I have a Shimano 105 R7000 GS (medium cage) rear derailleur and would like to mount a 12-25 cassette. According to its specs, this should not be possible.

I am just wondering: why not? A GS-cage will manage a 25t sprocket fine in many cassette combinations and in reality go as low as 11t. I understand, there is a problem in the other direction and a max on how many teeth it can handle on the highest sprocket.

Would anyone know, why a GS derailleur cannot handle a 12-25 cassette?

Thanks Morten

  • Yes, it is conservative. On my road bike the vendor even supplied a 34T cassette with an SS (r3000) derailleur. I did have to adjust the screw, else the jockey wheels interferred with the sprocket. – Vladimir F May 7 '20 at 13:28
  • @user7761803 You're right. Sorry, I missed that part lower down on the spec. I've never seen it listed that way before. – Noah Sutherland May 7 '20 at 17:22
  • Please post answers as answers. Check this metapost for more. rpg.meta.stackexchange.com/a/6534/63719 (yes RPG but still relevant) – Criggie May 10 '20 at 20:24

The key aspect of the GS cage is the length of the cage. The other key aspect of derailleurs is the geometry of the parallelogram.

Because the design has been adapted to suit more modern gearing with larger max sprockets, it moves downwards at a steeper angle, with room to accommodate the larger cogs. Therefore if a tighter range cassette is fitted, the guide pulley is now further away from the largest cassette than older GS derailleurs on the same cassette.

This is going to reduce the accuracy of the shifting a little if you can't move the guide pulley up enough using the b-tension screw. In practice of course, Shimano are famously conservative and breaching the spec might not give any issues. The 12-28 cassette is guaranteed to work, the 12-25 is not.

  • 2
    I believe that a similar question has already been answered on this site. Just too lazy to search! ;-) – Carel May 6 '20 at 20:04
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    The 12-25 could work in practice, as you imply. I don’t have one regularly installed, but the indoor training studio I frequented over the winter supplies wheels on Powertap hubs to clients. These frequently looked to have 11-25 or similar cassettes. My Shimano GS RD seemed to shift them fine. – Weiwen Ng May 7 '20 at 0:45
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    +1. Another thing to note here is that the R7000 SS version still calls out a more normal 25t for its minimum. Basically they've taken the GS version and made it wide-gearing-specific and left the SS as traditional racing, approximately. – Nathan Knutson May 7 '20 at 2:10

The GS cage will perfectly well handle the 12-25 cassette. Shimano is notoriously conservative with their specs and say that the smallest sprocket should be 11. It will handle a 12 and anything between 25 and 32 as the largest without trouble. You may have to give the B-screw some slight adjustment

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