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Rear derailleur problem

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Rear derailleur problem

Old 05-18-19, 10:14 AM
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Slugless
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Rear derailleur problem

Hi, I've got a problem with a 105 rear derailleur that is kind of hard to google because I'm not sure what the right terms are or even how to describe the problem in a few words.

It's a bike that hasn't been used for a while, was lent to a young family member, and I think something happened while they had it and they might be too embarrassed to of told me.

The trouble is that the top pulley wheel isn't being pushed under the cassette. So the top pulley makes little or no contact with the chain because it is too far rearward. It seems like a working rear derailleur want's to compress in on itself and this one doesn't.

My guess is that the spring where the top pulley wheel joins the parallelogram does this job. Maybe somehow it's been forced too far out it's normal position and the spring gave way or lost where it anchors? Is the reason I'm not finding any info on this because it just works, isn't user serviceable and requires no setting/adjustment. So even confirmation that it's toast or being set right about how it actually works would be great.

Thanks in advance.
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Old 05-18-19, 10:23 AM
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Photos? From what you're describing, it sounds like something to do with the B screw, for non-user serviceability, not sure what you meant by that, as mechanical RD's are a very user serviceable part.

You leant it to someone and it came back broken, would take out the rear wheel, and re-install, then look/compare vs a known working one. realistically, from your description, taking it to an LBS to get it fixed would probably be the easy solution, they may be able to ID the issue just by looking at it (this is where photos come in if asking on a forum)
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Old 05-18-19, 11:03 AM
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The pulleys are typically held in place by springs. It is possible that the spring that controls the pulley has gotten loose. Take the derailleur off and look, and I bet you can figure out what has gone wrong. When you do this, if you do not see an obvious spring, try rotating the pulley cage one full turn and see if that does the trick.
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Old 05-18-19, 11:27 AM
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There is a spring inside the Der. where it bolts to the hanger that can become gummy and make it hard to pivot. You can try to flush it out with WD-40 or take apart and clean and grease it.
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Old 05-18-19, 11:53 AM
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Originally Posted by davidad View Post
There is a spring inside the Der. where it bolts to the hanger that can become gummy and make it hard to pivot. You can try to flush it out with WD-40 or take apart and clean and grease it.
^^This

If you take it apart, be warned, reassembly can be tricky as the clip that holds the spring in place has to be set while the spring is under tension.
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Old 05-18-19, 12:03 PM
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Is this in the case in most of the "usable" gear combinations?
Or is the bike in the smallest or largest ring/cog combination?
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Old 05-18-19, 01:27 PM
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Originally Posted by Aubergine View Post
The pulleys are typically held in place by springs. It is possible that the spring that controls the pulley has gotten loose. Take the derailleur off and look, and I bet you can figure out what has gone wrong. When you do this, if you do not see an obvious spring, try rotating the pulley cage one full turn and see if that does the trick.
Thanks very much. This put me on the right path. There is a pin that stops me rotating the cage but I can now see how it works. I need to get the spring in there positioned in a particular hole and rotate it with the cage pulled out enough to clear the stop pin. That description isn't very good but it's exactly as you said. It's quite frustrating and never quite makes it past the pin without slipping. But I've learnt lots and I'll figure something out.

Thanks again. And thanks for the other replies as well, much appreciated.
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Old 05-18-19, 11:32 PM
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Originally Posted by DOS View Post
^^This

If you take it apart, be warned, reassembly can be tricky as the clip that holds the spring in place has to be set while the spring is under tension.
I didn't say it would be easy.
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