#1




seat tube angle
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. 
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#2




seat tube angle
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 
#3




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 able is different, there would be a very small adjustment needed. T. 
#4




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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