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Thread: Setback, Seat Tube Angle and Length....

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    Default Setback, Seat Tube Angle and Length....

    Is there any general rule of thumb cocerning how much set back you will lose/gain with a changing seat angle? I'm looking at some off-the-rack frames and am trying to envision how my set up will work with the different geo. I know this will vary with the seat height, but I just want to be in the ball park w/o having to resort to trig or drafting it out. My seat sits about 82cm over the BB.
     

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    Default Re: Setback, Seat Tube Angle and Length....

    Yeah, it's called 8th grade math. It's the math of right angles and can be remembered by the acronym SOHCOHTOA.

    Short cuts are lame. Grab that calculator. It's 15 seconds max.

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    Default Re: Setback, Seat Tube Angle and Length....

    Isn't it SOHCAHTOA?
     

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    Default Re: Setback, Seat Tube Angle and Length....

    Quote Originally Posted by Paul View Post
    Isn't it SOHCAHTOA?
    Oops, you are right.

    SOHCAHTOA

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    Default Re: Setback, Seat Tube Angle and Length....

    Quote Originally Posted by Jonathan View Post
    Oops, you are right.

    SOHCAHTOA
    Wasn't that a volcano in the Pacific?
     

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    Default Re: Setback, Seat Tube Angle and Length....

    Hey, I'm a biologist...if it ain't squishy I don't touch it.

    Even easier: let my stats troll take care of it.
     

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    Default Re: Setback, Seat Tube Angle and Length....

    Lewis,

    Trig is both easy and informative, yes?

    82*cos(72) = 25.3
    82*cos(72.5) = 24.6
    82*cos(73) = 24.0
    82*cos(73.5) = 23.3

    Looks like about 6-7 mm per half-degree at 82 cm.

    Cheers,

    Will
    William M. deRosset
    Fort Collins, CO
     

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    Default Re: Setback, Seat Tube Angle and Length....

    Trigonometry Calculator

    here's a trig calculator - side C will be your seat tube length and angle B your seat angle, side A will give the resultant setback of the frame.
    This won't give your saddle behind the BB distance but you'll be able to note the difference in the frame setback which will obviously apply further up at the saddle
     

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    Default Re: Setback, Seat Tube Angle and Length....

    Here's a post for reference where GrantM calls out a roughly 9mm horizontal change for each 1 degree of STA change.

    http://www.velocipedesalon.com/forum...tml#post411567

    So a 73* STA with a 25mm setback post would get you in a similar position as a 74* STA with a 16mm setback post. There's also saddle rail length and position, saddle height, and a general margin of error to deal with. And don't forget to adjust TT length in your comparison if the bikes have different STAs. If you know your current saddle setback (from BB), seatpost setback, and reach it shouldn't be too tough to map that to different off the shelf frames.
     

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    Default Re: Setback, Seat Tube Angle and Length....

    trig is the way to go. Setback 1 = seat height * sin(90 - seat tube angle 1), setback 2 = seat height * sin(90 - seat tube angle 2)
    in fact, the small angle approximation of sin is pretty close for these angles, so you can do this

    delta Setback = seat height * (seat tube angle 1 - seat tube angle 2)/60
    max error is about 3mm for a 900mm seat height and angles between 65-75 degrees

    every device that can post on this board has a calculator that will do either calculation
     

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    Default Re: Setback, Seat Tube Angle and Length....

    General rule of thumb: about a cm/degree at a saddle height for someone about 5'10". Give or take.
     

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    Default Re: Setback, Seat Tube Angle and Length....

    For those without a calculator...

    -g
    Attached Images Attached Images
    EPOst hoc ergo propter hoc

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