Proportional-Sized Frames This all changed when the Japanese got serious about the U.S. bicycle market, and modern bikes are generally built with "proportional sized" frames. This means that the smaller sizes have shorter top tubes, and the larger sizes have longer top tubes. This is generally a great improvement, particularly for riders of "average" proportions. A couple of other factors have made it harder to be sure of frame sizing. One is the fact that bikes come in more sizes than they used to. Where they used to come in increments of two inches, they often come in 2 or 3 centimeter increments now. Measuring Frame Size 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. An additional complication is that the height of the bottom bracket varies over a considerable range, typically anywhere from 10.5" to 13"! Thus even frames that use the same system for figuring the top of the seat tube may have widely disparate stand-over heights. Bottom line: seat tube "frame size" numbers are nearly meaningless unless you know how they are measured! Top Tube Length: More Important Than Seat Tube Length! When people speak of bicycle frame sizes, they generally speak in terms of the seat tube length. As mentioned above, this used to be the only variable, but with proportional sizing it no longer is. I would submit that seat tube height is no longer the most important frame dimension. More determinant of the actual way the rider will sit on the bike is the top tube length. It is obvious why you shouldn't have a bike that is too tall to stand over with a reasonable safety margin (although even this sizing practice was not universally accepted for the first 30 or 40 years of the diamond frame.) On the other hand, why shouldn't you ride a "too small" bike? "Because the seat and handlebars will be too low!" That was a good objection ten years ago, when tall seatposts were a rarity and quality handlebar stems were available in a variety of forward extensions but only one (short) height. All that was before the mass production of the mountain bike. Now 250 mm and 300 mm seatposts are stock items, and a variety of excellent handlebar stems are available There are a number of sizing systems available today, which require various measurements of the cyclists body and reccommend frame sizes on this basis. Probably the best known of these is the New England Cycling Academy FitKit. I use this system myself, but not in a blind, rote manner. The FitKit makes reccommendations for a particular seat tube length, and a range of top tube lengths with corresponding handlebar stem extensions. For instance, for a particular rider, it might suggest a 58 cm seat tube with a combined top tube and stem extension of 66 cm. This 66 cm might be from a 61 cm top tube with a 5 cm stem, or a 54 cm top tube with a 12 cm stem, or any other combination that adds up to 66 cm. Any of these combinations will give an equivalent posture on the bike. One or two combinations are particularly reccomended because the more extreme variations of stem length can cause a bike to handle strangely because of the positions of the hands relative to the steering axis. The "by-the-book" fitting method would then be to select a suitable bike with a 58 cm seat tube, measure the top tube, and install the reccommended stem. I would submit that this approach is due to the old fashioned fixation on seat tube heights. Better, in my opinion, to find a bike with the ideal length top tube, fit the reccommended stem, and not worry about the seat tube size, within reasonable limits.
