In article , Chris Taylor wrote:

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.

Trying to remember my high school trigonometry:

Assume that original seat tube length = 22 inches ~ 55.9 cm.
Seat tube angle = 73.5 degrees.
Consider a right angle triangle, where the seat tube represents the
hypotenuse and the other 2 sides are the x & y axes.
By trigonometry, x ~ 15.87 cm, y ~ 53.58 cm. [*1]

If you make the top tube shorter, this will result in a steeper seat
tube.
e.g. If we shorten the top tube by 1 cm, we get a new value x ~ 14.87
cm, which results in seat tube angle ~ 74.5 degrees. [*2]

Try lengthening the top tube by 1 cm. What are our new values?
new x ~ 16.87
new seat tube angle ~ 72.5 degrees

Through trial and error (it is easier when you are using a spreadsheet
than it is to properly calculate it), to get a seat tube angle of 71
degrees, we need to lengthen the top tube by about 2.6 cm, or roughly an
inch.

The numbers will change, depending on the length of the seat tube.

HTH.

------------
*1 y = [seat tube length] * sin (seat tube angle)
x = [seat tube length] * cos (seat tube angle)

*2 new x = old x - 1 cm
new y = old y (we are letting the new length of the seat tube to fall
wherever trigonometry says)
new seat tube angle = atan ( new x / new y )
------------

--
K.A. Moylan
Canberra, Australia
Ski Club: http://www.cccsc.asn.au
kamoylan at netspeed dot com dot au