Ok I know we have been down this road before with normal sized persons but here we go...

I am striving for what could be called the perfect fit on my road bike... which may or may not mean a custom frame down the road but lets talk performance aero bike fit.

I ride the following bikes...

61 cm cervelo P2K, 75 degree seat tube angle, traditional tt geometry, 58 cm seat tube, 60.1 cm top tube, seat pushed all the way forward. 83.2 cm stand over, 110 mm invert stem.

63 cm trek 2100, 73 degree seat tube, 60.5 cm top tube, seat tube (no value avalable), seat posistioned in the center. 84.5 cm standover 130 mm invert stem

I have found the "perfect" power wise seat tube angle on my road bike, saddle posistion and seat tube "angle" (73 degree standard road bike). We are then looking at two factors, seat tube length and top tube length. The main difference between the tt bike and the road bike are rider posistion so lets compare simular posistions, the arms 90 degrees to body, stretched out posistion (i.e. aero bars vs 90 degree hands in the hood posistion (decending)).

Both posistions feel about the same on both bikes, basically the fit is very identical with the two stems, flat back, very agressive (like being in the drops). I could not go any lower on either of the bikes, basically the most aero posistion.

Historically then I would fit in the two frames that I have, and 63 would be fine. The problem is both bikes have very long seat posts on standard geometry. This means I could ride theoretically say a 64 or 65 cm road bike with a longer seat tube and top tube with a shorter (say 120 mm) stem. This would mean that I would have a higher top tube vs my current bike. I see peoples fit where they have a bike with say 5 cm of seat post showing (very little) and a good reach on the bars (maybe not so stretched out).

Then what is the ramifications of continuing to ride a 63 cm bike vs going with a 65 or 66 cm custom bike with a 110 - 120 mm stem vs the 130 (should probably be 140mm) stem on my 63 cm bike? Will the handling be better with a shorter stem combo? What about responsivness?

Remember the bike is used for criteriums, and road races. It is not designed to be a "comfort" or century bike.

Thanks as always...