Di2 wont stay in lowest gear.

chrisped

My Ultegra Di2 has recently developed a very annoying behaviour when i climb. When shifting to the lowest gear (11-28 10sp cassette) the chain will run cleanly, but after 5-10 seconds of pedalling the rear derailleur will shift back to the second largest wheel on the cassette. Sometimes it may even get more confused and shift back and forth over the 9th and 10th wheel on the cassette. I can see and hear the rear derailleur actually initiating the shift.

Obviously, this defeats the purpose of electronic shifting completely. Through microadjustments, I can trim it to stay in the lowest gear, but then I'm not able to get into the highest.

What is going on with my rear derailleur? Why does it have a mind of it's own? I know that the front derailleur does trimming on its own, but does the rear derailleur do something similar to make sure it is what it thinks is the right gear?

Thank you in advance, guys

- Christian

bbrap

Might be worth checking the limit screw has not moved preventing the mech from going as far as it thinks it should.

trailflow

Sounds like the low limit screw is in too much and the safety feature is kicking in and dropping to the next cog. Back it off a tiny nudge. It should be as close as possible to the stop but not touching. When the chain is on the biggest cog on the cassette.

The micro-adjustments are for centering the chain in the middle of the cassette to stop any clatter. You may need to also re-set the micro-adjustments to the centre of its range. Before doing the above.

bikes`n`guns

Yes, the rear does trim as well.

It prevents excess wear. It should only happen when on the big ring at the front.

964cup

Either limit screw or bent hanger. The low limit screw needs to be slightly unwound from touching the stop once you have the RD in the right position. Note that the high limit screw for your highest gear (smallest cog) should be unwound one full turn from touching. In both cases this is because the mech will shift slightly over centre to ensure the cog is engaged, then trim back. If you can't get clean shifting with limit screws set correctly and trim function not maxed out, you have a bent hanger.

kingstonian

I had a slightly bent rear hanger - once straightened, the Rear shifter did this. Ended up having my LBS take a look and they upgraded the software and re-calibrated it, and it has shifted like a dream ever since. The bloke charged me a fiver for his time.

meanredspider

Yup - I'm going with a bent hanger too. I had this issue after an Off - new hanger sorted it.

hsiaolc

Bent Hanger. Either bend it back or get a new one.

If you don't know how to do it take it to the shops and get it fixed.

ugo.santalucia

Sounds likw it's been hacked...