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  1. #1
    mtbr member
    Join Date
    Jul 2004

    Settling road vs mountain external BB (Shimano)

    Or maybe not. But I see this question coming up every now and then and I heard different answers from different people. A guy at local bike store (big national chain starts with "P" and ends with "e") said "no difference" with straight face. He proceeded to say he orders road BB for everything. Mmm, ok... So I got a hold of XT BB and road (SM-FC4500, Tiagra) BB and did some comparisons. Disclaimer - I have no road bike, so I don't know if Tiagra is a decent grade or not.

    In the following comparison pictures, left is XT, right is Tiagra. Road cups are slightly thicker than those for mountain.

    Right cups:

    Left cups:

    XT inner cover:

    Tiagra inner cover:

    Left 2 are XT, right 2 are Tiagra - XT has more material inside the cups where Tiagra is CNC'ed deeply.

    Both XT, left is right cup and right cup is in left (sorry). Obviously, right cup has more threads - therefore spacers are intended to go onto right (drive) cup, I think.

    So my conclusions:

    1) Rroad BBs, in Shimano's case, are meant to go onto 68mm ~ 70mm shells. Any longer (i.e., 73mm shells like most mountain frames), and inner cover will NOT seal one or both side of the BB since the road inner cover itself is about 5mm shorter than that of mountain.

    2) If you want to use road BB for 73mm shell, then you will have to get a hold of inner cover meant for mountain, else your BB will get water/mud and will be destroyed.

    3) Right cup on mountain BB has about 2.5 more pitches. Therefore, BB spacers (2mm, 2.5mm) are meant to go onto the right cup by design. Putting spacers on left could, in theory, compromise the BB integrity.

    Or maybe not?

  2. #2
    Appalachian Singletrack'n
    Reputation: Endomaniac's Avatar
    Join Date
    Jan 2004
    Thanks for this, I have always wondered the difference and what compatabillity there realy is.

  3. #3
    mtbr member
    Join Date
    Aug 2007
    I think there is a 2.5mm difference in the external thickness of both of the cups between road and mountain. This means that if you install a Road-BB on a 68mm wide BB shell (road and some older MTBs) you will have the same overall width as a MTB-BB on a 73mm wide shell. I did a quick measurement and this seems to be the case. So this means that a Road-BB would fit a MTB _if_ it had a 68mm shell, but if you tried it on a 73mm wide shell the crank spindle would be 5mm too short. Been looking for some Shimano tech docs on this but not seen any yet.

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