Originally Posted by
SoSmellyAir
While it is true that (a) Seat Tube Angle ("STA") for road bikes almost always falls within a narrow range spanning only one degree, and (b) Head Tube Angle ("HTA") likewise generally falls within a similarly narrow range of about two degrees, those few degrees may add up to a significant difference in Effective Top Tube ("ETT"). For example, comparing my existing 2016 Synapse with the 2022 Synapse I just ordered, both in size 54 (generally considered as a medium):
2016 - Reach: 378 | Stack: 570 | HTA: 72.0 | STA: 73.9 | ETT: 542
2022 - Reach: 381 | Stack: 570 | HTA: 73.1 | STA: 73.0 | ETT: 555
This comparison made me realize that I may need to order a shorter stem to replicate my existing fit. But one may not appreciate the difference by comparing Reach alone (i.e., 3 mm difference in the above example). Thus, ETT is a key geometry number for visualizing frame size, unless you can do trigonometry in your head; I cannot.
Bikes that many years apart can have quite a bit of different geometry between the same models in the same size. Sometimes the change is even greater and the bike gets put in a difference fit class to appeal to a different market set of buyers..
But generally for the fast down and dirty method to decide if a bike is going to give the very aero or very relaxed fit I might desire, I'll look at frame stack and reach. Once I have some picked out I might get more fiddly with the other things as well as try to estimate what the real reach will be from the saddle to the bars.
Some of the bike geometry sites let you enter in all the custom data to see that. But I find it easier to do with the simple CAD software that's available. Actually I use the 3D modeling sofware Alibre, but I never take it to the extrusion process to make it 3D. But the sketch up part is very simple to show what is needed to know.