Thread: Frame Geometry
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Old 08-18-08, 07:52 AM
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Timmi
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Bikes: Turconi, made by Vanni Losa, and a roster of ever-changing other bikes.

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center of gravity

Originally Posted by Nessism
Seems obvious to me, and obviously you, but the old urban legend that a low bottom bracket "adds stability" is what a lot of people believe.

One of my favorite experiments to make my point is to tell people to take a yard stick and balance it with their palm underneath. After they get the hang of it, I tell them to try the same thing with a 12" ruler; the yard stick is much easier to balance because the CG if further away from the pivot point. This same thing applies to bikes; a high CG is more stable, but harder to turn. Truth be known though, the total range of effective BB heights on bikes is quite narrow so as to make this discussion somewhat unimportant. Interesting internet banter but not overly significant in the real world.

We must be careful when making such interpretations.
A stick (or any other object for that matter), is no harder to balance no matter what the length (height). It is harder to balance for a HUMAN. What I am saying, is that when you take into account our neuro-motricity responses, fine motor movements and larger motor movements in relation to reflexes (in delay and amplitude), a stick at a certain length is easier to balance, merely because it is easier to move our hand around several inches at a time than it is millimeters at a time.

However, when you are steering the handlebars, we are no longer concerned with fine motor movements such as in trying to balance a pencil or ruler - the movements are slower and more controlled (unless you feel like flying over your handlebars). We must think of the effect on lateral movement of the contact patch under the bicycle in relation to the vertical CG. What is the sideways deviation off a straight course of the front wheel's contact patch in relation to the CG height? And how fast and by how much do we need to move our arms to turn the handlebars. If you have deviation data, and divide it by the CG height, you can derive a probable percentage difference between two BB heights. I don't have that data, so this is all I'll say for now. But I suspect that what we are looking at is more like comparing two yardsticks in slightly different lengths, and definitely nothing of the amplitude of what was suggested. And yes, in this case, steering geometry definitely plays a very large factor.

Still haven't found my "dream geometry" btw. LOL ;-)
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