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I'm trying to convert a bike to a Shimano GRX 1x setup using the following components: RD-RX812, CS-M8000 (11-42T), CN-HG701-11 (and FC-RX810-1, 42T). The problem I'm stuck at is when shifted to the easiest gear (i.e. the 42T), the topmost rear derailleur jockey wheel sits too close to the cassette, causing the chain to not run smoothly from the cassette through the rear derailleur (see picture).

enter image description here

Can you help figure out what I am doing wrong here? Is the chain too long? I have seen images of GRX setups where the lower jockey wheel is pulled much more forward when in the biggest chainring (example). At the same time I am a bit reluctant to cut my chain, as the current length is exactly what Shimano recommends (adding 4-6 links to the length when wrapped around the biggest rings but not through the derailleur). Any pointers are most appreciated.

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  • Did you adjust the derailer B screw at the back? I.e. not the limit screws.
    – Weiwen Ng
    May 30, 2020 at 10:17
  • Nope, I did not. As it turns out, that was exactly my mistake. Thanks!
    – nbenn
    May 30, 2020 at 11:33

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You need to adjust the derailleur B-screw. That adjusts the distance between the top jockey wheel and sprockets. Winding in the B-screw will move the jockey wheel away from the sprocket. The gap should be ~5mm.

See https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment. The last section deals with B-screw adjustment.

You are correct that you should not change the chain length.

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  • Thanks for the tipp, this solved my issue. Out of curiosity, what is up with the (to my eyes a bit extreme) chainline in the linked article? In my setup, the chain now runs smoothly, but the derailleur is pulled nowhere near as far forwards as in image #3. Is this an indication for too long of a chain? Or is the chain in their setup too short? Or else? Or does none of this matter?
    – nbenn
    May 30, 2020 at 11:31
  • @Chain length is very important. The more extreme derailleur extension is probably because they are running a double chainring, so the derailleur need to extend and retract more to deal with greater sprocket differences than on a 1x system.
    – Argenti Apparatus
    May 31, 2020 at 10:21

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