GRX RX825 Di2 hesitates shifting from 9th to 10th sprocket

Tech Tech Help
1

Hi everyone!

I’m hoping to tap into the collective wisdom here.

I have a Rose Backroad FF with a GRX RX825 rear derailleur, a Classified hub, and an 11–34 cassette.

I’m having an issue where shifting from the 9th to the 10th sprocket causes the chain to hesitate and skip for a couple of seconds before it finally shifts. It always happens on that shift (9th → 10th) and nowhere else. Every other gear shifts perfectly.

So far I’ve tried:

  • Removed the cassette, cleaned it thoroughly, and reinstalled it with the lockring torqued to 40 Nm.

  • Checked the derailleur hanger bolts – all tightened to the correct torque.

  • Fine-tuned the indexing using Shimano’s procedure in the E-Tube app (adjust in the 5th sprocket, back off until it rubs against the 4th, then add five clicks).

  • Checked the B-gap.

  • Checked the high and low limit screws.

The bike hasn’t been crashed or taken any impact.

Any ideas on what else I should check? Has anyone experienced something similar?

Thanks in advance!

2

Hmmm… ? chain length adequate? Not too short so that last shift under excessive chain tension?

3

Nope, the chain length is correct according to Shimano’s manual (big chainring to big sprocket, plus the specified extra links).

4

Can you check the spacing between the cassette cogs? The spacer could be a smidge too narrrow.

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5

The cassette is a one-piece unit, so there are no spacers.

It’s really strange that only the 9th cog is causing problems. Everything is fairly new, factory-installed, and the bike hasn’t been crashed. I removed the cassette to inspect it, but I couldn’t find anything obviously wrong.

6

Sounds like a hanger alignment issue. Get it checked out/straightened.

7

Cassette wear? Do you have another cassette you could try to see if this is specific to the one cassette?

8

Ah, thanks! I haven’t checked that. It could be the derailleur hanger! I’ll take a look.

9

Hope not. The cassette is almost new, with about 1,000 km on it. The bike has a Classified drivetrain, so unfortunately I don’t have a spare cassette lying around.

10

Might be worth measuring the chainline and making sure it’s where it’s supposed to be. Could be a spacer in the wrong place for your particular setup.

11

Is the chain slack, or taught, in the smallest sprocket? It should have tension in it across the whole cassette.

12

Hanger alignment. Dealt with this before. Buy a hanger tool and you’ll tba m yourself.

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