try out the old one. The downside is the cassette looks worn and a new chain might skip.


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## RUFUSPHOTO (Oct 14, 2010)

bigbill said:


> The rear derailleur is easy to check out. If the chain was wadded up and twisted, it might have a bent sideplate. Nine speed chains are cheap, no need to try out the old one. The downside is the cassette looks worn and a new chain might skip.


The cassette is far from worn.


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## Guest (Apr 27, 2011)

It doesn't look like any spokes are overly damaged when looking at the first picture.

It does look like the derailleur hanger broke off, not the derailleur itself.

So, maybe the skewer was tightened down too much, or the hanger bolts, or the hanger had a crack in it from being set down on the ground before the bike was built-up, or something like that...


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## bigbill (Feb 15, 2005)

RUFUSPHOTO said:


> The cassette is far from worn.


That is true for the 12-15. Look at the shark fins on the bigger cogs. It looks like the rider predominately uses the bigger cogs/lower gears. Big cog, big ring riding puts a lot of stress on the rear derailleur and hanger. Maybe they just spin in the small ring.


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## GearDaddy (Apr 1, 2004)

I've had this happen a few times on a mountain bike and a cyclocross bike. The reason each time was that something like a small stick got sucked up into the rear der cage, which stopped the chain from pulling through. Thus, when you continue to pedal forward the chain simply yanks the whole derailleur along with it until something snaps off.

If you have your rear der misaligned such that it is not shifting properly, then it is possible to cause the chain to come off of one of the jockey wheels in the rear der cage. A common way for this to happen is if you back pedal with the misaligned rear der. I've also seen this with really muddy conditions that cause the shifting to get out of whack. But, the effect is the same as if something gets caught in the derailleur, i.e. the chain will not pull through the rear der cage and instead it will just drag the rear der along until something gives.

It is possible that the limit screw setting on the inside is wrong and the rear der got caught in the spokes too.

The best case scenario is that just the dropout shears off. Usually, the rear der is just fine, and possibly the chain is undamaged too. It kind of looks like that is the case here.

I've had it happen where the dropout doesn't break, rather it bends a bit but it's the rear der's part that screws into the dropout that shears off. Usually, this has resulted in a pretty bent up rear der, chain, and dropout that all need replacing.


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## MShaw (Jun 7, 2003)

That dropout is supposed to break like that. Saves the expensive bits.

As a mechanic, its *usually* not our fault, but if you press the matter, you may get free labor out of it. Expect to pay $20-25 for that dropout.

AFA the parts being reused: If I inspected them and they were straight, I'd have no hesitation putting them back on my bike. 

Last time I did that was in about 89. Completely roached everything. THAT was an expensive lesson.

M


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## Keeping up with Junior (Feb 27, 2003)

*Obvious*

Well the problem is obvious in the first pic. It's painted right there on the chainstay. And to think, people complain about Asian carbon exploding.


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## durielk (Jan 8, 2011)

From the pictures the chain is wrapped around the behind the cassette down in the spokes, or the chain was broken. The RD has chain wrapped in a unique way that is not normal also. Pretty hard to tell. Looks like someone broke the chain and just wrapped it around everything to get it off the ground and on the car, as it doesn't even go around the crank rings. If that is the way the parts were after she stopped it would be important info, if someone moved everything to get it on the car, then hard to diagnose.


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## JustTooBig (Aug 11, 2005)

durielk said:


> From the pictures the chain is wrapped around the behind the cassette down in the spokes, or the chain was broken. The RD has chain wrapped in a unique way that is not normal also. Pretty hard to tell. Looks like someone broke the chain and just wrapped it around everything to get it off the ground and on the car, as it doesn't even go around the crank rings. If that is the way the parts were after she stopped it would be important info, if someone moved everything to get it on the car, then hard to diagnose.


the chain IS running through the cage of the FD, so clearly it's just been pulled off the chainrings, and would not need to be "broken" to be put back on. Equally clearly, a good loop of the chain is wedged between the cassette and spokes. Neither implies the chain has been taken apart. But that does nothing to answer the question: how/why did it occur? That'll probably never be known, so those questions aren't so important after all. Instead, what parts can be re-used / what has to be replaced?


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## fishman473 (Aug 2, 2004)

As other have said, this occurs regularly on MTB's. The hangers are designed to break before the derailleur is damaged. I've torn 5-6 derailleur hangers off of Al bikes before going back to steel. Bend and breaking hangers was so common for me over the 10 years I was riding my Al bikes that I would always carry a spare hanger with me. Never had to replace the derailleur, I think maybe once there was a bent link or two in the chain that needed swapping out (not even the whole chain). Just look it over before riding it again, be on the lookout for shifting abnormailities for the first few miles of use... it should be fine.


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## durielk (Jan 8, 2011)

The chain could have been broken where it is behind the cassette, you dont' know. Probably isn't, but I am not one to make assumptions based on limited knowledge. He asked the question on what has happened, so evidently he wanted to know. The fact that you think it is not so important, is not so important after all.


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