Shimano Alfine Crank on a MTB- Mtbr.com
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  1. #1
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    New question here. Shimano Alfine Crank on a MTB

    Hello everyone,

    I was thinking to converting my 26" mountainbike to a single gear on the crankset, and for that I was thinking of installing the Alfine FC-S501. But in the description it says "not for MTB frames". Has anybody had any experience with this crank on a mountainbike? Is the new bottom bracket a problem on a mtb frame?

    Thanks in advance

  2. #2
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    Somewhere in its spec it will have a chainline figure.

    If your mtb had a threaded BB of 68 or 73mm, then the usual chainline from the middle ring is around 48-50mm (+/- depending on spacers used).

    The Alfine crankset is probably designed to work with the Alfine hub which has a chainline of either about 42mm or 48mm depending on which sprocket you use.

    However on a singlespeed if you are using a cassette type hub, your chainline can be whatever you want, so I don't see a problem there.

    If you are using a freewheel, then you need to know its actual chainline. It can be adjusted slightly out with some thin spacers behind the freewheel, but if you need to take it closer to the centreline you'll need some washers on the driver side axle and use thinner ones on the other side, plus adjust the rim so it is still on the centreline.

    The main thing on a singlespeed to is to be sure you have a straight chainline, otherwise your are likely to suffer chain derailments.
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  3. #3
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    I posted on singlespeed because i thought the chances were high that someone had done some conversions already, i am keeping the rear derailleur and 8spd cassette, only converting to single crank for simplicity as it will be my daily commuter. Also, is it hard to install yourself?

  4. #4
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    It could be because the crank has low q factor. The q factor is the amount of "bend" in the crankarm. Sheldon Brown explained it:


    " Tread ("Q Factor")

    The tread, or "Q factor" of a crank set is the horizontal width of the cranks, measured from where the pedals screw in. The wider the tread, the farther apart your feet will be. It is generally considered a good idea to keep the tread fairly narrow. There are three main reasons for this:

    The hip joint is optimized for walking, and in normal walking the footsteps are pretty much in line, with little or no "tread."
    For standing pedaling, the farther out the pedals are from the centerline, the harder you have to pull on the handlbar to counterbalance the tendency of the pedaling force to tip the bike sideways.
    The wider the tread, the higher the bottom bracket needs to be to prevent clipping a pedal while pedaling through a turn.

    Older bikes were generally designed to keep tread to a minimum, but starting in the late 1970s there has been a trend to wider tread, for a variety of reasons:

    The popularity of triple-chainwheel cranksets has moved the right side outward.
    Front derailers designed for triple-chainwheels have a more 3-dimensional shape to the derailer cage, which requires more clearance between the large chainring and the right crank.
    Mountain bikes have wider-spaced chainstays for tire clearance, which requires moving the chainwheels outward so they won't hit the chainstays.
    Newer bikes with more sprockets in back move the chainline outboard."

    Simply put, theres a big difference between road bike chainstays and mtn bike chainstays. Some cranks won't clear the wider chainstays mtn bikes are built with so we can have those wider tires.

  5. #5
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    Thanks, that is another point which I hadn't thought about! I guess I will measure my frame against the dimensions of the Alfine crankset or better yet, order it somewhere where i can return it with no hassle.

    Thanks to both for the very useful info!

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