# Bicycle Mechanics - bottom bracket drop?

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TimeTravel_0
10-17-10, 09:26 PM
If a new version of a frame is described as having "5mm more bottom bracket drop" (so now 75mm, originally 70mm), does this raise or lower the stand over height? Or would any other calculation or length of the geometry be affected?

Bezalel
10-17-10, 09:32 PM
As long as the seattube angle and frame size stay the same any increase in drop will decrease the standover height by the same amount.

fietsbob
10-18-10, 12:32 AM
drop is the difference between BB axis and the wheel center axis.
it may chance the length of the seat tube as the axis of the BB is one end of the seat tube length spec.

standover in the center of a sloping top tube is a different measurement..

TimeTravel_0
10-18-10, 09:00 AM
I do not understand your post.

HillRider
10-18-10, 09:12 AM
Fietsbob's definition of bottom bracket drop is correct. It's the vertical distance between the center of the bottom bracket and a line connecting the two wheel hub centers. For a fixed seat tube angle and frame size, if the bb drop is greater, the seattube must be set lower so the standover height will decrease.

He then digressed into sloping top tube frames which really isn't germain to your question.

fietsbob
10-18-10, 09:26 AM
if the fork is substituted , like a short fork for a suspension fork, the BB drop will increase.
bottom pedal gets closer to the ground at BDC/ 6:00.

and the head tube of the frame gets steeper .. that changes handling,
fork rake needs to change then to make up for that..

FBinNY
10-18-10, 09:41 AM
Think of the bottom bracket drop as a way to measure the height of the bottom bracket off the ground. Often complete bikes spec the bottom bracket height, but that can vary slightly depending on the cross section of the tires used.

Specifying the drop from a line at the height of the axles gives a measure that's independent of the tires. ----- BB drop + BB height from ground = Wheel radius (assuming both wheels are the same)