Originally Posted by
Racing Dan
Simply comparing reach and stack does not disclose actual seat to hood distance and drop. You need to compare top tube, stem length, seat post set back and bar reach as well. Just comparing reach and stack is meaningless.
Flame away if you must :-)
No flame, but someone clearly misled you about stack and reach and your reaction is understandable given that misinformation. Let's start here:
https://www.slowtwitch.com/Bike_Fit/...er_One_95.html
https://www.slowtwitch.com/Bike_Fit/...er_Two_96.html
https://www.slowtwitch.com/Bike_Fit/..._Three_97.html
Yes, of course, that's Slowtwitch and thus focused on triathlon bikes. Those articles are the genesis of stack and reach frame measurements, and they explain the reasoning behind it. Specifically, they are strictly for comparing bare frame to bare frame; your concerns about "stem length, seat post set back and bar reach" are valid for fitting and comparing full bikes, buy have zero effect on the frames' stack and reach measurements. Top tube length is important, but is one of a few factors that contribute to reach. Once you take the (effective) top tube length and account for seat tube angle, you get the frame's reach. Of course, you can slide the seat forward or back, or use an offset or straight seatpost, but that's true on all frames, and stack and reach compare how the frames fit before you install the seatpost, saddle, spacers, stem, bars, and hoods.
Stack and reach are the distillation of the frame's (and only the frame's) geometry that contribute to fit, including top tube. seat tube angle, bottom bracket drop, head tube angle, head tube length, and so on. They don't tell you how the final build will fit; that depends on a number of other factors, including those you pointed out. They don't tell you how the bike will handle, either. They do give you the easiest and clearest system for determining whether a 56 in one brand fits more like the 55 or 57 in another brand, which is what they were intended for. You don't have to use them, you don't have to like them, but pretending they don't work is like pretending trigonometry doesn't work.