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Quote:

Originally Posted by rpenmanparker (Post 16400403)

... Anyone have a formula for this?...

Yes. You do. You learned it in your high school trig class, no?

Quote:

Originally Posted by Looigi (Post 16401501)

Yes. You do. You learned it in your high school trig class, no?

Not exactly. The formula I came up with was arrived at by assuming the "effective seat tube angle" or seat to BB angle (whatever you want to call it) didn't change with the small shift in the saddle position. Actually it is about 0.33 degree. I just ignored the change. To get it right you have to know the length of the arc between the old seat position and the new seat position. You can approximate that by just using the distance you shifted the saddle horizontally, but it is actually larger than that. The answer doesn't have to be so close anyway. But I can't remember studying how to find any of this exactly, except by RollCNY's method

I think I used to be able to do the math but I don't know.

Now I use the layman's method - I put a tape measure between the BB and the saddle and I rotate forward around the BB axle. As I rotate the tape measure I see that it needs to go up a bit.

Quote:

Originally Posted by carpediemracing (Post 16402738)

Yep. My purpose in doing a calculation was to just get a sense of the size of the correction needed. I wanted to tell whether I needed to worry about it or could just ignore it. When I actually picked up the wrench to do it, I used the tape measure method you mentioned to determine when I got it right.

Quote:

Originally Posted by rpenmanparker (Post 16400403)

I keep looking for the answer to this question, but all I can find is the reverse: the need to move a saddle forward to maintain knee position after raising it. That is not what I am asking. I want to know how much do I have to raise my saddle after moving it forward (for whatever reason). I'm thinking the reach to the crank will have been shortened as the saddle moves more directly over it, not much but some. Anyone have a formula for this? I moved my saddle forward 1 cm.

I did a really quick and dirty approximation that suggests the amount the saddle has to be raised x is about y(cos theta) where y is how much I move the saddle forward and theta is the seat tube angle. Assuming 73 deg I get roughly 0.3 of the distance moved forward (or backward and lowering the saddle). So for my 1 cm change, I would need to raise the saddle about 3 mm. Not only is that a very small amount, but my seat post is stuck, so I will likely ignore it for now. At least until I decide if I like the new knee position. I know the saddle height will affect this, but I will work it out as I go.

Nevertheless, I would be interested in hearing what other folks come up with.

Thanks.

Robert

That would be .3mm of upward movement or to be precise, a pair of new cycling shorts

Quote:

Originally Posted by ursle (Post 16402772)

That would be .3mm of upward movement or to be precise, a pair of new cycling shorts

No, I moved the saddle 1 cm forward or 10 mm. So 0.3 of that is 3 mm. Sorry for the confusion switching units, but I think I did write it correctly ignoring that.

Quote:

Originally Posted by rpenmanparker (Post 16402962)

No, I moved the saddle 1 cm forward or 10 mm. So 0.3 of that is 3 mm. Sorry for the confusion switching units, but I think I did write it correctly ignoring that.

mybad, that's a lot of upward movement, missed it;)

What is your knee angle (goniometer) and length from seat to pedal, and length from pedal to patella (posterior)

Quote:

Originally Posted by Kamau (Post 16405275)

What is your knee angle (goniometer) and length from seat to pedal, and length from pedal to patella (posterior)

I don't know. It's more empirical for me.

Quote:

Originally Posted by rpenmanparker (Post 16405510)

I don't know. It's more empirical for me.

Moving your seat height the same as your saddle will maintain the position of the knee. If the saddle is moved 1cm forward, the saddle must go 1cm higher to maintain knee position.

Picture this exaggerated version where your knee starts at 90 degree angle while seated, pedal frozen in place. Imagine the saddle moved forward all the way to where the knee originally was, now raise the saddle straight upward the exact distance it traveled forward. The knee is now in its original location.

Edit: After drawing it out, I see that this will only work at 90 degree angles. The multiplier isn't a single number.