Originally Posted by my58vw
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...
my58VW....both positions of your two different bikes might feel the same but are completely different in their set up. The fit isn't near identical in spite of what you wrote. Fit really starts with KOPS..."a reference only" whether you choose well in front of KOPS say for your TT bike or even with or behind KOPS for your road bike. By intent or happenstance your respective KOPS positions for your two bikes are completely different. The math is pretty simple as you are very capable of performing with your training.
Your 75 deg. seat tube angle/seat pushed forward TT bike is a far cry from your 72.5 degree Trek road bike with seat pushed back...opposite ends of the KOPS spectrum. The question is whether you set respective seat positions up for a power versus comfort differential between the two bikes or because they have a slightly different top tube length...which I might add isn't much different between the two. As you know your seat position relative to your cranks affects your reach which is a function of top tube and stem length. Also do not discount drop from the seat to the handleabars as this is a major contributing factor to reach as well.
If you want a scientific approach...find a simple 2-D CAD program my58VW and compare the two bikes graphically. You can do this inside of a half hour and the difference will be apparent and it will be easy to set up one bike to be like the other and entertain any alternative bike geometry you are considering...even a custom.
HTH,
George