Road Bike Bottom Bracket Drop

For years brands and all the content around bicycles referred to bottom bracket height.

Road bike bottom bracket drop. The bottom bracket drop is the difference between the height of the wheel axles and the centreline of the crank axle. This is the distance from the ground up to the center of the bottom bracket. Read on to find out how to size up your road bike or mountain bike frame. However bottom bracket height is easier to visualize and so is widely used.

A greater bottom bracket drop lowers your centre of gravity and provides more stability but that needs to be balanced against the need for pedal clearance. To figure out bb drop measure the height from the middle of the bottom bracket and the height to the center of the rear wheel axle. My strong road bike was built with a bb drop of 75. A road bike typically has a bottom bracket drop of approximately 70 74mm.

It is the measurement from the center of the bottom. Contrast this with a typical drop of 55 65cm for popular production cross bikes. There is nothing magical about the bottom bracket height relative to the axle height a bike does not hang from the axles rather it balances upon the tires. When determining the geometry of my own frame i went with a relatively large bottom bracket drop of 70mm resulting in a bottom bracket height with 650b wheels and 2 6 inch tyres of just 290mm.

This measurement has phased away and you will notice that bottom bracket drop bb drop is what it has been replaced with. My rock lobster cross bike was built with a bb drop of 70. The bottom bracket height of a frame is often specified as bb drop because that metric is fixed while the the actual height of the bottom bracket above the ground varies with the tire size. Once a frame is built this measurement is fixed and never varies therefore it is the most accurate.

Power delivery can drop off. Shimano was an early advocate of moving the bearings outside the bottom bracket shell. Road bike bottom bracket standards. An ml sized giant tcr advanced 3 road bike has a bottom bracket drop of 65mm for example while a giant tcx slr 2 cyclocross bike has a 60mm bottom bracket drop across all sizes.

70 is in the normal range for a road bike. Cyclocross bikes earned a higher bottom bracket back in the days when racers competed with clips and toe straps. The thinking being that i want the stability under braking combined with the agility in the corners that this would afford. Bottom bracket drop is the measurement from the bicycle s wheel center to the center of the bottom bracket.