SRAM RD B-screw gap adjustment
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  1. #1
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    SRAM RD B-screw gap adjustment

    SRAM installation manual has a picture that suggests adjusting B-screw on SRAM rear derailleurs for 6 mm gap between the large cog and the upper jockey wheel. However, it is not immediately clear from the picture how exactly this gap is supposed to be measured. Is it the absolute gap between the cog and the jockey wheel (measured along the line connecting the centers) or is it the apparent gap observed along a horizontal line of sight from behind?

    Gap.jpg

    Which measurement is meant by the manual?
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  2. #2
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    the one on the left.
    use a torque wrench

  3. #3
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    What I observe during my attempts to adjust the B-screw is as follows.

    In my original B-screw position I measure about 8-9 mm if the aforementioned gap


    The RD switches fine, no problems whatsoever. Yet, just to try things out I wanted to reduce the gap to the recommended 6 mm.

    In an attempt to reduce the gap, I unscrewed the B-screw, which made the upper jockey wheel to orbit around the biggest cog (28). However, as the jockey wheel was travelling around the cog the gap decreased only very very slowly


    In order to bring the gap into the 6 mm ballpark I had to push the upper jockey wheel way up to the point where the chain wrapped around the 28 cog for more than 180 deg.


    This configuration looked weird. And in this B-screw position the RD refused to switch to the smallest cog (12).

    So, I returned the B-screw back to its original, more natural looking configuration with 8 mm gap. Again, the RD works fine in that position.

    I just have some questions out of curiosity. The fact that I can't reduce the gap to the recommended 6 mm without "distorting" the RD beyond its operational range - what is the cause of that?

    Could it be simply caused by the fact that I'm using a WiFLi derailleur on a "normal" cassette? (I.e. 6 mm would be achievable on a 32 cog, but not on a 28 cog).

    Or does it mean something else, like some spring inside the RD losing tension or my chain being too long?
    Last edited by AndreyT; 08-31-2014 at 02:27 PM.
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  4. #4
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    you said it refused to shift to the lowest cog after you correctly set the B tension screw. Did you check the limit screw? The limit screw improperly set will make it so it won't shift into the lowest cog.
    use a torque wrench

  5. #5
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    Yes, I did. The limit screw has nothing to do with it.

    I put my B-screw back to 6.5 mm gap configuration (jockey to 28 cog) and took some pictures.

    This is the RD, viewed from behind after being asked to shift from the 13 cog to the 12 cog

    IMG_7166.jpg

    This is what the H limit screw looks like at the same time

    IMG_7155.jpg

    As you can see, the limit screw is not even engaged yet. There's like almost whole 1 mm of space between the end of the limit screw and the "landing" area it is supposed to bump into.

    In other words, the movement of the RD in this case is not limited by the limit screw.

    And this is the same thing viewed from the side

    IMG_7160.jpg

    It is obvious that on the smaller cogs the gap between the jockey wheel and the cogs gets so small, that there's no free segment of chain between the cog and the jockey wheel anymore. Such RD cannot possibly shift further. It is mechanically impossible.

    In fact, it actually almost looks like the 12 cog teeth and jockey wheel teeth are overlapping/interfering with each other.
    Last edited by AndreyT; 09-02-2014 at 09:31 AM.
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  6. #6
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    did you figure out what the problem was? I've had the same problem in several of my builds and I had to play a lot with the B screw. I agree the limit screw has nothing to do with it.

  7. #7
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    Quote Originally Posted by anthro88 View Post
    did you figure out what the problem was? I've had the same problem in several of my builds and I had to play a lot with the B screw. I agree the limit screw has nothing to do with it.
    No, it must be just how that Apex WiFLi derailleur works with that cassette. As I said above, it shifted fine with the original gap of 8+ mm. I just found a RD position that made it "look right" and used that setup for a while. Later I replaced the Apex RD with a Force RD (medium-cage).
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