Framebuilders - Frame Size and Tube Angles

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Stickney
07-29-10, 10:48 AM
I am not a frame builder, but realized that perhaps this was a better forum than the road bike area to pose this question.

I have a question on comparing different frames' geometry -- would appreciate some guidance.

I have a bit of confusion in how to analyze top tube length in relation to seat tube and head tube angles. Here are three frames, what differences in fit would I need to take into consideration for each frame? Any guidance/explanation on how to analyze these variable would be really appreciated. Thanks!

Frame 1:
Top Tube = 59 cm
Seat Tube Angle = 72.5
Head Tube Angle = 73

Frame 2:
Top Tube = 59 cm
Seat Tube Angle = 73
Head Tube Angle = 73.5

Frame 3:
Top Tube = 58 cm
Seat Tube Angle = 72.5
Head Tube Angle = 72

NoReg
07-29-10, 06:46 PM
On my frame size the difference between 72 in the seat and 73 is 1/4 inch, if I recall correctly. So the seat tube angles are pretty much a wash since they could be adjusted out if one was starting from median settings. The head tube angles are not going to have much effect on fit here, except to start the bars a tiny fraction fore or aft. As far as non-custom frames are concerned these are all pretty much within a stem size, so you can pretty much ignore them.

So one would expect Frame 2 to have the longest top tube reach, then Frame 1, then Frame 3. Assuming that the seat tube lengths are the same, the seat position/seatpost offset are adjusted to yield the same over the pedals position for each frame. You could adjust away these differences because they are minor in the world of off the peg bikes. That is not the same thing as saying these are minor position adjustments, just that it ought to be possible to work with them.

Get a free 2 D cad program. Learn to do simple sketches, where you center a radius on your BB for seat tube length, then make a line representing ST angle, then repeat from the intersection for top tube angles and lengths. you can import frame photos and lay geometry over them, then compare to your stock drawings by overlaying different frames. You will quickly see what constitute major and minor changes.
Stickney
07-30-10, 07:48 AM
Thanks Peterpan1 -- that is very helpful!