Originally Posted by
hubcyclist
posted this in the fitting forum, but didn't really get a lot. i've always been under the impression that top tube + stem length (or frame reach + stem length) could be kind of interchanged (i.e. like a 54cm with 110 stem equals 56 with 90 stem). of course, all this assumes similar saddle/handlebar drop and most things being equal. i tried a 54 last week and as far as reach is identical to my current 56, and in a brief test ride felt like a better fit and more comfortable. i'd like to take one out for a longer test ride to see how it holds up over a normal ride for me, but i've been wondering if there is anything a shorter top tube that could be more comfortable for me compared to an equivalent reach and bar height on a bigger top tubed frame.
edit: there's probably too many other factors, from handlebar design to seat tube angle that I'm missing. so i definitely oversimplify here
You're right, if you measure from saddle to bar (takes into account TT, stem, and saddle forwardness sort of), you should be able to interchange.
Bars make a huge difference in reach at the drops (most saddle-bar measurements are to the tops of the bars). Drop, reach are significant variables.
Saddle design makes a difference. You may sit 20 cm back from nose on one, 25 cm back on another. Use the same make/model saddle to eliminate this as a variable.
Even with the exact same reach, say bar to drops, I think a bike with a shorter top tube feels better. There's more weight on the front wheel making the bike feel more stable at speed. Within reason of course.