Join Date: Jan 2009
Bikes: More than 1, but, less than S-1
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A shorter seat tube can provide a smaller frame triangle and less bottom bracket flex. But, at the expense of a longer seatpost, which, other things being equal, will be more flexible than a shorter equivilent, place more stress on the frame and subsequently be slightly weaker than a shorter seatpost. As long as you have more than the "minimum" amount of post insertion, most don't get too concerned about it. The longer post, with it's increase in flex can also provide a slightly more comfortable ride.
If you were enquiring with regard to using a longer or shorter post within the same sized frame, yes, slightly more insertion may be stronger. It depends upon how close you are to the minimum insertion point with the shorter post and how much excess the longer post would provide.
Birth Certificate, Passport, Marriage License Driver's License and Residency Permit all say I'm a Fred. I guess there's no denying it.