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Old 12-21-2019, 06:29 AM
marciero marciero is offline
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Originally Posted by giordana93 View Post
I respectfully disagree. OP asked whether seat height changes based on sta--when maintaining same reach, drop, and setback. And the answer is no (0) because saddle position--where you butt sits--depends on both height and setback, x and y from the bottom bracket. These numbers are independent of the seat tube angle. Your calculation, whereby seat height should drop with slacker angles, is correct if you do not maintain the same setback (x goes down as y goes up), but if you keep the setback the same, the height should remain the same. The position of the saddle rails on the post will move (forward if the frame is getting slacker) and indeed you may need a seatpost with more or less setback to get it in the right spot, but the height relative to the bb should not.
My calculation for drop in height does in fact assume the same setback. The point is that to maintain the "same height, drop, and reach" you do need to raise the seatpost, as well as slide the saddle on the rails. In the diagram, in order to keep the saddle at point A you need to raise the seatpost by an amount delta=L2-L1 and slide the saddle forward. If you dont do that, and only slide the saddle forward to achieve the same setback, the vertical height will drop by h and the distance to the bottom bracket will change by a bit more
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Last edited by marciero; 12-21-2019 at 06:58 AM.
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Old 12-21-2019, 11:34 AM
giordana93 giordana93 is offline
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agreed as long as we don't confuse "raise the seatpost" with change the seat height.

In a real world bike fit we aren't just pushing a fixed tube with an anchored seatpost that constitutes a fixed length line segment. When setting up a new frame (with possibly longer or shorter seat tube depending on sloping top tube, seat lug design, etc) you put the seat at the same height and setback relative to the bb as before.

When you write above
With 77.7 saddle height at 75 degrees, I am getting about 78.5 cm. to go to 73 degrees and about 78.9 cm to go to 72 degrees.
you are actually showing how much leg extension increases from the increased setback of a shallower angle if you don't push the saddle forward while maintaining the same "seat height" (vertical measure above bb). Think about it, 78.9 at 72 degrees is waaay longer a leg extension than 77.7/75 because you've raised the saddle over a cm AND increased the setback. 78.9 vs 77.7 isn't the difference in seat height--it's the difference in leg extension resulting from the increased setback of a shallower seat tube and a static seat height and you would have to subtract, not add, the 1.2 cm (lower the seat) to maintain leg extension while increasing the setback because of the shallower angle. Anyway, OP seems to have gone missing for a few and I fear we may be going down a rabbit hole of just us two saying the same thing with different terms
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Old 01-07-2020, 06:38 PM
Old School Old School is offline
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Join Date: Dec 2019
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Originally Posted by scottcw2 View Post
Maybe I should re-phrase the question.

With a saddle height of 77.7 from center of BB to top of saddle and a ST angle of 75*, what should the saddle height be for a 73* ST angle and a 72* angle if I want the same leg extension?
The difference would be 0.0053 of an inch. (seriously)
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