Originally Posted by
rruff
ETT doesn't give you meaningful reach, since the STA effects it quite a lot...
Yes, it does - and I have quantified by how much and shown how those differences can be accounted for. With all due respect to you (and none whatsoever to Campyfanboi), I think you misunderstand my point, and that at the crux of it, we don't really disagree that much.My point is simple: if you have a preferred ETT and HT that doesn't require any extreme saddle positioning or stem size, then you can get any typical road bike (i.e., with a STA range of 72.5 to 74) to fit. I hve never said that the reach is going to be exactly the same on different bikes with different STAs - I realize that (my example earlier shows a 2cm difference in reach between a 72.5 and 74 STA bike with a 82cm saddle-BB length). The only reason I've brought this point up is to counter Campyboi's exceedingly sweeping statement that there is no value in using ETT and HT to size a bike. Precisely BECAUSE bikes have a pretty largish "fit zone", ETT and HT are accurate enough (even if they aren't precise) to get the right fit for a lot of people.
in addition to HTA and HT. Most people don't realize that HT length effects reach on two frames that otherwise have the same STA and ETT. That's one reason why "stack" and "reach" are not so great for sizing. A 30mm HT difference is ~10mm in reach.
Not sure I follow your your point that stack and reach are not great for sizing - do you mean taken individually, i.e., 2 bikes with the same reach but a different stack will have a different effective reach? If so, yes, agreed.
For instance, I'm 6ft with average proportions, but like a lot of drop from the saddle to bar. Because this is "unusual" it is the controlling factor in sizing. I ride a 54cm frame with a 145mm head tube, 130mm -10 stem (no spacers), and 20mm offset post with the saddle back about as far as it will go. If I didn't want so much drop I could ride a 56, 58, or even a 60 quite easily.
Ha, I've had to learn geometry lessons b/c I have freakishly long legs and an absurdly short torso. I need a sportive-style head tube to get an aggressive (5" saddle to bar drop) geometry. I need a "short and tall" bike - the Roubaix, with a 185mm HT on size 56 would be an aggressive fit for me.