Originally Posted by galen_52657
Differences in frame seat tube angle through the size range are not just for KOPS. It is also done to keep the fore-aft center of gravity relatively constant through the size range. But, there are also plenty of manufactures who use 73 degree seat tube angle though the whole size range and let people fiddle with seatposts if the want too.
I think associating crank length with leg length just makes sense, but I think it most likely benefits the short riders who have been using cranks that are to long more than tall riders who have been using cranks that are to short.
I know about the wheelbase being held constant and I agree with that, but I maintain that longer cranks only work as well as shorter cranks if the knee at the top of the stroke isn't closer to the trunk.
Steeper seat tube angle helps keep this hip angle relationship. The longer top tube on the smaller frames in turns lets the cyclist rotate his body forward about the BB to make it all work.
KOPS is BS anyway. I think there is an ideal knee to spindle relationship, but it has nothing to do with KOPS. It has to do with how the levers and pivots that make up the leg are oriented around the BB. You rotate the trunk of the cyclist forward about the BB and the ideal position of the knee to spindle is also rotated forward.
Originally Posted by galen_52657
. But, there are also plenty of manufactures who use 73 degree seat tube angle though the whole size range and let people fiddle with seatposts if the want too.
Only LOOK does this. Maybe a couple of manufacturers I don't know about, but the main thrust of the industry is to adjust tube length and STA throughout the range (makes sense too considering there is only a token 5mm spread in crank length, but a huge variation in leg length).
Tall cranks need high forward seat(to maintain hip angle- think "saving your knees" too) with long top tube to compensate for forward rotation. Industry has been doing this for years to help small riders with short legs adapt to 170mm cranks.
Now how are you going to apply a "proportional" crank theory to tall riders when they don't ride their bikes in this position? You can't.
P.S. I agree with you Galen that taller riders would be better served with longer cranks, but making this happen would require some drastic frame changes (not just BB height) that might even change the wheelbase too.