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I have a [cheap and old] Falcon Indexing System (rear derailleur). There seems to be not enough tension on the B-knuckle pivot point so the cage runs into the large cogs. There is no B-knuckle adjustment.

Questions: Can that spring go bad with time? Is there another way to adjust the tension I might be missing? Can I just replace the spring or am I better off replacing the whole derailleur?

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  • You'd have to "borrow" the spring from an identical Falcon unit. May 27, 2020 at 14:02
  • The question can the spring go bad with time might also be a fit for the physics or engineering SE sites. I can say that if derailer tension springs can go bad, chances are that people would replace the bicycle well before then, or sometimes just replace the derailer. Thanks to manufacturer OEM pricing power, new bikes are often cheap enough that people just buy new. Spare parts for cheaper items probably aren't available from stores, so someone inclined to keep the bicycle would probably just replace the derailer.
    – Weiwen Ng
    May 27, 2020 at 14:17

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It takes a lot for springs to weaken from normal use but not break. There are applications where that's more of a thing, but little if anything on bikes unless maybe it's of particularly bad quality.

Usually if a derailleur spring has this happen it's because the bent nubs of the spring that engage the other part get bent or broken, or the holes they poke into get distorted, or slop at the joint allows them to slip out of the holes, or the spring has become distorted and weakened by being forced too much.

Some derailleurs allow disassembly of the knuckle but yours almost certainly doesn't. Just replace it. New similar derailleurs are extremely cheap.

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