Rocky Mountain Microdrive 28T BCD?-
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  1. #1
    mtbr member
    Join Date
    Jun 2019

    Rocky Mountain Microdrive 28T BCD?

    We recently bought a Growler Jr 24 for my daughter which has the 155mm cranks and 28T front chain ring. My daughter is short, and I need to swap out the cranks for shorter, probably 130mm or 127mm cranks depending on what I can find.

    What is the BCD of the Microdrive cranks? I measured with calipers and they look to be 76mm BCD but I want to make sure I'm not missing something.

    Anyone know if it ISO taper or JIS taper? It is a seal cartridge bearing bottom bracket.


  2. #2
    ups and downs
    Reputation: rockyuphill's Avatar
    Join Date
    Nov 2004
    The tricky thing is that the four bolts aren't equally spaced, the space between the close pair is about 52mm and the other two are about 56mm apart. So they are on a 76mm BCD, but not 4 equally spaced holes.

    You're likely going to end up replacing the cranks, chainring and BB to get what you want.
    I'm a member of NSMBA and IMBA Canada

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