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I have a new rear derailleur (Microshift Advent X) which uses somewhat oversized cog wheels with 12 and 14 teeth. I am known to wear these wheels during the winter season, so I am already planning for spare parts.

I have a small stash of smaller 11 teeth pulley/jockey wheels and I wonder if I could install them when the time comes, instead of looking for matching larger spares. The benefits are that the 11-teeth parts are quite more widespread, but what are the possible problems that this could create?

Here are some negative things that I can imagine, but maybe I am missing something or underestimating the consequences.

  1. Clearance problems between pulley/jockey wheels, derailleur cage and chain passing through them. I can imagine that squeezing larger cogs could cause that, but with smaller ones the clearance should increase.

  2. Increased chain friction because of sharper angles when links are wrapping smaller cogs. I could not care less about it. The bulk of my friction problems in that area come from clogged bushings/bearings inside the pulley wheels; something that better seals, not more teeth, should help with.

  3. Change of the optimal chain length. I can imagine that with fewer teeth I'd need to remove a few links from the chain? Or is it the other way around, and the chain must grow?

  4. Worse chain retention (chain jumping inside the derailleur). I wonder if this will be noticeable given how constrained the chain inside the cage is, and with clutched derailleur keeping it tensioned.

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    I think the typical recommendation of Kogel and Ceramicspeed is that you should add links when installing their large pulley wheels. From experience, going from dual 11t to 12/14t pulleys doesn’t seem to need additional links despite the recommendation. Actually, I size my chain using the big ring/big cog plus 2 pins method, which bypasses the RD cage entirely.
    – Weiwen Ng
    May 10 at 11:09
  • 5. Increased clearance between jockey wheel and sprocket. Since shifting relies on the angle between chain and sprocket, I guess that increasing the clearance (and by that decreasing the chain angle while shifting) could lead to problems with shifting. I think, that would be the main consideration (apart from having to take a link out of the chain). May 10 at 13:10
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    @WeiwenNg Mechanically, the difference between 11ti11t and 12t/14t should be exactly one link: The 12t has 0.5 teeth extra and the 14t has 1.5 teeth extra to wrap the chain halfway around each wheel. That's a total of 2 teeth, i.e. 1 link of the chain. May 10 at 13:14
  • 6. Less allowable range for the cassette: Wrap around of the chain around the jockey wheels adds to the amount by which the derailleur can lengthen/shrink the chain. If your derailleur is already at its limits with your cassette, smaller jockey wheels and a shortened chain may break its neck. May 10 at 13:20
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    If I remember correctly, I used some generic 10 t pulleys on a Shimano XT derailleur that originally used 11t pulleys, on a 3x9 speed bike. I noticed no negative effect whatsoever (somewhere between year 2000 an 2004) If I did that on my current XT derailleur (bought circa 2010) the chain would rub on a cage plate "protrusion" that goes towards the other plate.
    – Jahaziel
    May 10 at 15:45
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I am now in position to answer my question, as I've obtained practical results of an experiment described in the question.

I managed to destroy an upper pulley wheel when it met a stub. The rest of the derailleur was undamaged.

Broken upper pulley

The damaged guide pulley had 12 teeth (not 13 teeth as I originally miscounted). I only had 11-teeth replacements from SRAM in my reserves. So I went ahead and replaced it. I also shortened the chain by 2 half-links as I felt it was just a bit too long as well.

The main adjustment that I had to do to make it work was to ensure that the cage would not touch the cassette when climbing from the next-to-biggest cog to the largest cog. The B-tension screw of the derailleur was turned until there was a gap between these parts. A smaller diameter of the replacement part brought the cage closer to the cassette, so that had to be accounted for:

B-tension-affected distance

During the next two weeks, the repaired derailleur worked without noticeable differences compared to how it shifted before.

I have just received and installed a replacement 12-teeth upper pulley wheel from the manufacturer (the part is not available aftermarket yet). Again, I have not noticed any difference in shifting.

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  • Nice work - thank you for the completion. TBH probably didn't even need to remove any links in the chain.
    – Criggie
    May 27 at 23:43
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When people put in wider than stock pulleys, one possible outcome is the chain winds up getting jammed between the tension pulley and the cage. That can result in a destroyed derailleur and/or frame. This may not be common per se, but I've seen it enough that I'm very conservative about never using anything but the original thickness.

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    In an ironic twist of fate, I've just destroyed my upper pulley wheel when it met a tree :-( i.stack.imgur.com/dufpR.jpg Let's see if I will be able to make the derailleur work with the smaller 11t replacement. May 11 at 13:13

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