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Is the Shimano Dura-Ace 7900 10 speed cassette with a ratio of 11-27 cassette compatible with a dura-ace 7800 rear derailleur?

Some people say its fine but more important to me is what Shimano says on the matter. Some online retailers specifically state that the DA7900 11-27 ratio cassette is not compatible with the 7800 rear derailleur.

I believe that I saw on Shimano documents that it was not, due to chain wrap problems occurring but cannot find that material through google so am reaching out to you for help.

If you could direct me to material relating to this compatibility it would be very much appreciated!

  • GS model then for sure. SS? – paparazzo Sep 23 '15 at 21:53
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    Someone can verify this: Theres something peculiar about the 7800 freehub (larger splines?). I'm guessing that its not a derailleur issue, but if you own a 7800 freehub, you can't put a 7900 cassette on since it has normal splines. – Batman Sep 23 '15 at 23:04
  • I tried to fix the language but may have changed the text too much. Feel free to edit further or revert. – RoboKaren Sep 24 '15 at 3:49
  • Thanks for highlighting that cage length matters Frisbee --> No, my bike has an "SS 7800RD" – MartinCB Sep 24 '15 at 10:23
  • Hi Batman, are you saying that the 7900 cassette can move/jump on the 7800 free hub? – MartinCB Sep 24 '15 at 10:27
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The RD-7800 manual states that the largest sprocket for a SS is 27T.

Manual here https://static.bike-components.de/cache/dl-rd-7800-2-pdf-ce95c6782564d2bbf75cca831ae0bcbf.pdf

  • Hi Josh, I thought a 10 speed cassette is a 10 speed cassette, dumb :) problem for me is that Shimano say that DA7800 SS can't take an 11-28T and I believe not even a 11-27T. Some online retailers warn people of the 7900 cassette, that it won't fit on the 7800RD in the 11-27 or 11-28 ratios. Why would they warn unless there are incompatibilities stated from shimano? I can't find the material shimano issued on the subject but need it. Thanks again! – MartinCB Sep 24 '15 at 10:49
  • Hey @MartinCB I just added the manual to my answer. It states that the capacity of an SS is 27T – Josh Click Sep 24 '15 at 17:51
  • Hi Josh, I see where your coming from on the matter,it says the largest sprocket can be 27 and the smallest can be 11. But thats what it can handle, not the spread (i.e. 11-27T) you can have running on the same cassette at once. I was messaging with Shimano and they said 11-27T Da7900 is not compatible with the 7800RD, but the 12-27 would be, its a small difference (I know) but a massive difference at the same time. Thank you for your help Josh. >>>> I'm waiting for a response from Shimano as to the official documentation that expresses the spread of the 7800RD. – MartinCB Sep 24 '15 at 20:08
  • further to that, i just copped it now, is the total capacity of the SS is 29 teeth. so for a compact front (53-39=14) and then an 11-27t (27-11=16)..... then add the 14 & 16 = 30. thats a capacity of 30 when the SS has a total capacity of 29... this capacity measurement also takes into account the front cogs too(but I think the front cogs generally have a spread of 14, even if its not a compact). – MartinCB Sep 24 '15 at 20:15
  • 53-39t chainring is a standard crank, sorry – MartinCB Sep 24 '15 at 20:32
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The derailleur and cassette are compatible; in the sense that they will work together when they are installed on your bike, and the derailleurs are adjusted correctly.

Shimano states them to be incompatible, but this is not to say they would not function; rather they would not function as Shimano intended them to. This is because Shimano designs their drivetrains as a set, rather than individual components.

In the case of the 7900 cassette and 7800 derailleur with 53-39 chainrings, it should function , based on details that are not part of the derailleur design. Such as if you know how to shift through your gears correctly. If you shift into the largest chainring, and then onto the 28t rear cog, there is a chance that the drivetrain could bind. This is based on things like chain length, chainstay length, rear dropout design, and how far the B-tension screw is turned in on the derailleur. Since Shimano can't control these aspects of your riding experience, they simply state that the two components are not compatible.

My professional recommendation to a customer who wanted to use the 11-28 cassette, but didn't want to buy a new derailleur would be this:

  1. Install the cassette and derailleur.
  2. Measure your chain to the correct length (using Park Tool's 3rd method), but do not cut the chain yet.
  3. Add an additional link (outer/inner) to the total chain length.
  4. Cut the chain, install, and check the shifting function.
  5. Adjust the B-tension screw in on the derailleur so that the chain and upper jockey wheel on the derailleur do not grind against the largest rear cog.

If for some reason you can't adjust the drivetrain noise out, then forget it. Any of the above factors may make it impossible. It's unusual, but it does sometimes happen. If there is excessive slack in the chain in the small ring/small cog combination, it's also probably not a great idea. However, I would not worry a great deal; Shimano SS cages have never been compatible with 28t cogs, but tons of people run them all the time. It's for legal protection that Shimano states this. What you really need to run an SS cage and a 28t cassette is mechanical know how (or a good shop mechanic!)

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