Originally Posted by MichaelW
Looking at frame geometry charts, it seems that all frames from the smallest to the largest, are built the same height off the ground (ie bottom bracket height). There is less than1cm difference between a tiny women's frame and a clydesdale and that distance is set to be correct for a medium frame.
Would it make sense to select a crank size proportionate to the leg, then design a BB height to give safe cornering clearance. This would mean that every rider has their pedal at the same height off the ground, but the BB would vary by several cm depending on their size.
If you look at v small frames, you see riders with the saddle and BB way too high off the ground. Imagine scaling up the smallest size to the largest with a proportionate BB height. (ie increase every dimension by 50%), It goes from 26.6 to 39.9cm
You have touched on the dilemma all frame builders face. Building a traditional road frame with a level top tube you can only lower it until it merges with the down tube and you are left with a tiny head tube. When you have reached this point the only way you can make the seat tube any shorter is to raise the bottom bracket. But as you so rightly point out the last thing a short person needs is a bike with a high bottom bracket.
However now it is acceptable to build a frame with a sloping top tube, building small frames got easier. As for crank length as ‘seely’ mentions in the post after yours; the difference between the longest and shortest length is not that great. Also the modern clipless pedals offer more ground clearance; so yes bottom brackets could be lowered. But to do so lengthens the down tube and the chainstays so you build a frame that is not quite as stiff. So it all really depends on what you are designing the frame for.