Originally Posted by chaadster

I don't think there's much accurate in that statement, because the phrasing is awkward, the term "traditional bikes" needs definition and is arbitrary at best, and the use of quotations around the word quoted raises questions as to what you're meaning to say there.

However, I think I know what you're trying to say, and it seems to be calling into question the comments I made upthread, so although you didn't quote me, I guess I need to make a defense of my comments!

Traditional frame sizing was based on length of the seat tube, but because of the customary practice of building a "square" frame, i.e. one where ST and TT were same length, it came to be a shorthand (I guess) for the combined measurements critical to fit.

It worked for several reasons:
- lack of variety in frame design
- less concern with concept of fit
- fewer options for adjusting fit (e.g. bar options)

As technology developed and attitudes shifted, everything changed in frame design and our approach to fit. Even before Giant commercialized compact geometry, builders were stretching TT lengths and playing with angles, so the ST sizing concept was blown as a useful tool, but rather than it getting dropped, we learned to adapt by brand, like in the way we all knew Lemonds had long TTs.

At the same time geometry was changing, components were advancing to make changes fast and easy, but also limiting in some ways (think ahead stems) and our approach to bike fit became more scientific. Tlogether with the preponderance of compact geo design, these elements really made talking frame size by ST length utterly ridiculous.

But what to use instead? S/M/L based schemes seem less ridiculous because it drops the pretense of specificity, but doesn't tell us any of the numbers we need for fitting, so what to do? It's back to the geo tables, then, but that's a lot of numbers and requires some math when comparing frames from different builders with different geometries to know who's M is more medium.

Stack and reach measurements tell us something no other measurements do: where the HT is relative to the BB. It's handy because if you know where you want your bars to be relative to the bottom bracket, your golden, and you can compare different frames more easily.

So S&R are nice to have, but they don't describe fit; you still need to know how you want a bike to fit and have the numbers to figure it out and give S&R meaning. It would certainly be good if all geo tables included the info, and we started to work from them as a baseline....but there's still the question, "what size frame do I need?" I don't think that's ever going to be an easy answer again!