Originally Posted by
urbanknight
I respectfully disagree. You measure saddle height from the BB to whereever your butt hits the saddle. Any other method is wrong, because setback is a factor, even on a non-setback post.
In the traditional bike fitting sense, estimation of the saddle height is as measured along the seat tube. Of course measuring directly works, too, but that doesn't mean it is the only way.
Originally Posted by
urbanknight
As for the equasion, you're dealing with an arc. You want the saddle position to move in an arc in order to retain the same distance from the BB, but sliding it forward in the post moves it linearly, so you have to correct for the variance. The reason the original seat height plays a factor is because the linear movement makes a different outcome in arc degrees depending on how long it is. Seat tube angle is not needed because the distance moved will determine the angle CHANGE, regardless of starting angle.
I re-looked at my analysis and your points here are correct. So my equation is not accurate; everyone, please ignore it.
Originally Posted by
urbanknight
In reality, it's easier to just measure the seat height before you move it, then adjust it to the same height after the move. I promise you, if the OP does it, there will be less than 1mm to move up.
I agree with the first part of this statement. But whatever the result, the difference shouldn't make a significant difference on the fit.