Originally Posted by
jimmuller
There are several ways to answer that. Saddle height can be adjusted easily so if the ST is 2cm shorter, well you just pull the seat post 2cm further out.
If all frames and people were built to the same proportions one could pick the appropriate ST length and the TT would scale with it. But neither bikes nor people are built that way!
The real answer is that the TT determines how stretched out your back has to be, how much weight is on your hands, how your weight is distributed front to rear, etc. You can't adjust the TT like you can saddle height except by swapping out the stem, and that has a few other side effects. You can move the saddle fore or aft on the rails but that also affects your knee position over the pedals and crank. (I think that is an overrated feature but who am I to argue?) So it's easier to start with an appropriate TT.
Thanks for the explanation.
It's funny, I've had to replace the stem on every used bike I've bought for my wife and I except my '87 Peugeot Traithlon I recently picked up. Our other bikes are all early/mid 90's Trek mtbs, and every one of them had a crazy long stem - even my wife's 15.5" 930! I often wonder what designers in the 90's were thinking with these long top tube, long stem configurations. I'm almost 6ft tall and I was still WAY too stretched on both my 950s before I swapped out the stems to something shorter. They all fit well now, but of course it was a PITA since none of these bikes had threadless stems, lol.