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Is Shimano RD-5800 rear derailleur compatible with a 7 speed cassette?

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Nope. The RD-5800 is the previous generation 105 level derailleur that works with 11 speed shifters and cassette.

You need a derailleur intended specifically for 7 speeds. The only 7 speed models Shimano makes are in the Tourney range.

Apparently, at least for 7-9 speeds, Shimano road derailleurs are compatible with 7-9 speed shifters as the combination of shifter cable pull and derailleur ratio will index the cage correctly. This does not work for 11 speed though.

  • I don't think so. The derailleur is not indexed (unlike the shifter) such that new a derailleur is compatible with old (7-speed) shifters. See blog.artscyclery.com/science-behind-the-magic/… – Christian Lindig Apr 9 '18 at 17:22
  • 11-speed derailleur can be installed together with shifter indexed for 7 speeds, but the combination will not index correctly for 7-speed cassette. Check "derailleur ratio" under the link you posted. 10-speed road derailleur or any Shimano derailleur for 9 or less speeds will work. – ojs Apr 9 '18 at 17:32
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    Yes the shifter does the indexing, and pulls a certain amount of cable per sprocket, but the derailleur also has a derailleur ratio which is how much it moves the cage per cable pulled. AFAIK there used to be compatibility across different Shimano speed drivetrains, but 11 speed broke that. – Argenti Apparatus Apr 9 '18 at 17:37
  • @ArgentiApparatus 5800 was superseded by R7000 a week ago, so it's not really the last 105 anymore. – Klaster_1 Apr 10 '18 at 2:33

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