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daka I ended up putting around with a pen, a pad, and a calculator. Oh, and a CAD program that I've input all my bikes into.
I developed a few formulas and ratios to create a simplified (but probably pretty accurate) way to measure, based off my 620 as the "standard," the ideal chainstay length for riders on other sizes of bikes. There are, of course, many factors that also affect overall rider balance and ride quality, and I am aware of many (and have written on as much).
I have a set sit bone setback number, and that point, along the wheelbase of the 620, split between the front and rear wheel at a 1:5 ratio. That is to say, that longitudinally, from the sit bone point to the rear axle was 16.5% of the wheelbase, and from the sit bone point to the front axle was 83.5% of the wheelbase.
From there I made a formula to calculate the sit bone setback point of a rider of/on a 56cm frame (as you gave as an example). The "bogey" or example bike I used was a 1986 Schwinn Paramount road/race bike. 410mm chainstays, 73° HT angle and 40mm of fork rake. Good, long-range geo, at least in my experience. Well wouldn't you know? That 1:4 ratio reappeared! [Side bar: geo varies so much per road bike/brand, and a slightly shorter top tube would bring those ratio numbers even closer to that of my 620, but for the sake of simplicity, the 1:4 is fine to claim here] Maybe that was a lucky guess--the 56cm size--maybe not! Whatever it is/was, it ended up being pretty much spot on. So as far as neutral balance/rolling goes, a rider meant for a 56cm frame on a 56cm frame, in theory, feels very similar to me, a rider meant for a 65cm frame on a 65cm frame.
Post edited on 1/11/21 to dig down and find frame size : chain stay length numbers that mimic my 65cm/47cm setup all the way down to a 47cm frame. Correcting ratios, recreating formulas, etc. Maybe this is why I was a designer? Eh, I'm just a bit rusty. Getting it right eventually!
More mental food to chew on, but this was/is pretty fun.
