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I have a RD 7900 derailleur and a standard 105 28t casette.

After replacing my wheelset I observed after I put my bike on the workstand that the chain is not sliding in perfectly into the casette cogs in the lowest (28t) gear, all other gears are fine.

It makes some sort of grinding noies because, spoiler alert, the casette teeth are grinding on the chain rings and "forcing them" into place.

The derailleur is indexed and high low screws are properly set. I also dont think the derailleurs bent but I cant make that judgement with my bare eye.

How do I adjust my derailleur so that the teeth on the casette will fit perfectly in the chain ?

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By definition, if the chain is not running on all cassette sprockets properly, the indexing and/or limits are not properly adjusted. Low limit setting being off would be my guess.

Run through the full indexing adjustment procedure. Park Tool Company has the best instruction video.

You can check the derailleur hanger by lifting up the rear wheel and looking down a line from the derailleur cage to the chainrings. The cage should look parallel to the rings.

  • I've followed the video and I'm still having the issue, what else can I try ? – zython Apr 14 '18 at 21:36
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Usually shifting to the largest sprocket should be trouble-free. You just pull cable in until you hit the low screw.

Make sure that the low screw is properly set (i.e. far enough out), there is proper tension on the cable and the B-screw is far enough in. The derailleur’s pulley wheel should not be in contact with the sprocket, there should always be a few millimeters of space between it and the sprocket.

If all of that doesn’t help: Maybe you’ve made a mistake when installing the wheelset? A nut or spacer too much on the drive side? Is it a 11 speed wheelset? If so, have you installed the spacers for the 10 speed cassette correctly? You should need a 1.85mm and a 1mm spacer as described here.

  • tried with additional spacer from my old wheelset (so 1.85+1mm) and still having this issue – zython Apr 14 '18 at 21:37
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    So just to be clear: The derailleur is able to move far enough towards the wheel/spokes and you are actually able to shift to the largest sprocket? Is there also sufficient space between pulley wheel and sprocket? How’s the chain angle? Where does the grinding noise come from? Maybe the front derailleur is not allowing enough chain angle? – Michael Apr 14 '18 at 21:42
  • its not a cross chain issue, I have this issue only in the small ring (front) and largest cog in the rear, the noise comes from the casette teeth grinding on the chain links because the casette is not "fed" the chain properly, I have tried turning the b screw a few turns clockwise as to move it (the derailleur) away from the casette but literally nothing changed – zython Apr 14 '18 at 21:45
  • maybe I was too quick with my b tension statement, I turned that thing as far in as I trusted it, and lo and behold the noise is gone, if you have other recommendations please post them – zython Apr 14 '18 at 21:50
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    I’m surprised myself but apparently with 28 teeth you are at the maximum of the RD 7900 derailleur. It could well be that you need the b screw all the way in. I’m using an RD 5701 derailleur at its limit of 30 teeth and it is a tight fit which only gets worse with age and wear. – Michael Apr 15 '18 at 6:48

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