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  1. #1
    Senior Member 1jacktripper's Avatar
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    Geometry conversion from compact sloping frame

    Hi everyone,

    Not sure if I'm in the right forum. If not, I apologize, and I'd appreciate directing me to the right forum.

    I am wondering about the following: if a compact sloping frame has a 42cm seat-tube length, I imagine it does not automatically mean I'll need a 42cm size regular frame, correct? By "regular", I meant non-sloping traditional horizontal frame.

    Follow-on question: how do I, if it's possible, convert the measurements of such a compact sloping frame into a regular frame? That is, can the geometry of a compact sloping frame tell me what geometry I'd need in a regular traditional horizontal frame in order to achieve similar fit?

    Thanks very much!

    Jack

  2. #2
    Decrepit Member Scooper's Avatar
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    Giant has a brief treatise on relating bikes with compact geometry to bikes with traditional geometry HERE.

    The Giant TCR Advanced Team compact geometry is HERE.

    Hope this helps.
    - Stan

  3. #3
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    Seat tube length and top tube slope are incidental products of a design aesthetic and/or the need for standover clearance- they don't have an effect on the geometry (ht/st angles, etc) of the frame at all.
    To compare your compact Giant (ugh) with a traditional looking road frame (bonus ) measure horizontally from the top of the stem to the seatpost.
    happy bike hunting!
    Last edited by Live Wire; 06-18-08 at 09:38 PM.

  4. #4
    Senior Member 1jacktripper's Avatar
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    Quote Originally Posted by Scooper View Post
    Giant has a brief treatise on relating bikes with compact geometry to bikes with traditional geometry HERE.

    The Giant TCR Advanced Team compact geometry is HERE.

    Hope this helps.

    Thanks, Stan. This is very helpful!

  5. #5
    Senior Member 1jacktripper's Avatar
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    Quote Originally Posted by Live Wire View Post
    Seat tube length and top tube slope are incidental products of a design aesthetic and/or the need for standover clearance- they don't have an effect on the geometry (ht/st angles, etc) of the frame at all.
    To compare your compact Giant (ugh) with a traditional looking road frame (bonus ) measure horizontally from the top of the stem to the seatpost.
    happy bike hunting!
    Thanks, Chauncey! I mentioned the seat tube length because I have a shorter-than-the-average-Joe legs. But from what you are saying, it sounds like I just need to measure from top of the stem to the [top or junction of?] seatpost?

    Great website, by the way!

  6. #6
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    Quote Originally Posted by 1jacktripper View Post
    Thanks, Chauncey! I mentioned the seat tube length because I have a shorter-than-the-average-Joe legs. But from what you are saying, it sounds like I just need to measure from top of the stem to the [top or junction of?] seatpost?

    Great website, by the way!
    Jack,
    Thanks.
    Yes, measure from the top of the stem(the bolt that goes in the steerer) to the center of the seatpost while holding the tape horizontally. This will give you the "effective" top tube measurement regardless of the tt slope. This is also the way most manufacturers measure tt length and the main measurement a custom builder uses to tailor a frame to fit.

  7. #7
    Senior Member 1jacktripper's Avatar
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    Hi,

    Not sure if this is going to come out right, but I'd greatly appreciate any help. Also, I am wondering if I'm posting on the wrong sub-forum here, so much apologies if that is the case.

    I currently have a compact frame with the measurements as attached, and it fits me really well, but I am wondering if such measurements would translate to the 49cm measurement (for traditional/horizontal toptube geometry) in the second page of the attachment.

    If not the 49cm, maybe the 50cm?

    Or is it not as easy as I'm making it out to be?

    Thanks much!

    Current frame geometry:



    New frame geometry:

    Last edited by 1jacktripper; 07-02-08 at 01:15 PM. Reason: Adding Image files

  8. #8
    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by 1jacktripper View Post
    Hi,

    Not sure if this is going to come out right, but I'd greatly appreciate any help. Also, I am wondering if I'm posting on the wrong sub-forum here, so much apologies if that is the case.

    I currently have a compact frame with the measurements as attached, and it fits me really well, but I am wondering if such measurements would translate to the 49cm measurement (for traditional/horizontal toptube geometry) in the second page of the attachment.

    If not the 49cm, maybe the 50cm?

    Or is it not as easy as I'm making it out to be?

    Thanks much!
    Maybe this will help.

    For the drawing on page 1 of your attachment, it appears the manufacturer is using the intersection of the top tube and the head tube (instead of the top of the handlebar stem) as the datum for determining the location to measure horizontally to the extended seat tube centerline to derive the virtual frame size.

    I've marked up your drawing, and using a Gerber Variable Scale, measured what would be the center-to-center dimension of the seat tube (from the center of the bottom bracket shell to the intersection of the center of the virtual horizontal top tube and the center of a virtual extended seat tube. That dimension is 51.8cm.

    If you used the top of the handlebar stem as the datum, the intersection of a virtual horizontal top tube with the centerline of an extended virtual seat tube would be 60cm.

    If you look at the drawing, you'll see that this manufacturer drew the virtual top tube c-to-c measurement as 534mm, or 53.4cm, and that he used the head tube/top tube intersection for the datum.

    - Stan

  9. #9
    Senior Member 1jacktripper's Avatar
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    Quote Originally Posted by Scooper View Post
    Maybe this will help.
    ...

    I've marked up your drawing, and using a Gerber Variable Scale, measured what would be the center-to-center dimension of the seat tube (from the center of the bottom bracket shell to the intersection of the center of the virtual horizontal top tube and the center of a virtual extended seat tube. That dimension is 51.8cm.

    If you used the top of the handlebar stem as the datum, the intersection of a virtual horizontal top tube with the centerline of an extended virtual seat tube would be 60cm.

    ...
    Stan,

    Thanks much! If I am understanding you, it sounds like you are saying that I should be looking for a farme size with a seat tube measurement of 51.8 cm and top tube 60 cm? Hmm...if that is the case, then I guess none of the options in the new frame geometry would be appropriate then. Bummer...

  10. #10
    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by 1jacktripper View Post
    Stan,

    Thanks much! If I am understanding you, it sounds like you are saying that I should be looking for a farme size with a seat tube measurement of 51.8 cm and top tube 60 cm? Hmm...if that is the case, then I guess none of the options in the new frame geometry would be appropriate then. Bummer...
    No, no... Sorry, I guess I wasn't clear enough. You should be looking for a frame with a 51.8cm (or 52cm) seat tube measured center-to-center, and with a top tube of 53.4cm center-to-center.

    Disregard the 60cm dimension; that was just to show you what the c-c seat tube would be if you measured horizontally from the top of the stem instead of from the top tube/head tube junction. It just confuses the issue.
    - Stan

  11. #11
    Senior Member 1jacktripper's Avatar
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    Quote Originally Posted by Scooper View Post
    No, no... Sorry, I guess I wasn't clear enough. You should be looking for a frame with a 51.8cm (or 52cm) seat tube measured center-to-center, and with a top tube of 53.4cm center-to-center.

    Disregard the 60cm dimension; that was just to show you what the c-c seat tube would be if you measured horizontally from the top of the stem instead of from the top tube/head tube junction. It just confuses the issue.
    Got it! Thanks, Stan. That's too bad, then...I saw this nice 49 cm frame on CL. Oh well, at least now I know the more exact spec's. Off to shopping I go!

  12. #12
    Senior Member 1jacktripper's Avatar
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    [Bump]

    I've recently inherited a frame, and I'm wondering if you wouldn't mind sharing with me your thoughts as to whether if my conclusion that the geometry is close enough to what was suggested in this thread.

    The new frame I found has the following geometry center-to-center:
    top tube: 21.5 inches, or 54.6 cm
    seat tube: 20 inches, or 50.8 cm
    bottom tube: 24 inches, or 61 cm
    head tube (tip to tip): 3.5 inches, or 8.9 cm

    The suggestion in this thread so far was to find a top tube with length of 53.4 cm and seat tube of 52. I figure if I play a bit with the stem, bar (perhaps a moustache bar), and seat post length, I can make this new frame work. I'm thinking of converting this new frame into a fixie/townie/commuter (max 15-mile ride), so perhaps the frame doesn't have to be at very dialed-in fit.

    What do you think? Thanks very much!

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