One reason most framebuilders use
BB Drop instead of
BB Height (above the ground) is that using BB drop, the drop is part of the frame design which doesn't ever change, but BB height is quite variable depending on wheels and tires. For example changing the wheels on my Super Sport from 27" (630mm BSD) rims to 700c (622mm BSD) lowered my BB height by 4mm not counting the smaller cross-section of my 25c tires compared to the stock 27" x 1-1/4" tires. I use SPD pedals now instead of the OEM rattrap pedals, so the shorter pedal spindle minimizes the impact of the 4mm lower BB.
