BB Height Vs.Drop

Bottom bracket height affects bicycle stability and pedaling clearance, and is an important specification to consider when choosing a frame or otherwise looking at geometry charts. Generally speaking, road and touring bikes have low bottom brackets for stability over the long haul, while track and mountain bikes sit higher for ground and cornering clearance. Confusion may arise when comparing geometry specifications from different manufacturers as some quote bottom bracket height and others quote bottom bracket drop, here we explain the relationship between the two.

Bottom Bracket Drop - Defined as the vertical distance between the wheel axle and the center of the bottom bracket spindle. Bottom bracket drop is a fixed measurement of how far the bottom bracket hangs below the axles, determined by the frame geometry and does not change with tire choice. Convention states that bottom bracket drop is expressed as a positive number when the bottom bracket is below the plane of the wheels as shown above—a bottom bracket drop of zero would be level with the axles, a negative bottom bracket drop would place the bottom bracket above the axles. Bottom bracket drop can be determined by subtracting the bottom bracket height from the radius of the wheel and tire combination.

Bottom Bracket Height - Defined as the vertical distance between the ground and the center of the bottom bracket spindle. This is a relative measurement, in that bottom bracket height is influenced by tire volume. In practice the tire choices available for most frames do not vary enough to drastically alter bottom bracket height, but in the case of cyclocross and mountain frames that can accept a wide variety of tire sizes it is worth considering how your tire choice may affect the quoted bottom bracket height. Bottom bracket height can be determined by subtracting the bottom bracket drop from the radius of the wheel and tire combination.