Earlier in the decade I wrote an article on how bikes are sized, and the gist was that we ought to be looking at bikes from the point of view of head tubes and top tubes, and that's it. -- no more seat tube sizing.
I wrote this because it doesn't matter where the seat tube is. It used to matter, and I'm old enough to remember when it was useful. When I started racing -- in the era when we nailed our cleats onto the bottoms of our shoes -- all road bikes were made the same way, with 1 1/8" diameter down tubes and 1" top tubes. Top tubes were horizontal and were not "dropped." There was no such thing as "compact geometry." All stems were "minus 17 degrees," which is to say they were parallel to the ground. The only sort of bike was the standard steel road race bike. Neither aluminum nor carbon were available to frame makers and the one and only titanium bike made was the Teledyne Titan, out of commercially pure titanium.
The world has changed. We have global warming, terrorist attacks on U.S. soil, and bike fit nomenclature that makes no sense. Heck, in my last years at QR our sizing system was "center to air," which meant we measured from the center of the bottom bracket axle to the center of some vapor point we made up in our heads. We didn't want to spend half our day answering questions about our fit nomenclature, so we just figured that if a particular size was made for a guy who would ride a 59cm normal bike we'll call it a 59cm. It was a minor detail to us that the bike didn't actually measure 59cm anywhere on it. And we were right. Very few people called us on it
In fact, some of our competitors found the solution elegant, and adopted it themselves. Nowadays virtually no tri bike maker today offers a dimensional sizing scheme where the dimensions match the size name. In other words, you may be looking in vain for a 57cm bike that measures 57cm anywhere on it.
Alternatively, there are companies that have opted for T-shirt sizing, but that's not entirely useful either. What does "Large" mean in terms of bike size?
That's why a half-decade ago I whined for a system that simply measured the bike's top tube and head tube. But with the latter measure, you've got to take into consideration the bottom bracket placement and the wheel size. In other words, if the bottom bracket is placed 7cm below the centerline of the bike (a line drawn through the two wheel axles), and another bike has 5.5cm of drop, then these two bikes will fit differently even if they have the same head tube length. The rider will have a more aggressive position on the bike with the 5.5cm of drop, all other things equal, because what matters is the head tube's top relative to the bottom bracket. And, of course, a 650c-wheeled bike is going to have a much longer head tube than a 700c bike with the same basic geometry. So simply measuring a bike's head tube length leaves questions unanswered.
All this came home to roost when I was compiling the manuals and curriculum for our first-ever F.I.S.T. tri bike fit workshops. There are three fundamental tasks of a bike fitter. First is to determine what sort of geometry a subject needs—simply put, ought the subject be fitted more along the lines of a road bike, or is a more aggressive, tri-specific position appropriate?
The second task is to find those points in space that determine where the rider's weight ought to rest. Forgetting front-end measures for a moment (clip-on length, armrest width, base bar tilt, stuff like that) a subject's proper bike geometry and size is determined by where his weight will rest in space. That weight is displaced over three spots, the bottom bracket, the saddle, and the armrests. You place your subject on a fit simulator, like the one at right. You move the saddle up and down, fore and aft, same thing with the adjustable stem, and presto, you've got your subject positioned. Once you know where the saddle and armrests ought to be in relation to the BB, the position's dialed. You're done.
Yes, you're done in theory, but you've only generated a determination of where the rider ought to be "in space." To be sure, this is the most important part of the fit. But what bikes are made in production that conform to these points in space? What custom geometries flow from these points in space? This was fairly easy in the old days because, as I said, everything was very standard. But not now.
In 2003 I proposed an entirely new nomenclature for bike sizing, and in the intervening period a lot of bike makers have started to provide these metrics (we're compiling a geometric database that includes these measures). It's built around the idea that there are two parameters that really, truly, determine a frame's length and its height: how high a frame's head tube top sits above the bottom bracket and how far the head tube top sits in front of the bottom bracket. These measures are what we call stack and reach.