Ask a Mechanic | Mis-Shifting Rear Derailleur with New Rear Wheel

Question: I’ve gone through your video on Di2 rear derailleur adjustment, and the chain seems to settle nicely onto the various cogs with minimal noise. However, when I’m on the very largest cog, after about 3 seconds, it slips to 2nd gear all by itself. I started having these problems a couple weeks after I moved to some new wheels. Any idea what’s going wrong? From: Arelene

Answer: Your rear derailleur is autoshifting because the Di2 system overshifts slightly to make the shift faster and smother, especially at the extremes of the cassette. Then it settles down into the right position a couple of seconds later. That’s what you are experiencing.

The new wheelset is what caused the problems. The position of the cassette, left to right, is slightly different on each manufacturer’s hubs. So, every time you change wheelsets, you really should retune the rear derailleur, whether it is Di2 or mechanical. Sometimes you get lucky and the replacement wheel has the cassette in the same spot, but this is rarely the case. With your new rear wheel the cassette sits slightly further inboard than with the previous wheel. Just reset your limits and tune your indexing as shown in our video on Di2 rear derailleur tuning and Shimano mechanical rear derailleur tuning and you’ll be good to go.

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