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seat tube angle



 
 
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  #1  
Old November 18th 11, 12:42 PM posted to aus.bicycle
Chris Taylor[_3_]
external usenet poster
 
Posts: 12
Default 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  
Old November 18th 11, 04:00 PM posted to aus.bicycle
K.A. Moylan
external usenet poster
 
Posts: 38
Default 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  
Old November 19th 11, 02:59 AM posted to aus.bicycle
Tomasso[_8_]
external usenet poster
 
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 able is different, there would be a
very small adjustment needed.

T.


  #4  
Old November 19th 11, 03:01 AM posted to aus.bicycle
Tomasso[_8_]
external usenet poster
 
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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