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New RD-R8050GS overshifting on big sprocket

JoachimvadsethJoachimvadseth Posts: 7
edited June 2019 in Workshop
Replaced my bent (I think, since I still had shifting problems after I replaced the mech hanger) RD-6870 Di2 with a brand spanking new RD-R8050GS (GS since I use 32T+ cassetts these days), but when fidling around with the limit screws for an hour or two the RD wont change into the largest sprocket. Limit screw set perfectly on both sides but when shifting to the big sprocket (32T) in shifts perfectly on to it and then clicks a couple of times and then shifts back down the cassette, not aligning with the 2. sprocket at all. Micro-adjustment is set to 16/16 clicks so that should be centered.

After some reading I understand that this is a safety mechanism to stop the RD from over-shifitng, wich is dosent do, but it think it does.

Ran the "hold button for 5-10 sec, RD shifts all up and down cassette"-thing (whats it called again, crash-something?) and limits where correct. Shifting to the 2. sprocket and back to the 32T, and the problem is still there.

No chain on the bike for now since I thought it was easier to set the limits first.

It dos not help to anti-clockwise turn the limit screw 1/4 or 1/2 a turn to fix it. Ive tried. Only thing that works is so that the RD overshifts the 32T with about 1-2mm.

Thanks for any tips and tricks in this dark hour of need.


  • cossyrushcossyrush Posts: 41
    I would say that the top limit screw needs to be wound out a little bit. The mech can’t shift up properly to the big gear, so to save the motor it shifts back down to gear 2.
  • As I already have mentioned the H limit screw is set perfecrly. If I unscrew it more the RD will hit the wheel when overshifting before setteling on the 32T.

    However, after some more googleing and studying the Di2 Compability Chart, its seems that my SM-EW67A junction A-box is not supported on these shifters, neither the battery.

    So,if I go with a new SM-EW90 junction A-box and Y-split cables for my shifters and a new internal BM-DN110 battery things might get working? I dont know what type the juncion B-box is, but that is just basicly a 'dumb box' to connect the wires, right?
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