Thread: seat tube angle
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Old November 19th 11, 03:01 AM posted to aus.bicycle
Tomasso[_8_]
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Posts: 33
Default seat tube angle

"Chris Taylor" wrote in message
...
is or has anyone out there got a cad program or able to help me work out
the difference in seat tube angle.
I'm looking at a frame with a 73.5 deg seat tube, if I get the frame with
a 1cm shorter top tube and set the saddle back 1cm what difference in
angle would that make. I'm looking for a frame with a 71deg seat tube.


You need to allow for the head tube. It is at an angle 72 - 74 deg, and not
very long.

Let s = length of seat tube, t = length of top tube, h = length of head
tube, d = length of down tube.

Check whether distance are to centre of tube or end of tube and make an
allowance if end of tube. Sheldon Brown has a long discussion on the
different approaches have existed on the market. Also you need to allow for
a sloping top tube - the angle of interest is "as if" horizontal top tube.

Then use the Cosine Rule.

Your triangle is A = s - h, B = t, C = d. The angle you want is the angle
opposite C (down tube), let's call it c.

Refresher of Cosine Rule:

C^2 = A^2 + B^2 - 2ABcos(c).

Ie, c = arccos( (A^2 + B^2 - C^2)/2AB). Arccos = cos^-1 on some calculators.
You need to choose degrees for Arccos (not radians) or convert.

Example (in cm): s = 52, t = 54, h = 12, d = : ie A = 40, B = 54, C = 57

c = 72.945, ie, 73 deg (almost).

Increase top tube to 55 and

c = 71.77 deg, ie just a bit under 72.

Increase top tube to 56 and

c = 70.61 deg, ie, under 71 deg.

If the head tube angle and seat tube angle are different, there would be a
very small adjustment needed.

T.


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