Are you normalizing saddle X-Y as you change crank length to maintain some sort of normalized flexion at the bottom of the pedal stroke? Because saddle position would affect both your angles. You have sort of a multivariate problem, not just a static optimization.

I'm a little confused, though because mentally I see shorter crank length reducing knee flexion. Unless you're measuring the opposite angle I'm thinking of.

This is just me scribbling.

Yes, the saddle moves as the crank length changes, mostly to maintain knee angle at the bottom of the stroke, also to make the same compensation at the forward most point.

I'm not measuring knee flexion in the classic sense, but rather how I describe. The actual angle made by those three points will increase as crank length shortens. I probably shouldn't call it knee flexion, or I should measure it in the current medical manner and change the equation.

What do you think though? Is there a relationship between these angles that could be used to drive crank length choices? I say there is.

You should ask H2Ofun /Frank Day to share their meticulously collected data.

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I think there is, but I'll have to think about it more (and read your angles more carefully). I once started a set of equations that could be used as a "fitter's starting point." E.g. the inputs were some basic measurements and constraints (limb lengths and angle constraints), then it would solve for saddle and bar XY to start at. I *think* I included crank length, but used it as an input rather than something to solve for like you are. Will have to think about it more....

yeah, yeah, I don't think it's perfect or even close as written. I do think those two angles work together to cause restriction at the top of the stroke, knee flexion is more important, and shortening the cranks improves the situation for both.

I'm doing my masters thesis on this topic. I'll send a pm to confer if you like.

Absolutely. Email is coachdave@davidluscan.com