How Do You Measure A Bike Frame?
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  1. #1
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    Question How Do You Measure a Bike Frame?

    How is a bike frame measured?


    From exactly what point to what point is the measurement to be taken?


    If I'm not mistaken, one point is the center of the bottom bracket or the crank. What is the top point? Is it on the seat tube where an imaginary extending from the center of the top tube intersects it; or is it the top edge of the seat tube; or something else?
    Last edited by TomBrooklyn; 05-17-2013 at 11:12 PM. Reason: spelling correction

  2. #2
    Cranky Old Bastard
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    "When you know absolutely nothing, anyone who knows 1% more than nothing sounds like an expert."

  3. #3
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    Tom I see you are new here. Did you ever hear of something called Google?

    Bike Frame Sizes, Geometry, Angles and All That!
    We just don’t realize the most significant moments of our lives when they’re happening
    Back then I thought “well there'll be other days”
    I didn’t realize that was the only day
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    One.
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  4. #4
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    Seat tube length measurement can vary, based on manufacturer. Most companies will specify either "center to center" or "center to top" or "virtual".

    You're correct about one method. If it's C-C it can be an actual measurement of the length of the seat tube to the top tube intersection. If it's C-T is can be to either the top side of the top tube or to the end of the seat tube.

    If it's virtual, it's as you describe, to an imaginary intersection with a horizontal top tube. The virtual size is useful for comparing brands/models.

  5. #5
    .je
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    Quote Originally Posted by Peter P. View Post
    Seat tube length measurement can vary, based on manufacturer.
    This and other dimensions are tricky to compare between brands. I measured up stack on two bikes: A Trek 1 series, and an Opus.

    The Trek has a head tube dimension of 150mm, but when I measured the tube, it was 130mm, and the remaining 20mm were the upper and lower cups of the external headset. I'd emailed Trek, who told me that the stack is measured to the frame, not cup, which is at odds to what I just measured. Maybe that person wasn't familiar with the external cups, since almost no new bike uses them anymore. I then measured the stack to the frame (not the 10mm cup), and got 545mm (official dim is 555mm). This head tube, and stack include the attached external cups, and don't include the 15mm cone reducer above the headset. Spacers are extra to all that as well, with a 15mm head start.

    The Opus has an official head tube dimension of 140mm, but when I measured the tube, it was 120mm (this bike does not have external cups in the headset, it is all integrated with the head tube), and the remaining 20mm was the cone reducer. I measured the stack to the frame, which was 530mm iirc. Spacers are on top of this 530mm.

    Isn't it enough to drive you nuts?

    This is an example of why comparing stats in a spreadsheet doesn't tell the whole story of how a bike fits you, why you'll always be advised to go try out the bike before buying.
    Last edited by .je; 05-18-2013 at 09:48 AM.

  6. #6
    wim
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    Quote Originally Posted by TomBrooklyn View Post
    How is a bike frame measured?
    If you're asking "how do bike companies arrive at the size numbers for their frames," the answer is "in all the ways you mentioned, plus a few more." This means that, for example, company A's 54 cm bike could be smaller or bigger than company B's 54 cm bike, with company C's 54 cm bike being different from those two. Because of this inconsistency, you can't use nominal frame sizes any longer in establishing your fit when looking at different brands or models.

    What hasn't changed is the fact that bike fit essentially is established by the geometric relationship of three points in space: where you plant your butt, where you put your hands most of the time, and where your feet contact the pedals. If you have established the relationship of these three points in space, you have your fit regardless of bike brand or model.

    Because it's not all that easy to establish that relationship and because of the inconsistent and often confusing ways in which companies size their bicycles, a whole new industry called "bike fitting" has sprung up in recent years. Its practitioners ("fitters" or "professional fitters") will fit you to a bike for a hefty fee.
    Last edited by wim; 05-18-2013 at 07:46 AM.

  7. #7
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    Frame stack and reach, the most important measurements in determining frame fit:

    Technical FAQ: Setting your bikes up identically
    Bike Geometry, Sizing and Fit - Cervélo
    Stack and Reach Design
    Tech Center

    To measure easilly: With the bike standing vertically on a flat surface (floor), measure from the floor vertically up to the center of the BB and up from the floor to the top center of the head tube. Subtract to get the stack. For reach, stand the bike vertically on a flat surface and back the rear wheel against a vertical surface (wall). Measure horizontally from the wall the the center of the bottom bracket and from the wall to the center top of the head tube. Subtract to determine reach.
    ... 'cuz that's how I roll.

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