Rear derailleur short cage vs. long cage???
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  1. #1
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    Rear derailleur short cage vs. long cage???

    Hi,

    I have a 105 rear derailleur and and 105 rear cassette; and an Ultegra compact Crank 34/50 gearing, and a Dura ace front derailleur. Since I put on the Ultegra and dura ace in the front it really hasn't been shifting smooth--I had it adjusted at the bike shop a couple of times, still not smooth in the rear.

    So I think I might get a Dura Ace rear derailleur and a Dura ace rear cassette.


    I was wondering what the difference is between a short and long cage rear derailleur?

    What would you recommend based on what I've got on my bike?

    I'm going for the RD-7900 rear derailleur and a Dura Ace 7900 rear cassete--will they work with my dura ace front derailleur and ultegra compact crank?

    I think my chain is 105 or less--would it make a noticeable difference to upgrade my chain?

    Thanks

    Jay

  2. #2
    wim
    wim is offline
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    Stop the madness.

    - Worn chains don't shift smoothly. So if it's worn, replace it with a new 105 chain if you like Shimano chains (I prefer KMC chains).

    - Long-cage rear derailleurs have a larger "total capacity" than short-cage derailleurs, meaning they can take up more chain wrap. Total capacity needed for a particular setup is (largest rear - smallest rear) + (largest front - smallest front). Shimano long-cage (GS) derailleurs have a total capacity of 39; short-cage derailleurs (SS) 33. There is no Dura-Ace 7900 GS derailleur. If you have the standard 11-25 or 12-25 cassette, you don't need a GS rear derailleur: (50 - 34) + (25 - 11) = 16 + 14 = 30.

    - Substituting ever-more expensive parts is not the solution of your problem. The Dura-Ace 7900 cassette would be a particularly bad buy: it costs a fortune, shifts no better than 105 or Ultegra, but wears much quicker than these perfectly good lower-level components. Find and pay a competent mechanic instead of throwing money at a seller of bicycle parts.

  3. #3
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    Could be one of the following
    1. You mechanic is not very good.
    2. RD Hanger is out of alignment, this can cause mild to severe degradation in shifting performance.
    3. Poorly lubricated chain or a stiff link.
    4. Worn chain.

    #1 or #2 would be my most likely suspects.

    Shawn

  4. #4
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    Shawn and Wim are right. You're throwing expensive upgrades at a problem without getting to the root of it. There's little performance difference between 105, Ultegra and DA stuff. The main differences with the more expensive stuff are durability over the long run and lower weight components.

    The 7900 series cassette is one of the lightest available.... Thanks to using titatinium cogs. But that makes it about twice as expensive to buy and it'll wear out in about half the time. Just about any other 105/Ultegra/Sram cassette would be a better purchase for most people.

    No need for a long cage RD unless you have larger than 28T biggest cog on the cassette (27T with some earlier short cage RD's), which I seriously doubt would be the case with a compact crankset.

    Use Shawn's list for trouble shooting. I'd also add...

    5. Incorrectly routed, poor quality, kinked, or damaged RD shifter cable.
    6. Mis-aligned front derailleur. Be sure it's well aligned with the chainrings and has about 1 to 3mm of clearance with the largest chainring. Sometimes people mistake FD noises for problems at the rear. Also a chain dragging on the FD cage can cause issues at the rear.
    7. Missing 10 speed shim behind the rear cassette (on the free hub). This is often a 1.2mm shim (I think) on most 8/9/10 speed freehubs. Mavic freehubs use a thicker, proprietary shim. Some other freehubs might differ. But, if this shim is missing, the indexing won't be right and you won't be able to adjust shifting across the cassette's full range.

    I'm assuming here that it's all newer 10 speed stuff you're using, not mixing older 8 or 9 speed bits with newer 10 speed. Mixing might cause some issues... but many people have made them work quite well, too.

    If you are working on the bike yourself, there are some good books on bike repair and the Park Tool website has a lot of good repair tutorials.
    Last edited by Amfoto1; 07-28-2010 at 07:49 AM.
    Hey, I'm not going bald... I'm getting more aero.

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  5. #5
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    I second replacing the cable. I have no idea how mine got kinked once, but it sure made for some interesting shifting. Good shifting at the top of the cassette (25) and the shifting just degraded the further I got down the cassette.

    Also replace the housing and make sure that the housing is cut pefect, this crisps up shifting also.

  6. #6
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    Another vote for replacing the shift cable. Rear shifting on a 10-sp Ultegra set-up in our house degraded. I adjusted it back to normal, it degraded again, and I adjusteed again. Soon after that the rear shift cable broke inside the shifter mechanism. The LBS could fish-out the head of the cable, but it could have been much worse. I'd replace that cable before your next bike ride.

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