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  1. #1
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    shimano rear M952 b-screw preload?

    I'm having problems re-installing a 2004 xtr m952 rear derailleur. My newer m960 wrapped itself around the cassette and I figured I'd use this one I had laying around. I replaced my hanger and chain and when installing this derailleur I back the b-screw all the way out and then tighten the derailleur into the hanger. Then I line up the top and low cogs and then put the chain through, which I took from the old chain and verified using the shimano 2 extra link method. The problem is when I try to adjust the b-screw so the derailleur does not hit the low cog I have to turn it all the way in for it to have any effect and this almost takes the b-screw off its hanger pressure point. I'm running a 11-34 XT cassette with standard XC chain rings up front. Should I be preloading the b-screw spring when installing the derailleur? If so how? I actually did try unloading the chain tension and then when tightening the derailleur rotating it clockwise as far as it would go with no b-screw tension, and this seemed to work slightly better.
    If you're not falling, then you're not riding fast enough!
    Ibex Asta Pro SE & Giant XTC-2

  2. #2
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    The problem here is derailleur cage length. I suspect that the m960 (if it was working properly) that you took off is longer than the m952.

    The solution is to use the longer cage version of m952 (SGS) or to use a 11-32T cassette or to prevent the currently fitted m952 from going onto the biggest 34T sprocket.

    You took the right steps regarding setting up the B-screw the first time round. It is only meant to be used to prevent the upper pulley wheel from hitting the sprockets and not for chain tensioning.

    Gear shifting works best when the upper pulley wheel is close but not touching the sprockets so that means the minimum possible adjustment of the B-screw. If the B-screw is screwed all the way in then less chain is wrapped around each sprocket and will cause mis-shifts.

  3. #3
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    Standard, thanks, but they are both the SGS long cage versions. They are identical in every way except the rapid rise has some newer pulley's on it. For the 952 I set it up and rode around a little last night and everything seemed to work perfectly, I'm real anal about mechanical stuff and I could spend hours or even days adjusting and thinking about it. Maybe the older 952 is just a little stiff from sitting around covered with dirt for so long. When I replaced it a few years ago it was covered with dirt and dust and I just threw it in a box, I probably should disassemble it and clean and re-grease it. One thing I am learning is that I might like the shimano top normal shifting better than the rapid rise low normal, the shifts seem to be a little harder but it shifts solid with no skipping noise. The first thing I noticed last year about the rapid rise derailleurs, I have an XT on HT and an XTR on FS, is that they are very hard to adjust on the stand and then you need to readjust cable tension when riding. I'm wondering since the rapid rise are so finicky concerning cable tension I'll bet the movement of a FS bike always keeps them out of adjustment? That's my theory, for now anyway.
    If you're not falling, then you're not riding fast enough!
    Ibex Asta Pro SE & Giant XTC-2

  4. #4
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    Not sure if this will help but I've noticed a serious problem with B-screw adjustment on every Shimano RD I've used (LX, XT, 08XT). The problem is this: There is a horshue clip on that holds the pivot assembly in place (where the RD bolts to the hanger) and it makes contact with the hanger before the RD is actually against the hanger. I ground this piece down and it helped some but I think the piece with the "notch" in it needs to be deeper to "capture" the hanger and allow the B screw to do it's job and Rotate the RD away fro the cassette. I know this sounds confusing but if you look at the RD with it off the bike, I bet you'll see what I kean.


    Red

  5. #5
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    red, I hear what you're saying and at first glance I thought the same thing when I first started looking at the shimano derailleurs, but I'm pretty sure the derailleur is supposed to tighten on the inner point which is what it pivots around. Then the outer section does not lock up on the hanger because it's pivoting for the b-screw tension which is transfered to it through the internal spring. I'm 99% sure this is the case because I disassembled and saw how it worked on my m960 that got wrapped around the cassette and was bent to hell. I could actually rebuild the m960 with the m952 cage and b-screw components, but I think I might like the shifting action of the top normal 952 better.
    If you're not falling, then you're not riding fast enough!
    Ibex Asta Pro SE & Giant XTC-2

  6. #6
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    baraant, are you sure that the crankset is a standard 44/32/22T ? The reason I ask is because I just doublechecked my book of measurements to confirm the capacities of the derailleurs.

    m960 sgs: capacity=43T, min=11T, max=34T, front chainring difference=22T

    m952 sgs: capacity=43T, min=11T, max=34T, front chainring difference=22T


    capacity required by your derailleur based on your chosen crankset and cassette:

    (44-22) + (34-11) = 45T


    I know that with extra tweaking some bikes with a 44/32/22T crankset and 11-34T can be made to work, but the number (45T) does tell you that the derailleurs (m960 and m952) do not have have enough chain slack take up capability to work properly - which is why you had to fully screw in the B-screw.

  7. #7
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    standard, good advice. I know the front is 44-32-22, I'm going to have to check the rear to see if it is a 34 it might be a 32. Is there any other shimano derailleur that would handle this better? I thought the long cage version was made for a greater capacity.
    If you're not falling, then you're not riding fast enough!
    Ibex Asta Pro SE & Giant XTC-2

  8. #8
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    Standard, yes I do have a 34T on the cassette. Is there any shimano derailleur that woould handle this set up or is it just the nature of the beast?
    If you're not falling, then you're not riding fast enough!
    Ibex Asta Pro SE & Giant XTC-2

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