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  1. #1
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    Dura Ace 7900 Shifters and 7800 Rear Derailleur Compatable?

    Hi all. I just installed the new Dura Ace 7900 brake/shift levers due to the old ones wearing out after 7 years. The charts I've read on Competitive Cyclist and other sites say you need the 7900 front derailleur, which I also bought, but that it works with the 7800 rear derailleur. I've put the cable on both sides of the fixing bolt, but I can't get it to shift without skipping a cog in the middle of the cassette. Can anyone tell me where I'm going wrong? Thanks for any help out there.
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  2. #2
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    You need to get the matching 7900 rear derailleur. 7900 & 7800 Dura Ace are incompatible. Different cable pull. 7900 was designed to only work as a stand alone. Its not compatible with any other group.

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    Quote Originally Posted by gamara View Post
    You need to get the matching 7900 rear derailleur. 7900 & 7800 Dura Ace are incompatible. Different cable pull. 7900 was designed to only work as a stand alone. Its not compatible with any other group.
    I thought I read somewhere that people were stockpiling 7800 rear deraileurs to use with their 7900 groupos because they didn't like the flex in the 7900 rear deraileurs.

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    Quote Originally Posted by gamara View Post
    You need to get the matching 7900 rear derailleur. 7900 & 7800 Dura Ace are incompatible. Different cable pull. 7900 was designed to only work as a stand alone. Its not compatible with any other group.
    Nope. The front derailleurs are incompatible, the rears are fine. Here's a Shimano Tech document. Look at page three. All the shifters are listed in the same flow chart block because all of them will run the derailleurs down stream.

    http://bike.shimano.com/publish/cont...ty%20Chart.pdf

    What made you post this?
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    Quote Originally Posted by gamara View Post
    You need to get the matching 7900 rear derailleur. 7900 & 7800 Dura Ace are incompatible. Different cable pull. 7900 was designed to only work as a stand alone. Its not compatible with any other group.
    That is not true, the 7800 rear derailleur will work fine as long as you don't go beyond 27 teeth in the cassette.

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    Quote Originally Posted by Kontact View Post
    Nope. The front derailleurs are incompatible, the rears are fine. Here's a Shimano Tech document. Look at page three. All the shifters are listed in the same flow chart block because all of them will run the derailleurs down stream.

    http://bike.shimano.com/publish/cont...ty%20Chart.pdf

    What made you post this?
    I've set the high and low travel of the derailleur, and I set the barrel adjuster to shift well on the inside cogs, it starts skipping around the 5th or 6th gear down. If I set it to shift well on the 12 tooth (10th gear), then it's fine shifting up until again, it starts skipping a gear here and there. It worked great before the shifter change, which is why I'm confident it's not the cassette or the derailleur. It sure seems to me that it's not compatable, but I know the charts say it is, so it must be something that I've missed, or I'm not doing right.
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    Quote Originally Posted by ozarkbikeracer1 View Post
    I've set the high and low travel of the derailleur, and I set the barrel adjuster to shift well on the inside cogs, it starts skipping around the 5th or 6th gear down. If I set it to shift well on the 12 tooth (10th gear), then it's fine shifting up until again, it starts skipping a gear here and there. It worked great before the shifter change, which is why I'm confident it's not the cassette or the derailleur. It sure seems to me that it's not compatable, but I know the charts say it is, so it must be something that I've missed, or I'm not doing right.
    Check your derailleur hanger.

    Also, if the 7800 shifter had enough mileage to wear out, your derailleur springs and pivots might also be worn beyond accuracy. You are combining a brand new shifter with well worn parts - that doesn't always work.
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    Make sure that the tab on the washer under the cable attachment bolt is correctly aligned before tightening the cable bolt. Also be sure that the shift lever is shifted to the proper spot before installing the cable. (been there, done that on both)

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    Quote Originally Posted by Kontact View Post
    Nope. The front derailleurs are incompatible, the rears are fine. Here's a Shimano Tech document. Look at page three. All the shifters are listed in the same flow chart block because all of them will run the derailleurs down stream.

    http://bike.shimano.com/publish/cont...ty%20Chart.pdf
    That's an old chart because it doesn't even show 6700 & 5700 but if I'm reading it correctly, it definitely shows that 7900 is a stand alone drive train with the exception of the cassettes being compatible between all groups. Check for yourself & follow the flow lines. From what I know, I'm positive that 7900 & 7800 should not mix. But of course there will always be someone that can figure a way to make things work by chance depending on their chain & cassette setup. But as far as I know, the only backwards compatible group is 5700.

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    Quote Originally Posted by gamara View Post
    That's an old chart because it doesn't even show 6700 & 5700 but if I'm reading it correctly, it definitely shows that 7900 is a stand alone drive train with the exception of the cassettes being compatible between all groups. Check for yourself & follow the flow lines. From what I know, I'm positive that 7900 & 7800 should not mix. But of course there will always be someone that can figure a way to make things work by chance depending on their chain & cassette setup. But as far as I know, the only backwards compatible group is 5700.
    "As far as I know." Know, how? I work on this stuff for a living, where did you get your information?

    The chart clearly shows that, for rear shifting, the 7900 shifter is in the same block as all the others. If you compare that to the front shifting chart you'll see it grouped separately because those aren't cross compatible. If the front and rear shifting had the same lack of compatibility, the charts would look the same. They don't.

    And what "chain & cassette setup" are you talking about? All Shimano and SRAM ten speed chains and cassettes have identical spacing and widths. What magic cassette is going to change anything?


    You heard the wrong thing somewhere, you don't have any personal experience with it and you can't site a single tech document to support your theory. Why disseminate bad information?
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    if I'm reading it correctly
    You're not. As Kontact points out, it indicates the 7900 RD is only compatible with the 7900 STI, but that the 7801 RD is compatible with the 7900 STI. Not sure what the chart being old has to do with it. It used to be compatible, but now it's not? Shimano is always very conservative in their compatibility charts, so if they indicate it will work, it will work.

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    Quote Originally Posted by gamara View Post
    ..... if I'm reading it correctly....
    You're definitely reading it WRONG ....

    The 7900 shifters are compatible with the 7800 RD, as Kontact has rightly pointed out. Its the 7800 FD which is not compatible.

    In fact if you scour through the Cyclingnews picture archives you'll even see some pro teams using that setup when 7900 first came onto the market.

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    Thanks, guys, for your input. I'll check my cable routing and fixing-bolt again.
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    Correct

    Quote Originally Posted by gamara View Post
    You need to get the matching 7900 rear derailleur. 7900 & 7800 Dura Ace are incompatible. Different cable pull. 7900 was designed to only work as a stand alone. Its not compatible with any other group.
    Just found this 6 year old thread and wanted to say, you are 100% correct. The 7800 RD DOES NOT work correctly with a 7900 shifter as I just confirmed this with NOS parts on a perfectly straight frame. It does seem to be a pull-ratio issue. Could simply be that pre-7900 stuff is compatible e,g,. 7700 RD with a 7800 shifter. 7900 does work with the same period Ultegra iteration - 6700. These are critical facts LOL.

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    Quote Originally Posted by crewdoglm View Post
    Just found this 6 year old thread and wanted to say, you are 100% correct. The 7800 RD DOES NOT work correctly with a 7900 shifter as I just confirmed this with NOS parts on a perfectly straight frame. It does seem to be a pull-ratio issue. Could simply be that pre-7900 stuff is compatible e,g,. 7700 RD with a 7800 shifter. 7900 does work with the same period Ultegra iteration - 6700. These are critical facts LOL.
    Something is wrong with your setup.

    Rear derailleurs of 7800, 7900, 6700, 6600, 5500, 6500, 1055 all have the same actuation ratio. With the exception of 7400, all Shimano road groups have used the same RD actuation ratio going back to 1985 600, 6 speed through 10 speed. That only changed with 11 speed.


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    Quote Originally Posted by Kontact View Post
    Something is wrong with your setup.

    Rear derailleurs of 7800, 7900, 6700, 6600, 5500, 6500, 1055 all have the same actuation ratio. With the exception of 7400, all Shimano road groups have used the same RD actuation ratio going back to 1985 600, 6 speed through 10 speed. That only changed with 11 speed.


    I read the same F&%$ing chart dude and I have ample experience with charts and graphs trust me. It doesn't work. Period. And I agree that the chart indicates compatible. Setup is a NOS 7800 RD on a new Bottecchia steel frame. Now, I will tell you, the shifters are the somewhat obscure 7900 downtube shifters but that iteration is not excluded per the chart so it should work. It doesn't. I put the 7900 RD BACK on there and all good. I also positively confirmed I have a 7800 not the similar-looking 7700. I would welcome the dialogue if you want to get the parts and try it. Maybe that 7800 will work with a "brifter"...? I'm definitely intrigued and I'll try that thing with a different shifter and get back to you either way. I would be as suprised too if a Shimano tech doc turned out to be wrong but Ihave evidence to that effect.

    Joe
    Last edited by crewdoglm; 4 Weeks Ago at 09:33 PM. Reason: Addendum

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    Quote Originally Posted by crewdoglm View Post
    I read the same F&%$ing chart dude and I have ample experience with charts and graphs trust me. It doesn't work. Period. And I agree that the chart indicates compatible. Setup is a NOS 7800 RD on a new Bottecchia steel frame. Now, I will tell you, the shifters are the somewhat obscure 7900 downtube shifters but that iteration is not excluded per the chart so it should work. It doesn't. I put the 7900 RD BACK on there and all good. I also positively confirmed I have a 7800 not the similar-looking 7700. I would welcome the dialogue if you want to get the parts and try it. Maybe that 7800 will work with a "brifter"...?

    Joe
    They aren't remotely rare. It is one of two DT shifters Shimano makes, and you can buy them all over:

    Shimano Dura-Ace 7900 10sp Downtube Shifter Set | Chain Reaction Cycles

    "Fully compatible with 10-speed Road drivetrains"

    And if you read the reviews you'll see people using it with 105 derailleurs and such.

    Something is wrong, but it is not the compatibility. Check that you have the cable routed to the fixing bolt correctly. It goes on the back side, closer to the spokes.
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  18. #18
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    Other things to check:

    That your BB cable guide is tight.
    You don't have an SL-7700 9 speed shifter (marking on the black plastic.)
    You have a Shimano or SRAM 10 speed cassette installed.
    You hanger is properly aligned.
    The jockey pulley screws are tight.
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    Yes... those are the basics... Check. The compelling feature of this is that a 7900 RD works perfectly whereas the 7800 simply refuses to shift cleanly into all positions particularly the furthest outboard. Shifting inboard, from that point, the derailleur moves 2 positions with a single click. Again, this is a new derailleur and problem vanishes when I put the 7900 back on there. It certainly behaves like pull ratio is the culprit. Stumped for the moment.

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    BTW Chain Reaction is not a canonical source.

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    Can't wait 'til this thread gets dredged up again in another 6.5 years. Thanks for reminding me I have NOS 7801 shifters I need to sell...

  22. #22
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    Quote Originally Posted by crewdoglm View Post
    BTW Chain Reaction is not a canonical source.
    Nope. It is just a source of a not at all rare SL-7900 shifter.

    Your description of the shifter downshifting different than upshifting sounds like something is sticking, not a shifter incompatibility.

    Did you check that the chain was routed properly and on the pulleys?
    Is there packing material inside the parallelogram?
    Is the cable frayed inside the housing?
    With the chain off does the derailleur move easily?
    Is the cassette a wide gear range that the derailleur could be catching on?


    You have a mechanical problem. You just don't have a compatibility problem. You'll note the OP never came back to the thread to say it wouldn't work, Shimano says it will work and I have made it work myself in a bike shop. I don't know why you've latched on to Gamera's posts which aren't even from his own experience.
    Last edited by Kontact; 4 Weeks Ago at 01:46 AM.
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    Mind Blowing

    Quote Originally Posted by Kontact View Post
    Nope. It is just a source of a not at all rare SL-7900 shifter.

    Your description of the shifter downshifting different than upshifting sounds like something is sticking, not a shifter incompatibility.

    Did you check that the chain was routed properly and on the pulleys?
    Is there packing material inside the parallelogram?
    Is the cable frayed inside the housing?
    With the chain off does the derailleur move easily?
    Is the cassette a wide gear range that the derailleur could be catching on?


    You have a mechanical problem. You just don't have a compatibility problem. You'll note the OP never came back to the thread to say it wouldn't work, Shimano says it will work and I have made it work myself in a bike shop. I don't know why you've latched on to Gamera's posts which aren't even from his own experience.
    Believe it not Kontact, I tracked down that tech doc before getting a 7800 RD basically to confirm what I already knew. Yes... the Shimano chart indicates compatible and no, none your patronize-the-novice trouble-shooting applies because, shocker, I'm actually experienced with **** as you are. It would appear for the moment, that either the guidance is wrong or the specific RD I'm installing has a defect/anomaly. Neither of those are very likely; can we agree at that point? Anyway, one solution would be to duplicate or exclude the problem with a different 7800 RD; I don't have another. I'll play with it some more AND I'll actually post it here if I missed something e.g., the "B" limit screw, evil spirits. Not too big to admit I missed something or learned something.

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    Quote Originally Posted by crewdoglm View Post
    Believe it not Kontact, I tracked down that tech doc before getting a 7800 RD basically to confirm what I already knew. Yes... the Shimano chart indicates compatible and no, none your patronize-the-novice trouble-shooting applies because, shocker, I'm actually experienced with **** as you are. It would appear for the moment, that either the guidance is wrong or the specific RD I'm installing has a defect/anomaly. Neither of those are very likely; can we agree at that point? Anyway, one solution would be to duplicate or exclude the problem with a different 7800 RD; I don't have another. I'll play with it some more AND I'll actually post it here if I missed something e.g., the "B" limit screw, evil spirits. Not too big to admit I missed something or learned something.
    I'm not sure why you're insulting me because you can't get your derailleur to work. I'm sorry I suggested anything. Good luck.


    And you aren't "as experienced" as I am. Had I run into this and was seriously entertaining a difference in actuation ratio, I would have gotten out the calipers and measured the difference in fixing bolt location between the derailleurs that would demonstrate actuation ratio differences. I have modified shifters in the past to change their ratio and combined multiple different brands of shifter, derailleur and freewheel spacing long before there was an internet to discuss it.
    Last edited by Kontact; 4 Weeks Ago at 10:02 AM.
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    Quote Originally Posted by Kontact View Post
    I'm not sure why you're insulting me because you can't get your derailleur to work. I'm sorry I suggested anything. Good luck.
    Copy all thanks. Suggest with empathy and respect and instead of snarky condescension and these conversations will go better. Merry Christmas.

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