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.
This analysis is wrong. If the saddle is placed in the same position relative to the BB, the reach will only differ by 3mm. Your analysis compares reach to the seat post at the stack height. Obviously, a steeper STA just requires the saddle to be moved back further on the post to place it in the same location, relative to the BB. All measurements must be taken relative to the BB.