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I had a flat on the side of the road. So I took my wheel off to replace the tire with a temp to get me home. When I reinstalled the wheel, the derailleur "opened up" completely...what have I done/forgot to do? Can I fix it or is it done?

enter image description here

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    You're on a smaller gear at the rear end and your chain is tight. That should not happen, and it's potentially a serious problem, whatever is causing it. If you were to try shifting to a larger gear on the rear now, the chain would try to be even tighter, which won't end well. I don't see how it can be a problem with the derailleur - something else is pulling the chain tight - which doesn't really make any sense. Check the chain - where is it? Post pictures that show the entire chain - where it's sitting, what gear(s) it's on. – Andrew Henle Sep 12 '20 at 13:14
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    Does Acera have offset jockey wheels? To me, it looks like the guide pulley is missing. There’s also what looks like a bend midway on the derailleur cage, although that may be intentional as well. – MaplePanda Sep 12 '20 at 17:20
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    Try removing the wheel again and see if the derailleur can pivot around the mounting bolt. In the photo, that pivot seems to be rotated too far clockwise, which is why juhist suggests the possibility of a broken spring. I also think you may be missing the B-link; the derailleur in a more “normal” position would be awfully forwards. – MaplePanda Sep 12 '20 at 17:25
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    We can't see from the photo how the chain passes around the chainwheel(s). This chain can't possibly be long enough for this bike because it could not possibly allow the rear derailleur to move the chain up to any of the yet larger sprockets. If you were riding the bike, stopped, removed the rear wheel and replaced it, you must have tangled the chain up in the chainwheel, front derailleur or have it around the bottom bracket shell somehow. Compare the chain path of your bike now with another bike that doesn't have this issue. – Stuart Cox Sep 15 '20 at 23:56
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    Its been a couple of months, how have you got on with this ? Did you fix it? Please do post an answer showing what the cause was. – Criggie Jan 13 at 11:25
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enter image description here

Figure 1. The way Shimano imagined it.

It looks like you have the first link of the mechanism mounted 90° clockwise. Try again.

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  • Why would he have removed the derailer while changing a tire? – Daniel R Hicks Jan 29 at 21:18
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    That upper pivot is sprung to rotate the derailleur counterclockwise. Perhaps the spring got dislodged, allowing the derailleur to rotate to its current more clockwise position. – MaplePanda Feb 4 at 7:22
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Almost certainly a spring has popped loose in the derailer.

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Remove the chain.

When you put it back bring the chain above the gear.

Put the chain under the second gear then put the chain back together.

It should fix it, or the spring is broken, or the spring popped loose.

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  • The chain is properly routed. – Daniel R Hicks Feb 4 at 3:41
  • i see the chain not on the gear – ryan Feb 4 at 17:33
  • people take off the wheel to change the popped innertube faster – ryan Feb 4 at 17:39
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    Taking the wheel off would not change the routing of the chain through the derailer. – Daniel R Hicks Feb 4 at 18:51

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