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Thread: Fork length/Headset stack height/etc...

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    Default Fork length/Headset stack height/etc...

    I have a stupid question: when I see a frame and want to decide if it fits me, I always miss this piece of the puzzle.
    I know that my colnago's 173mm headtube with a CK headset and a -8deg stem puts the bars at the right height without spacers.
    Now if all frames had external headsets and the same fork length it would be easy: a 173 headtube is perfect, a 163 headtube requires 1cm of spacers, a 153 needs 2cm, etc.

    But to complicate things most new frames have integrated headsets. Should I simply subtract the total stack height of the CK headset to compare?
    Or fork length may change and partially make up for some of the external headset stack height?

    Please educate me...
     

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    Don't forget headtube angle.

    Of course what everyone needs is a stack measurement and a standard for it but it isn't available.

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    You are right, but for simplicity let's assume head tube angles are all 73 degrees.

    I know it's not correct, but headtubes are usually within 0.5/1 degree and are quite short so it should not make a large difference in the resulting height.
     

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    Head tube angle doesn't affect the reach enough to be consequential. 1-2mm in reality. No princess can feel that pea. BB drop changes the head tube length also. Head tube length is a meaningless measurement in a vacuum but the advent of sloping top tubes have people considering head tube length when looking at fit. Back in the good ol' days when top tubes were level and Archibald had teeth seat tube length determined the head tube length and then other variables like fork length and bb drop weren't relevant. I suggest a quick cocktail napkin sketch to determine the fit on a sloper. It's really not that difficult. You know your saddle height. The geo chart should allow for the other variables. Bust out the eighth grade geometry skills. No CAD necessary.

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    Curt,
    what you say is sound. However, I still miss one point: when I look at the typical geometry chart for a stock frame I don't see fork length and often BB drop is also omitted (but is rather easy to take into account once the information is found). So the only data I have is headtube length.

    Example:
    - my Colnago has a 173mm headtube, a CK headset (let say 30mm stack height), no spacers.
    - An imaginary Specialized (or Trek or whatever) has a 165mm headtube and an integrated headset (same top tube length and seat tube angle).

    Am I correct to say that with the Specialtrek I need 173+30-165 = 38mm of spacers to put the bars in the same place as the Colnago? (same stem angle/length)

    (I'm not looking at a half millimeter precision, just close enough)
     

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    Not sure about your equation. At first, I thought you were not considering that the integrated headtube will also include some of the height that you're giving to your headset, but I think you're right. For the relatively same size frame, one with an integrated headtube should be longer than one without, like your Colnago. So if your C is 175, the integrated should be around 190 or so.
     

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    Quote Originally Posted by frenk View Post
    Curt,
    what you say is sound. However, I still miss one point: when I look at the typical geometry chart for a stock frame I don't see fork length and often BB drop is also omitted (but is rather easy to take into account once the information is found). So the only data I have is headtube length.

    Example:
    - my Colnago has a 173mm headtube, a CK headset (let say 30mm stack height), no spacers.
    - An imaginary Specialized (or Trek or whatever) has a 165mm headtube and an integrated headset (same top tube length and seat tube angle).

    Am I correct to say that with the Specialtrek I need 173+30-165 = 38mm of spacers to put the bars in the same place as the Colnago? (same stem angle/length)

    (I'm not looking at a half millimeter precision, just close enough)
    Consider the top of the head tube as a point in space. Add the head set, desired spacers and stem dimensions. Draw a horizontal line through the bars. Draw a horizontal line across the saddle height. Measure the distance between those two lines. There's your bar drop. Where the bottom of the head tube is located is irrelevant for your purposes so fork length and drop aren't critical. Pencil, ruler, protractor and napkin are. Have fun and embrace your inner geometric mind. Bikes with integrated head sets just shouldn't be considered solely on principle.

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    Quote Originally Posted by Curt Goodrich View Post
    Bikes with integrated head sets just shouldn't be considered solely on principle.
    Is this a Curtpedia entry?

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