Ok, so, I know this topic has been discussed to death, but I'm looking for the high-level, majority opinion: when you say a frame is a 59cm sized frame, is that the measurement of the top-tube? Or the seat-tube? If the former, is it from center of seat-tube to center of head-tube? If the latter, is it from the center of bottom-bracket shell to center of top-tube?
Also, if you are sizing up a time-trial frame, is it true you want it to be a little smaller than you would normally ride on a regular road frame?
Generally, when you see a single number listed as a frame's "size" that number refers to the length of the seat tube .
A further complication is that nobody knows how to measure a bicycle's seat tube any more. Even leaving the inches/centimeters question out of things, there is the question of where the seat tube ends.
* The old standard system was to measure from the center of the bottom bracket to the very top of the seat tube.
* Some manufacturers have decided that this is too easy, so now many bikes are measured instead to the intersection of the centerline of the top tube with the centerline of the seat tube.
* Some other bikes that have seat tubes that protrude farther than normal above the top tube measure as if they were measuring to the to the top of a seat tube with normal protrusion.
* Some bikes are measured to the top edge of the top tube, even though the seat tube protrudes higher up.
* Some bikes with slanting top tubes are measured as if there were a level top tube, they use the length that the seat tube would be if it was as high as the head tube.
Anarchy reigns; I know of one bicycle line that made a running change in the middle of the year. You could have two bikes of the same make, model, year and nominal size, but one was 2 cm larger than the other! The only way to know was to measure them.
There has never been a standard method established. English framebuilders including myself usually measured center to top. Italian frames were usually measured center to center. With compact frames with sloping top tubes some are being measured differently yet again.
Just be careful when you are buying a frame that the buyer and seller are on the same page. If one is talking center to top and the other is thinking center to center; someone is going to end up with a frame about 2cm. too big or too small.
A further complication is stem length as many manufacturers take into account stem length in assigning size to a frame. For example: A bike maker may have "square" bikes where the nominal top tube length is the same as the nominal seat tube length. A size 55 frame may have a 110 mm stem. A size 57 frame may have a top tube only 1 cm longer than the 55 but with a 120 mm stem. The size 59 frame may have a top tube that is 2 cm longer than the 57 with a 120 mm stem.
So what it all boils down to is that you must either have the frame fitted to you by a knowledgeable person or do some significant test riding.
Or if you are a casual rider and can adjust to a bike that is close to the right size instead of needing wind tunnel perfection, you just need to know when your genitals are being crushed against the head tube when you straddle it... start large and move down in size until the pain stops.
I know the value of professional fitting, even though I haven't had one done myself, so please refrain from flames... If my humor is bad, just ignore it and move on, it won't bother you any more unless you read it again.
Yeah, the seat tube is the measurement, and it's ********. It's the single most adjustable piece of the bike. They should measure by top tubes. You should go ride some bikes and find one with a really comfortable feel (or get a real fitting) and measure the top tube (center to center along a horizontal since some compact frames would measure longer as the top tube slopes downward to the back). Get the stem size as well. Add the two together, and make sure your new bike has that top tube/stem sum (or get a new stem to make it so). The seatpost is pretty easy to adjust.
BTW: TT bikes generally have steeper seat tubes. Top tube length is generally shorter too since your elbows need to get pretty close to the bars.