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calling all math wizards....stem rise question(9 posts)

calling all math wizards....stem rise questionjoeblack
Apr 5, 2003 2:24 PM
What the difference in height between:

a 100mm stem with a 95 degree rise
and
a 110mm stem with a 80 degree rise
?
(what to know how much in spacers I'll need)
What's your head tube angle?SamDC
Apr 5, 2003 3:13 PM
The calculation's also gonna depend on what your head tube angle... not by much, probably, but if you want to be exact as possible, you might want to provide that too.

If you wanna try the calculation yourself, here's how to do it (I think... I don't have paper and pencil to confirm the logic, but it seems right):

1) Take the difference between your head tube angle and the rise angle of the stem (the difference should be positive and less than 90 degrees). This angle, in essence, is the angle the stem makes with a horizontal.

2) Figure out the SINE of that angle (use a calculator or look it up in a table).

3) Multiply the SINE of the angle and the length of the stem. You now have your rise!

Hope that helped!
Clarify...SamDC
Apr 5, 2003 3:16 PM
When I said that it won't matter much if you knew your head tube angle, I meant that you can assume it's about 73.5 degrees if you don't know the exact number (it could range between 73 and 74, but 0.5 degree ain't gonna matter that much).

Sorry for the confusion.
What's your head tube angle?joeblack
Apr 5, 2003 3:17 PM
72.5
This is what I got...SamDC
Apr 5, 2003 3:23 PM
For the first stem:
38.26mm

For the second:
14.36mm
This is what I got...joeblack
Apr 5, 2003 3:58 PM
thanks.

How are you the first part of the math?
Is it 72.5 - (-10) ?
http://www.habcycles.com/fitting.html (nm)geeker
Apr 5, 2003 7:37 PM
The change in stem will affect both vert and horizB2
Apr 6, 2003 7:48 AM
With a head tube angle of 72.5 degrees, the following DIFFERENCE will occur when swapping stems:

100 x 95 stem will be 2.39cm higher (vert) and 1.66cm shorter (horiz) than the 110 x 80 stem.

Bryan
apples to oranges....C-40
Apr 6, 2003 11:45 AM
why would you compare a 100mm 95 degree stem to a 110 x 80 degree?

The 100mm is shorter to start with and even shorter due to the additional angle. If the stems are not the same model, the steering clamp height may also be different, resulting in a error in the calculations.

Most folks want to raise the bars while maintaining approximately the same horizontal length. These two stems have significantly different horizontal length.

For the mathematically challenged, the formula for the horizontal length is: length x cosine(stem angle - head tube angle). The rise is length x sine(stem angle - head tube angle).