There are a couple of ways to remake sizing nomenclature so that it means something. What I'm presenting here is "stack" and "reach." Another way to get to the goal is via a method constructed by Enduro Sport in Toronto, a shop that is further down the road in tri bike fit than most in North America.
Enduro Sport has catalogued all the tri-geometry bikes it offers in terms of head tube length and top tube. But they've gone one step further. They've "normalized" all the seat angles at 78 degrees. In other words, let's say one bike has a 76-degree seat angle and a 58cm top tube in its 58cm size, and another bike that size has a 80-degree seat angle and a 55.5cm top tube in that size. Let's further say that both bikes have 13cm head tubes. In Enduro Sport's nomenclature, both these bikes might be the same for purposes of fit. Both might have 56.75cm top tube lengths when "normalized" to 78-degrees. Here's another way to look at reach: it's the bike's top tube length with the seat angle normalized at 90 degrees.
Back to Enduro Sports' sizing nomenclature: you might object with, "These bikes aren't the same, they have different top tube lengths." But they really don't. What is fixed in stone is whatever your proper position is relative to the BB. You can't be led by the geometry of the bike. You have to determine where you need to be "in space" and then find the right bike to place underneath you. Let's say your correct seat angle happens to be 78 degrees. If you buy the 76-degree frame you'll move the saddle forward on the rails, and/or change out the set-back seat post to a zero-offset post. In so doing you've effectively shortened the top tube length. Likewise, you'd have to move the saddle back on the rails of a set-back post on the 80-degree bike to get to 78 degrees, and you artificially lengthened the top tube. Therefore, in both cases you've really come up with the same bike, from the point of view of fit.
That's the elegance of Enduro Sport's approach, and the approach of any process that reduces the measures of any and all bikes to a sort of "Esperanto" that allows the entire industry to speak one meaningful language. Stack and reach is precisely that.
Why not simply use Enduro Sports' sizing scheme? This nomenclature would work fine for me, but for a couple of things. First, if you are just considering head tube length you are omitting two parameters, bottom bracket height and wheel size. You must know the wheel size before you can use the head tube dimension properly. Another way to view it is that you must also, perhaps, "normalize" the head tube to 700c (or to 650c if you prefer).
So, by using stack and reach you don't need to normalize anything. The information about the bike's size you need and don't have with stack and reach is whether the headset is integrated. If it is not, you must account for the head parts that stick up above the head tube top.
Stack and Reach
Reviewed by: Howard, Oct 10 2009 10:49PM
This 'normalization' really makes it simple to compare bike-to-bike measurements. I struggled with the different geometries versus claimed 'sizes' and this way of looking at a bike makes that inconsistency disappear. Normalization is indeed the right term. I now have a template against which I can compare any future bikes.
Good article - still valid 6 years later
Reviewed by: Bill, Apr 13 2009 8:19PM
I agree with many points, and to this day a 54 is a 52 is a 57.5 is a large. One thing I would add to the two basic measurements of stack and reach, is seatpost angle. at a point it becomes important because to adjust your "in the air" saddle position upwards, you also increase your reach, which, from manufacturer to manufacturer, can prove impossible to balance correctly with stack height.