Ask a Mechanic: Rear Derailleur Troubleshooting

Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at 

Question: I have watched your YouTube video on how to adjust Di2 rear derailleurs and I am having a problem that I don’t understand. When I go through the setup process at home, everything works just fine, but out on the road, I am encountering a well-defined shifting problem as described below.

When I am in the small chainring climbing a grade, as I shift down to a lower gear ratio and I get to the 7th & 8th cogs on the cassette, the rear derailleur won’t settle on a gear and it floats back and forth between the two gears, I can move past those two gears in either direction and everything settles down. It does not do this when in the big front chainring.

Do you have any ideas, suggestions as to what is going on and how to resolve the shifting problem? I’m in South Carolina, installed the Ultegra Di2 myself, and my LBS knows less about Di2 than I do. From: Wayne

Answer: Your problem is not unique to Di2. Mechanical rear derailleurs exhibit the same shifting woes, and the fix is the same for both. The fact that the problem you are experiencing occurs in the small ring and not the big ring, leads me to believe that you have a b-tension adjustment issue, chain length issue, and/or a hanger alignment issue. When you are shifted into the big ring, the rotation of the pulley cage changes the distance between the upper pulley and the cassette, which is why you are only having the problem in the small ring.

First check your chain length. Getting this right will minimize the influence of your chainring selection on your rear shifting. Here is a video on how to do that.

Once your chain length is right, check your derailleur hanger for alignment. Make sure the hanger fixing screws are tight also. A misaligned or loose hanger will change where the upper pulley lines up with the selected cog depending on which chainring you’ve selected. Lastly, play around with b-tension to see if this will improve consistency on your shifting. There is a sweet spot in b-tension that results in good shifting in both rings.

Here are a few more tips that may be causing your problem. Inspect the cassette for uneven wear. Make sure the cassette teeth aren’t damaged or bent.

Make sure that the whole cassette is made to work together. If the cassette is pieced together from two or more cassettes, different series and production runs have different shift aids that can conflict with each other. And of course, make sure your cassette lockring is tight.