Bottom Bracket Drop
What is Bottom Bracket Drop?
The vertical distance from an imaginary line intersecting the axles of each wheel down to the center of the bottom bracket. When comparing bikes with the same wheel diameter, a smaller bottom bracket drop will result in a higher Bottom Bracket Height (measured from the ground).
When evaluating Bottom Bracket Drop, it’s important to consider Wheel Size as well. Bikes with smaller diameter wheels require less Bottom Bracket Drop to maintain a similar rider position relative to the ground.
For this reason, Bottom Bracket Height can be a more useful measure since it takes a bike’s wheel size and tire width into consideration.
Why does it matter?
More bottom bracket drop may improve handling by lowering the bike’s center of gravity, thus improving stability and providing more responsive rear-end handling (steering with body weight). This also places the saddle closer to the ground which makes mounting/dismounting easier and can be an advantage for commuters who make frequent stops in traffic.
For applications where pedal strikes are anticipated (mountain biking/crit racing/track racing/cyclocross) a higher bottom bracket is desirable to increase ground clearance.
Some bike designs will adjust Bottom Bracket Drop proportional to anticipated Crank Length. For example, a very large bike size may have less bottom bracket drop so there is sufficient Pedal Spindle to Ground Clearance.