Bottom bracket height is something that can vary from bike to bike, even if the frame and fork are the same. The variable is the tires. Not all tires are the same height, which will effect bb height. That is why bb drop is used by manufactures more often that bb height, Bottom bracket drop is the difference between two horizontal lines. One line is between front and rear dropouts. The second line is the center of the bottom bracket. The difference between these two lines is bb drop Patrick http:www.TandemCycleWorks.com