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I use a Shimano Ultegra 6600 rear derailleur with short cage in a 11/28 cassette. The derailleur is at the end of its useful life and would like to exchange for a cheaper one, since I intend to exchange the complete groupset in the future.

I found a really cheap 105 5701 rear derailleur, but only the medium cage version is available, that is compatible with cassettes up to 30 teeth. If I use that medium cage derailleur with my 11/28 cassette, is it possible I'll have any performance issues?

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    Another question is why do you say that the RD has reached the end of its earthly life. Maybe you could just get a pair of new jockey wheels, which are in most cases the real consumables on a RD for less than the 5701. Also the medium cage version normally allow for 32 teeth max. – Carel Oct 2 '18 at 17:35
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    Hey @Carel! I change the jockey wheels last year, they are fine! The major problem is that after a crash the cage plate became bended (also a broken derailleur hanger). I was able to undo a little, it continues to work, but it is misaligned and I'm also afraid of breaking it up during a ride. – Confundir Oct 2 '18 at 20:17
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    The difference in cage length basically controls the max # of links that the derailer can adjust for. If you have a long cage and you never use the two largest cogs on your rear cluster the derailer still works, right? The only negative (besides size/weight) is that the longer cage will make for slightly less "crisp" shifts. – Daniel R Hicks Oct 3 '18 at 1:12
  • @Confundir: Remove the 6600 from the bike, disassemble the cage and straighten it off the bike. You can tell that it's true when the screws that hold the jockey pulleys fall in place without having to force them. If it's not too mangled it should be easier if you work on the two plates separately. I did it once with a cage that had suffered a 90° twist. – Carel Oct 3 '18 at 7:26
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Assuming the same length chain, let's think about the edge cases of front and rear chain ring/sprocket combinations:

  1. Big front/Big rear: You hopefully don't ride in this combination anyways but the RD cage would be pulled forward, but as it's already relatively inline with the chain there is no real difference..

  2. Big front/Small rear: Short cage would normally hang downwards, longer cage will take up more chain, it will therefore pull slightly more forward. I wouldn't expect an issue.

  3. Small front/Big rear: First gear, again, short cage that would normally hang downwards, longer cage will take up more chain, it will therefore pull slightly more forward. I wouldn't expect an issue.

  4. Small front/Small rear: Again, not a good combo to ride in, but in case you do, having a longer cage will actually be helpful here as the relatively shorter chain length will pull the cage forward.

All other cases would be between these extremes, that is, some form of 'it's going to be fine'.

Going the other way - using a short cage RD with a chain set for a long cage - may not work so well in case #4, but in that case a solution would be to remove links (much less potential for complication than when adding them ;)

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    It's not a good idea to assume that you will never use some ring-cog pairings. The derailleur should be able to shift to any combination of gears. In the heat of a ride you may easily get into a gear that you planned not to use, with dire consequences. – Carel Oct 3 '18 at 7:17
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    Its always possible to screw up while riding - we've all done something wrong at some point, or will. Its best to have enough chain to not bind when in big/big. – Criggie Oct 3 '18 at 10:12
  • @Carel, I'm not suggesting you should, nor do I suggest that a long cage RD will cause ride-stopping cross chain issues, so in that regard it looks like we agree :). Within reason, I think largest cog size determine chain length more than RD cages though. – Lamar Latrell Oct 3 '18 at 12:23
  • @LamarLatrell : Yes, both must match. Number of teeth and length of the cage determine the chain length. If the chain length is good for an 11-34 cassette it will also be OK for any cassette where the largest cog has less than 34 teeth. With modern RDs gear change smoothness and speed don't suffer. – Carel Oct 3 '18 at 16:45
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An RD-5701 rear derailleur should work just fine as a replacement for an RD-6600. I've used an RD-5701 with an 11/28 cassette for the last 6-7 months with no issues, including swapping my crankset from a 50/34 compact to a 53/39 a few weeks ago.

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    But you are using the medium cage version? The short cage I'm sure that work fine ... – Confundir Oct 2 '18 at 20:27

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