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  1. #1
    kfm
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    track/road frame geometry

    From a Singlespeed & Fixedgear thread:

    i want to design a frame with road geometry. does anyone have specific information as to how geometry is factored (beyond the assumed)? i've looked around the web a bit, & it seems like most frame builders work off of standards that they tweak a little here an there. i'm curious, because otherwise the process seems very abstract. lower bottom bracket, more fork rake, longer stays, looser seat & head tube angles... but how do i figure out how much (given my inseam & size).

    thanks

  2. #2
    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by kfm View Post
    From a Singlespeed & Fixedgear thread:

    i want to design a frame with road geometry. does anyone have specific information as to how geometry is factored (beyond the assumed)? i've looked around the web a bit, & it seems like most frame builders work off of standards that they tweak a little here an there. i'm curious, because otherwise the process seems very abstract. lower bottom bracket, more fork rake, longer stays, looser seat & head tube angles... but how do i figure out how much (given my inseam & size).

    thanks
    Try this: Bicycle Geometry 101

  3. #3
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    Very usefull program! Thank you!

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    If you use a program it will likely deliver certain outputs that are the programer's preferences. Basically just copying a known design. To develop your own preferences you need to examine your relationship to your own bikes; read and examine as much as you can about the bikes others are making; draw as many bikes as you can. Draw bikes by the major makers. Draw bikes from bike shows, try and blend features you would like to explore with baseline features of frames you already own. You want to be able to get a very clear idea of the frame you will build. You should know all the angles measurements, and options you have considered. It may sound like a lot, but over a few weeks or months of research you will enormously increase your understanding. It goes to a whole other level as you build your first frames because you will be forming and joining all the parts and you will have to get all kinds of measurements and make decision as it goes.

  5. #5
    Randomhead
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    if you want to do a fixed with road geometry, you probably still want to have a high bottom bracket.

    There are 3 points where the rider intersects the bike, at the pedals, the seat and the handlebars. Note that the stem and the seatpost/seat setback are all adjustable. So the seat tube angle and length are somewhat arbitrary, as is the top tube length. A lot of the design is determined by aesthetics, and some by practical concerns such as stability and control.

    You probably want to use standard parts, so the seat tube angle and height is determined within a certain range. You probably want to have a "reasonable" length stem, so that determines the top tube length. So, you pick a drop for the bottom bracket height, pick a headtube angle, and things are starting to become more fixed. The fork rake/trail is determined by the headtube angle within a certain range. The minimum chainstay length is determined by the wheel and desired comfort level.

    I barely hinted at rider fit in all of this. The leg dimensions and desired setback/seat post exposure will determine seat tube length, the torso length,arm and desired stem length then determine top tube length. Most successful designs range between 72 degree head tube and 74 degree head tube angle.


    Something like bikecad is very useful for all these decisions. Looking at successful frame designs is also very helpful as Peterpan1 points out.
    Last edited by unterhausen; 02-10-09 at 09:08 PM.

  6. #6
    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by Peterpan1 View Post
    If you use a program it will likely deliver certain outputs that are the programer's preferences. Basically just copying a known design. To develop your own preferences you need to examine your relationship to your own bikes; read and examine as much as you can about the bikes others are making; draw as many bikes as you can. Draw bikes by the major makers. Draw bikes from bike shows, try and blend features you would like to explore with baseline features of frames you already own. You want to be able to get a very clear idea of the frame you will build. You should know all the angles measurements, and options you have considered. It may sound like a lot, but over a few weeks or months of research you will enormously increase your understanding. It goes to a whole other level as you build your first frames because you will be forming and joining all the parts and you will have to get all kinds of measurements and make decision as it goes.
    With this program (Bicycle Geometry 101), you can input known design specifications (i.e. angles, rake, tube lengths) to reproduce a known design and tweak it from there.

  7. #7
    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by unterhausen View Post
    Something like bikecad is very useful for all these decisions. Looking at successful frame designs is also very helpful as Peterpan1 points out.
    BikeCAD is a very nice program, but the free, on-line version is quite limited in what it allows you to do, and the full version is pretty pricey unless you intend to build a lot of frames.

  8. #8
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    Here's a link to an older version of bike cad that lets you print the build specs for free:
    http://javaboutique.internet.com/bikeCAD/
    its not as pretty, but that doesn't really matter does it
    the chainstay and seatstay veiws are really helpful too

  9. #9
    Senior Member Road Fan's Avatar
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    Quote Originally Posted by unterhausen View Post
    if you want to do a fixed with road geometry, you probably still want to have a high bottom bracket.

    There are 3 points where the rider intersects the bike, at the pedals, the seat and the handlebars. Note that the stem and the seatpost/seat setback are all adjustable. So the seat tube angle and length are somewhat arbitrary, as is the top tube length. A lot of the design is determined by aesthetics, and some by practical concerns such as stability and control.

    You probably want to use standard parts, so the seat tube angle and height is determined within a certain range. You probably want to have a "reasonable" length stem, so that determines the top tube length. So, you pick a drop for the bottom bracket height, pick a headtube angle, and things are starting to become more fixed. The fork rake/trail is determined by the headtube angle within a certain range. The minimum chainstay length is determined by the wheel and desired comfort level.

    I barely hinted at rider fit in all of this. The leg dimensions and desired setback/seat post exposure will determine seat tube length, the torso length,arm and desired stem length then determine top tube length. Most successful designs range between 72 degree head tube and 74 degree head tube angle.


    Something like bikecad is very useful for all these decisions. Looking at successful frame designs is also very helpful as Peterpan1 points out.
    Is the classic road bike balance criterion, 40 to 45 % of bike+rider weight on the front, part of the design input?

    Road Fan
    Last edited by Road Fan; 02-21-09 at 09:43 AM.

  10. #10
    Senior Member Road Fan's Avatar
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    I guess the answer is "download the software and RTFM."

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