Dear George:
The seat tube size gives you the relative postion of saddle to bottom bracket-crank set- pedal. You then know whether you can get a comfortable leg extension with a reasonably sized seat post. BB drop and standover clearance are somewhat related, but can vary between two frames with the same seat tube length. I do agree with Harvey Sachs that top tube length and handlebar position are the primary determinants of comfortable "fit", but everyone has different levels of flexibility. I've owned and ridden frames with seat tubes from 60 to 65 cm, and top tubes from 56 to 61cm, and was able to get a reasonable level of comfort with all of them. But aesthetically, I want one fist plus two fingers of seat post showing, and a 90 to 110 mm stem. That "looks right", and the bike seems to handle better without extreme compromises to get it to fit. For me, then, seat tube should be 62 to 63 cm and top tube 58 to 59 cm. BB drop affects how the bike handles, but not too much how I fit on it. Lower is always better for me, since I don't race crits and don't have to pedal thru corners. Standover height I almost never check, but I have ridden plenty of bikes that gave me a little tickle as I straddle the top tube. You just have to remember to be careful getting off. Haven't done myself a mischief yet, knock on wood.
Tom Adams, Shrewsbury NJ
GCJ <p2vp26na@intergate.com> wrote: This may seem a naive question asked from the point of view of a consumer not a designer-builder. Ive been following this discussion and I keep wondering why the seat tube measurement is relevant at all if the ultimate goal is a particular standover height of the top of the top tube to fit a riders crotch measurement from the ground, or whatever height has been found comfortable from experience. Why would a measurement from the ground to the top of the top tube not be a more meaningful measure with which to state frame size? Are frames designed working backward from a desired height above ground, factoring in wheel with tire diameter, fork dimensions, desired bottom bracket to ground, etc. to arrive at the desired height? With a slanted top tube it would seem that a virtual top tube would have to be assumed for determining size. Why then would it matter what the seat tube dimension was or was called.
Sages have said that there are no dumb questions, only dumb answers. I ask in ignorance and hope to be enlightened.
George Jones Dallas, TX
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