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  1. #1
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    Square Taper BB size question

    I've got a frame with a 68mm shell. Currently chainline is around 50-51mm. I'm switching my M770 cranks to one with a narrower q factor (I think), which is also a square taper (XTR M900). I don't know much about square taper BB's, so just wanted to confirm the sizing.

    If I'm trying to retain the 50-51mm chainline, I would use a 68x113 BB, right? I would be using the ring on the middle mount. If I'm reading Sheldon Brown's chart right, the 113 spindle lenght should give me a 50mm chainline.

    Sheldon Brown's Bottom Bracket Size Database

    Square taper guru's, please confirm. Thanks.

  2. #2
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    Eliminate the guesswork and remove your bottom bracket. It should have the size marked on it. If not, measure it. The chainline should be the same if you keep the same length BB. I think the q factor for the M900 is 5mm less than the M770.

    Also, you may have the option of shifting your cleats (if you use them) towards the outside of your shoes, that will have the same effect as reducing the q factor, possibly by a greater margin than changing the cranks.
    I see hills.

    I want to climb them.

  3. #3
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    Quote Originally Posted by Stevob View Post
    Eliminate the guesswork and remove your bottom bracket. It should have the size marked on it. If not, measure it. The chainline should be the same if you keep the same length BB. I think the q factor for the M900 is 5mm less than the M770.

    Also, you may have the option of shifting your cleats (if you use them) towards the outside of your shoes, that will have the same effect as reducing the q factor, possibly by a greater margin than changing the cranks.
    ?? The M770, is an ext. BB crankset. But I think I have it figured out. The 68x113 should do it.

    And thanks for the tip on the cleats. Hadn't thought of that.

  4. #4
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    Quote Originally Posted by 2:01 View Post
    ?? The M770, is an ext. BB crankset. But I think I have it figured out. The 68x113 should do it.

    And thanks for the tip on the cleats. Hadn't thought of that.
    No prob. When you mentioned "...which is also a square taper" in your OP, I took that to mean the M770 was also a ST crankset. I've never used the M770. My apologies for the misunderstanding.
    I see hills.

    I want to climb them.

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