Originally Posted by
DaveSSS
As a mechanical engineer I certainly know my trigonometry, but everyone wanting a new frame shouldn't have to use it. Once you know your preferred stack and reach range, plus your desired STA, it only takes seconds to figure out whether a frame meets your needs. It's a huge improvement to have this info. I also know that the proper stack can be affected by the stem angle. Two of my frames have -7 degree integrated bar/stems with internal routing of brake lines. Other bars and stems would make for an ugly setup. I chose a 505mm stack and use the minimum 20mm headset top cover setup. If a -17 stem is an option, I would prefer a 20mm taller total stack, with the headset top cover.
OK, three dimensions should be considered. Still no trigonometry needed. Many of today's frame have proprietary seat posts with inadequate setback for me. If the fame has a 75 degree STA and anything less than 25mm of setback on the post, it's an immediate NO.
I understand that this is how you (and maybe others) evaluate a frame. That is fine. But it does not explain the contradiction between your two original statements (that I had highlighted above), which is all the more surprising given your apparent grasp of the trigonometry. To me, we seem to be essentially doing the same thing by looking at a third measurement in addition to stack and reach.