Originally Posted by
geepondy
Quick question on frame height rather than start a new thread. My friend let me borrow his 56cm "winter bike". When I straddle the bike directly in front of the seat, while I can put my feet flat on the floor (wearing socks only), the top tube pretty much comes up all the way to my crotch or very close so. When I straddle my old 54cm bike in the same manner, there is a good couple of inches clearance between the top tube and crotch. Which is supposed to be more correct for the proper size?
Either is fine, just whichever you're comfortable with. For sizing, the first baseline is "can you comfortably stand over it," and it sounds like both of these meet that bar (no pun intended). In the old days, most road bikes had flat top tubes, resulting in very high standovers with minimal seatpost showing. In the late 90s, compact geometry came to the fore, as manufacturers (led by Giant) tried to find ways to make their bikes lighter and stiffer. Most bikes these days are neither fully flat-top-tube designs, nor are they that close to true "compact" geometry. Most bikes don't even have top tubes that are very steeply raked these days either - I think that's a function of wind tunnel testing.
Once you know you can stand over the bike, the next three things are saddle height (as dictated by how much you can extend your seatpost), reach (horizontal distance to the handlebars), and stack (vertical height of the bars). The different sizes ought to have different baselines for reach and stack - ie, if you put the same stem with no spacers on two bikes in the same line, the smaller bike ought to be a little lower in stack and a little shorter in reach. All that goes out the window when you start comparing bikes across lines with different geometries - race oriented bikes tend to be long and low, endurance oriented bikes tend to be taller and less long - so for those, knowing what your body can work with is most important.