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Going through hell trying to figure out what's causing my ghost shifting. Before anyone asks: I trued the hanger already.

I suspect the GX derailleur is made of chocolate and breaks if a rock or branch so much as mocks it from a distance, but I'm having difficulty figuring out if it's out if true or not. Almost every surface has a curve to it making it really hard to tell.

The symptom is I can shift low (large) gears just fine but it will over-shift for low gears and they'll ghost really, really badly. If I shorten the cable so the high (small) gears shift, I get the same problem for the low gears (it will shift further than it should, again).

Any suggestions would be appreciated.
 

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Double check the hanger, that can be tricky, also the bgap and the chain. If you knocked it hard enough you may have damaged the mounting bolt assembly (need b-bolt kit) though you likely would have noticed that when removing it to straighten the hanger.
 

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The other issue with GX (vs some of the other Eagle 12 speed versions) is that the bushing where the derailleur bolts into the hanger can develop a good bit of "slop" over time. Not easily fixable. Easy to test though - just wiggle it a bit, and if there's too much movement (and the bolt is correctly tightened), it might be new derailleur time. Also, as above, Eagle 12 speed is very sensitive to alignment (vertical/hanger and b-angle).
 

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Say what? What manufacturer specs that?
That's what the SRAM supplied derailleur setting gauge indicates. I've seen 3mm but that's way too close. There's barely enough space for the chain between the two teeth.
The 3mm refers to setting up the b gap without a gauge and the chain is in the 2nd largest sprocket (42). This set up is difficult to get right because the derailleur is under the 42 while you're measuring the gap between the tallest tooth on the 50/52t sprockets from the tallest tooth on the derailleur guide pulley and in the sag position.
 

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That's what the SRAM supplied derailleur setting gauge indicates. I've seen 3mm but that's way too close. There's barely enough space for the chain between the two teeth.
The 3mm refers to setting up the b gap without a gauge and the chain is in the 2nd largest sprocket (42). This set up is difficult to get right because the derailleur is under the 42 while you're measuring the gap between the tallest tooth on the 50/52t sprockets from the tallest tooth on the derailleur guide pulley and in the sag position.
We were talking about the hanger alignment, I thought that's what you were referring to too. The b-gap spec is actually about 15mm for sram eagle.
 

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IME, I get about 500-600 miles out of great shifting from a Sram Eagle 12 speed cassette, chain, and chain ring. After that it works “OK” up to about 1000-1200 miles.

As stated above, these RDs develop a wobble at the bushing where they clamp onto the hanger. Replace that part if it wiggles at all.

Try pulling the chain off and cleaning it. Lube it every few rides.

And finally, replace the cable AND housing if it’s been a while.
 
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