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  1. #1
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    Dura-Ace RD-7800 B-Tension screw

    I am having an issue on one of my bikes, it seems the b-tension screw on my RD-7800 is not keeping the jockey wheel off the largest cog even when the screw is set all the way in. It is a short cage derailleur and I have a 11-28 cassette.

    It was making a bit of noise during a long ride yesterday, and since I was replacing my tires, I looked into it today. I had also taken it apart, cleaned and re-oiled the various bits. Would this be caused by RD being too small for 11-28 (purchased the bike with this configuration), or is the b-tension spring worn and needs to be replaced?

    If the spring needs to be replaced, would the RD-7900 b-axle and spring assembly work? Seems spares for RD-7800 are becoming more sparse and the pricing shows.

  2. #2
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    Quote Originally Posted by jason124 View Post
    s. Would this be caused by RD being too small for 11-28 (purchased the bike with this configuration), or is the b-tension spring worn and needs to be replaced?
    Did you buy it new or from a bike shop?

    Is the chain long enough but TBH it's like you suspected.
    Attached Thumbnails Attached Thumbnails Dura-Ace RD-7800 B-Tension screw-rd-7800-ss-.png  

  3. #3
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    It was purchased used. I did swap our the crankset from a 53/39 to a 52/36 but shortened the chain appropriately using the big-big +2 links method. But this was done months ago.

    A side note, I measured my chain for wear this morning and the .5% wear indicator can be forced between the links. Not sure if that's enough wear to cause the issue.
    Last edited by jason124; 10-16-2017 at 06:47 AM. Reason: Terminology

  4. #4
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    I bought a 12-28 a month ago, to use on my 7800 bike. It works fine with my 53x39 crank with no additional adjustment. The chain was originally set up to work with 12x23 and 12x25 cassettes, and will now also run in the 53x28 without any bother................Your chain might be hitting the jockey wheel because it's too long.
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  5. #5
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    Quote Originally Posted by jason124 View Post
    It was purchased used. I did swap our the crankset from a 53/39 to a 52/36 but shortened the chain appropriately using the big-big +2 links method. But this was done months ago.

    A side note, I measured my chain for wear this morning and the .5% wear indicator can be forced between the links. Not sure if that's enough WEAR to cause the issue.
    Chains don't stretch.
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  6. #6
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    Quote Originally Posted by cxwrench View Post
    Chains don't stretch.
    Fixed. I had just woken up when I replied.

  7. #7
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    Took it to the bike co-op yesterday to have a mechanic look at it. Turns out it was a combination of a tired b-tension spring and a rear cable housing that was too long and was putting pressure on the RD. Trimmed an inch off the housing and the issue was resolved.

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