## determining BB drop, with the BB height measurement?

Discuss light weight issues concerning road bikes & parts.
Catagory6
Posts: 399
Joined: Thu Feb 09, 2017 2:36 am
based on the standard 700c wheel with standard 23mm tire, it seems like it should be a very simple calculation.

bsd 622 + tire (23 x 2), multiply by 1/2 for the radius, then subtract the BB Height measurement?

so if the BB Height is 267mm, based on geometry charts, i would do:

622 (bsd) + 46 (23mm tire) = 668 (total wheel/tire diameter)

668 x 1/2 = 334 (total wheel/tire radius)

334 (wtr) - 267 (bbH) = 67 bottom bracket Drop in millimeters

is this correct? assuming that bottom bracket Height measurements are calculated with a standard 23mm tire.

BdaGhisallo
Posts: 2314
Joined: Wed Mar 17, 2004 1:38 pm
You could so a simple measure to confirm your calculations. Subtract the distance from the floor of the BB center from the distance to the wheel axle center.

Catagory6
Posts: 399
Joined: Thu Feb 09, 2017 2:36 am
BdaGhisallo wrote:
Fri Nov 30, 2018 7:05 pm
You could so a simple measure to confirm your calculations. Subtract the distance from the floor of the BB center from the distance to the wheel axle center.
yes, that would make sense, but i'm comparing geometry chart numbers, not real-world measurements. although, that does give me an idea

BdaGhisallo
Posts: 2314
Joined: Wed Mar 17, 2004 1:38 pm
Catagory6 wrote:
Fri Nov 30, 2018 7:23 pm
BdaGhisallo wrote:
Fri Nov 30, 2018 7:05 pm
You could so a simple measure to confirm your calculations. Subtract the distance from the floor of the BB center from the distance to the wheel axle center.
yes, that would make sense, but i'm comparing geometry chart numbers, not real-world measurements. although, that does give me an idea
Wouldn't it be so easy and convenient if all manfacturers simply gave the BB drop number for their frames and forgot all about BB height? There'd be no need to worry about tire size and the like. It would be so simple and clear... I see I have stumbled on the reason why they don't!

Calnago
Posts: 8542
Joined: Sun Nov 07, 2010 9:14 pm
Yes, BB drop is a simple measure that should be given by what it’s name implies (the distance the BB center drops below the axle center). But in its absence, as was just mentioned, the difference between the axle height and the BB center will give that number to you as well, providing you can pinpoint those measurements exactly.
Similarly, trail is a number often given but one of its dependent variables is tire size, or more specifically wheel radius. Therefore to compare between bicycles you need to know what wheel radius is being used and that can vary these days. especially where anything from a skinny 22mm tubular to a fatass 30 something mm clincher on a wide rim may be in use. Always gotta be comparing apples to apples when comparing geo charts etc.
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