Eccentric Bottom Bracket Location- Mtbr.com
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  1. #1
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    Eccentric Bottom Bracket Location

    I recently converted my Niner SIR9 to singlespeed, and for the first time am experimenting with ideal rotation of the eccentric bottom bracket.

    On my first go-around, I left the BB in the bottom half of the rotation (9-6 o'clock on drive-side) as obstacle clearance isn't a big issue for me and I like how the bike corners with the cranks lowered.

    However, now I'm thinking I should rotate the BB to the top front (ie 1-3 o'clock) to prevent chain tension from slipping as the bike rolls over obstacles.

    What do you think?

  2. #2
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    Quote Originally Posted by Canuckxc
    I recently converted my Niner SIR9 to singlespeed, and for the first time am experimenting with ideal rotation of the eccentric bottom bracket.

    On my first go-around, I left the BB in the bottom half of the rotation (9-6 o'clock on drive-side) as obstacle clearance isn't a big issue for me and I like how the bike corners with the cranks lowered.

    However, now I'm thinking I should rotate the BB to the top front (ie 1-3 o'clock) to prevent chain tension from slipping as the bike rolls over obstacles.

    What do you think?
    Personally I think a proper EBB won't slip so you should place the eccentric wherever you like.

    Over the years I've seen plenty of posts on this forum that ask the question, "To which position should I rotate the eccentric?" and often the reply is, "In the top half so you don't strike your pedals."

    But frame BB shell height varies as does crank length, so that answer is in fact a panacea. For instance, although I employ 195mm cranks (nearly an inch longer than "standard" 175mm arms) and run my eccentric in the lower half of the EBB shell, my pedals are still probably further from the ground while in the 6-o'clock postion than most. That's because my frame was designed with a high BB shell. It was designed around extra length cranks.

    I mention this because if this thread heads off in the typical direction, you might get some replys that suggest you run your eccentric in the top half of the shell when this is actually meaningless advice unless the person offering it knows your frame's BB height, crank length, your riding style and the terrain you traverse.

    I feel so much better getting this off my chest. Thanks.

    --Sparty
    disciplesofdirt.org

    We don't quit riding because we get old.
    We get old because we quit riding.

  3. #3
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    You're bb shouldn't slip/rotate when going over obstacles, regardless of where it is. I assume you are afraid of bashing your ring on a tree or something, and causing your BB to rotate (correct me if i'm interpreting your concern wrong). I've never had that happen on any bike with eccentric bottom brackets. Did you have problems with your BB rotating, when you were running your bike with gears?

  4. #4
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    I have the Niner BioCentric (and love it). I optimized my chain length and BioCentric position together so that I could easily switch between a 17t to up to a 20t cog without changing my chain (and have done so, the throw is amazing on this EBB). For me, the actual position of the EBB is irrelevant, though I know some will argue it affects your seating position and other geometry...I personally have not noticed any fit issues when I switched cogs and rotated the cranks, so there is my $.02.
    I no longer deserve a signature.

  5. #5
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    Quote Originally Posted by Sparticus
    Personally I think a proper EBB won't slip so you should place the eccentric wherever you like.
    -Sparty
    without a doubt

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