I moved my saddle 6mm forward how much do I need to raise my seatpost ?

does. not. compute.

You would probably lower it if you need to move it at all.

does compute and needs to be raised i just cant remember how much to raise it in ratio to moving foward

Let's see if I remember my geometry well:

z = sqrt[(H x cos (90-STA))^2 + (H x sin (90-STA) + S)^2]

Where z = new saddle height, H = old saddle height measured from the BB as a straight line to the top of the saddle, S = change in setback (use a positive value if moving forward) and STA = seat tube angle

Edit: This equation is inaccurate; see below.

fa63, nice try but seat tube angle is not a factor (remember seat height starts as a straight line from the BB) and you need the original seat height to accurately calculate it. It should come out less than 1mm if done correctly.

Seat tube angle should be a factor, provided that you are using a non-zero setback seatpost, because saddle height is most commonly measured from the BB as a straight line along the seat tube and not as a direct straight line from the BB to the saddle. Also, if all you want to know is the difference between the old and new saddle height, knowing the old saddle height is not necessary (i.e., you can re-arrange the equation in terms of z-H).

Edit: These statements are inaccurate, see below for urbanknight's explanation.

Get on and ride, if it feels off, move it.. repeat until it feels right.. that's how I do it anyway.

theres a misnomer.

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.

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.

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.

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.

I re-looked at my analysis and your points here are correct. So my equation is not accurate; everyone, please ignore it.


I agree with the first part of this statement. But whatever the result, the difference shouldn't make a significant difference on the fit.

Isn't trigonometry fun?

According to mathematical theorems moving the seat back increases the distance to the pedals but not enough(mm) to make a real difference that you would notice. Don't worry about it.

no

2cm. down and to the left.

Fair enough, as long as your measurement method is consistent for you, it will work, but for the purposes of this thread, your method ignores the exact factor the OP is trying to compensate for.

Going back to your original geometry calculations, the long side of a triangle will be somewhere around 600-900mm (seat height) and a second side is 6mm (the distance you're moving the saddle). You know the arc of that long side vs. a straight line is not going to make a big difference in only 6mm.

To the OP,

Give me your current saddle height measurement (from the BB to where you sit on the saddle), and I will tell you exactly how much your saddle height should change (when geometry / trigonometry fails me, I resort to AutoCAD )

I just did this. I moved forward 5mm and it was 1mm or less I needed to raise my saddle.

Smart man. Do let me know the results. I'm curious.

decided to move it 1 mm and see how it feels. I am a podiatrist I only need to count to 10 (more or less is a problem)

Hey!
Stop dragging logic into a perfectly pointless discussion.