Sunday, December 27, 2009

Geometry So far

Well . . . There you have it.
The top-tube length doesn't really mean anything it seems. First they are all measured by their true length, and not horizontal from center of head tube to center of seat tube . . . aka, Center to Center. . . . not that that perfect either, but if one BMX frame has a 11" seat tube, and another a 13" seat tube, with identical top tube lengths, then the one with 11" seat tube would have more reach (Cockpit) than the 13". I looked at a lot of geometrys. If the seat-tube was more laid back, then top tube was longer; if the seat angle was steeper, the top tube was shorter. Either way the downtube would be the same length. The down-tube length on a BMX frame would be more helpful in determining the reach of the handlebars.

Also Bottom Bracket height is really interesting on a 24". I found most 24" Bottom Bracket heights between 11-1/2" to 12". A 24x1.75" wheel is not really 24" tall, it's 23.625" or 23-5/8", Half of which is 11.81". So if you have a bottom bracket height of 12", that puts the bottom bracket above the wheel axle, and if you have 11.5, that puts it below the bottom bracket. so this brings another element into play. I chose 11.75, or right about level.
I'm not sure how much this little nuance will make in manualing the bicycle in reality. Mechanically there is a big difference. If the bottom bracket is below the axle of the rear wheel, it is harder to manual, than when it's above.

No comments: