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c40 and company... calculating STA? (long and technical)(2 posts)

c40 and company... calculating STA? (long and technical)Frith
Apr 1, 2003 6:53 PM
How should I go about calculating STA?...I've seen ways described that use setback as a measurement. I hesitate to use this one because I'm worried I won't be able to get an accurate setback measurement. Here's what I did instead...tell me if you think it's right. everything is measured c - c.
c=TT=58cm
b=ST=58cm
a=imaginary hypotenous line measured from center of BB to center of (head tube + top tube intersection)=70cm
this forms a triangle with sides a, b, and c.
I found the equation which finds an angle given the lengths of the sides...it looks like this
a^2 = b^2 + c^2 - 2bc cosA ...which rearanged looks like
cosA = (b^2 + c^2 - a^2)/2bc
= (58^2 + 58^2 -70^2) * 2 * 58 * 58
= 0.2717...
when I do an inverse cos on that number I get 74.23 which seems very steep to me... Is there something I'm missing doing it this way? Should I stick to the method that uses set back? if so how do I take an accurate measurement and what's the formula?
stick with setback...C-40
Apr 2, 2003 5:37 AM
The angle that you calculated would be unusually steep for a frame this size. 73 degrees would be much more common.

There is a drawing the includes setback on the Colnago geometry chart at www.trialtir-usa.com. It's the Sc dimension.

cosine A = setback/seat tube, where A is the seat tube angle (STA) and the seat tube is measured from the center of the BB to the center of the top tube.
Setback is the horizontal distance from the center of the BB to the intersection of the TT and ST.

To measure the setback, level the top tube and drop a plumb line from the top tube through the center of the bottom bracket. Mark the center of the BB on the TT and measure from this mark to the center of the seat tube. Divide this distance by the (c-c) seat tube length and you have the cosine of the STA. Then take the inverse cosine to get the STA.

Leveling the top tube only works on a frame which has a uniformly shaped top tube. Some of today's frames have top tube that are odd shaped (like C'dales). With a frame like this, the only thing you can do is place the complete bike on a level surface and hope for the best.