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Hi: I recently bought an X-9 medium cage rear while building up a DH bike. I also bought an SRAM PC-99 chain to go with it, and the chain came pre-measured to length.

Using the body set-screw adjuster, I set up the derailleur according to the instructions, so that there was the correct space between the cage and largest cog. This was about half the length of the set screw, or about the middle of the adjustment range.

WIth the bike on my stand, I noticed that the derailleur cage was pulled virtually straight when on the largest cog. I didn't notice any excessive binding or friction so I thought it should be OK.

I took the bike out for it's shakedown ride, and after a few charges down hill, I noticed the derailleur had fallen forward so that it was touching the chainstay, and the set screw was now pushed all the way back. The set screw is surrounded by a plastic bushing inserted through the derailleur body. I reset the screw to it's original setting, but it slipped back again after a few shifts.

Questions: was the chain too short, putting excessive pressure on the derailleur body and stripping the set screw?

Is the bushing stripping because of the excessive force? Is it designed to do this as a protective measure or is there something wrong with it?

Now that it's stripped once, if I put in another link on the chain to reduce the pressure, will the bushing hold the set screw position, or is it trashed and needs to be replaced?

Has anyone experienced this?
 
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