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  1. #1
    mtbr member
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    help understanding geometry numbers

    I'm going to be buying a new frame in a few weeks, probably have to order it sight unseen (and certainly without a test ride). I'm pretty average height/build, so I can ride most stock sizes without any trouble, but some geometries just feel better.

    I've currently got a Niner Air 9 which feels long/big to me, especially compared to my Yeti ASR 5, despite a difference in effective TT of only 4mm. It fits fine and I ride it well, but it just feels a bit bigger than I'd like.

    Is there a way I can read geometry charts of the bikes I'm considering and have a reasonable sense as to how the bike will fit/feel? How do things like effective TT, ST angle, and reach translate to fit?
    Jackson
    Waltworks 29er HT (coming soon)
    Yeti ASR5

    Specialized Roubaix
    Cervelo P2

  2. #2
    dru
    dru is offline
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    There's a million threads on this subject if you search. I do believe that you can duplicate the fit between your Yeti and Niner if the TT numbers are so close but this might require swapping stems and seat posts.

    Your seat tube angle affects where your saddle sits relative to the cranks so differences in this angle may be accommodated by choice of post (set back or not) and rail position of saddle in the clamps.

    Different seat tube angles affect the reach to the bars even if the virtual TT length is identical, but reach can be adjusted by stem choice to account for this.

    First off you need to place you saddles in the same place on both bikes relative to the cranks; follow below:

    I place the bikes near a wall and then use a square at a fixed point on the floor lined up with the centerline of the cranks so that the BB shells of both bikes are in the same place. I then measure how far the respective saddles are from the wall. Obviously move the post on the rails and swap posts if need be to make the measurements match.

    Once this is done measure how far away the bars are from the nose of the saddle. Swap stems if need be.

    Measure how high the bars are from the ground. Swap stems, move spacers or switch bars if need be.

    Obviously, if you are moving the bars up or down and back and forth you've got to recheck a few times before spending money on a new stem or bars.

    Doing the above I got both my MTBs identical and got my road bike's seat position to match. The bars on it are stretched out though.

    My 26er has a 650 mm TT, the 29er has 630 mm TT. The former runs a 0 deg. 110mm stem , the latter a 6 deg, 120mm, yet the bar, seat, cranks relationships match. (I run identical bars)

    The seat tube angles on the two bikes are close (72 vs 73) but in actuality differ a bit more since the 29er runs an EBB that puts the cranks way out of line from the seat tube centerline.

    It is so funny when I read about guys saying "I feel like I'm sitting 'in' the bike instead of on top" and are happy about it and somehow think it's because the frame is such and such size or has 29" wheels etc.

    They've very likely changed the relation between seat, cranks, and bars from one bike to the other instead and don't realize it.

    My 29er and 26er feel exactly the same aside from handling differences caused by wheel size.

    My road bike sure felt worlds different when I was riding it with the saddle 4 cm further back than the MTBs.

    Drew
    occasional cyclist

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