Shifting issues bewtween Large and Small chainrings

1. 

   jbjewitt
   

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   I built my own bike up from scratch. I have a compact DuraAce 7900 CRank running a 11/23 Ultegra 6700 cassette with a Dura Ace Chain. Dura Ace 7900 FD and RD. DA 7900 shifters.

   I can set the RD indexing perfectly when in the large chainring, however when I shift to the small chainring, it's off and I have to tighten the cable tension on the RD barrell adj. Can't seem to find a compromise position that works bewteen both.

   Rear D hanger was adjusted using the Park Tool.

   I set the chain length per the Zinn method (derailleur cage position.) When I tried to establish chain length per the Park Blue Book method, it was one link shorter.

   Would shortening the chain have any effect? Any ideas

   As a note - this particular bike does not have a cable tension adj on the downtube like my previous bikes. The cable goes inside the downtube through an internal housing. There's only a cable stop where normally a barrel adjuster would be.

   JJ

   Posted 1 year ago
2. 

   jpouchet
   

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   Miss-shifting in the 19/21/23 range? Check to see that the FD is moving inboard completely. A slight intermittent rub on the inside edge of the FD can cause a shifting problem.

   I know, have a similar setup and need to fix it this evening. Had a field 'fix' last weekend that required removing FD cable, cleaning out all the caked-in dirt and Gatorade (better inside the body not all over the downtube and FD).

   Didn't reseat the cable properly making the Big ring work just fine but the 11/23 in little ring are a problem.

   Posted 1 year ago
3. 

   zootracer
   

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   I had the same problem when I when to a compact, although mine is Campy. I kept on messing with the cable tension. I think the main thing is there is a bit of a jump between the 50-34. My main problem was shifting from the 34 to the 50. I could get it perfect on my work stand, but on the road was different. Not having a down tube cable tensioning adjuster might make it a little difficult. Also when shifting with a Compact you have to soft pedal while shifting between rings. More-so then with a double. Mine started shifting better when it a couple thousand miles on it.

   Posted 1 year ago
4. 

   Andy
   

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   b-tension maybe too tight? My shop recently told me that the derailleur hanger spec for road and mountain is different and that some frame manufacturers did not know this and have been using out of spec derailleur hangers. One Italian brand was using a really old pre index spec too.

   Lance who??

   Posted 1 year ago
5. 

   Vern Niehaus
   

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   The cables may have gotten tangled in the down tube of the frame. When installing make sure to only do one shifter at a time and make sure it is pulled tight before putting the other one in.

   Posted 1 year ago
6. 

   nightfend
   

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   Most likely it is one of the following:
   1.The derailleur is too high up in comparison to the large chainring. Check the space between the cage and the chainring teeth.
   2.The derailleur cage is not parallel with chainrings and chain.
   3.The tension is too high on the cable, which then will not let the cage move completely into the small ring. You can check this by noting whether the inner ring adustment screw actually does anything.
   4.You have really short chainstays (like on a TT bike).

   Posted 1 year ago
7. 

   nightfend
   

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   Ooops, nevermind. I misread your original post. I thought you were having issues with your front derailleur.

   To check chain length, you need to shift into the large chainring, and smallest rear cassette cog. Then check to see if both rear derailleur pulleys are roughly vertically parallel to each other. If the bottom pulley is further forward, then the chain is probably too short.

   Posted 1 year ago
8. 

   jpouchet
   

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   Thanks nightfend, that simple chain length test was something I was missing. Makes sense. Since I need to drop a new chain on the old bike pretty soon this will come in handy.

   Posted 1 year ago
9. 

   zootracer
   

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   Yeah, I misread the entire post also. Cutting the chain a few links short should not make a diff (I did that also) unless you do a lot of cross chaining. I'd try the B screw adjustment.

   Posted 1 year ago
10. 

    CaleyFretz
    

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    What BB type? If it's a standard external BB, you didn't throw any spacers in there, right? Sounds like it could be a chainline issue.

    Also, is the chain installed in the correct direction? 7900 chains are directional.

    Finally, I'd check the b-screw. You may need to pull the mech away from the cassette a bit. Andy is right, hanger spec varies wildly between frames.

    Caley

    Posted 1 year ago

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