Originally Posted by
Metacortex
According to the Schwinn Reporter (dealer newsletter) in May '73 in an article on frames the men's fillet-brazed bikes are supposed to have 73 degree HT and ST angles. I've measured those angles using the level top tube as a reference on several of mine, and it verifies for me to within less than half a degree. By comparison the electro-forged models (Varsity, Continental, etc.) were 70/70 degree HT/ST, and that measures out for me as well.
It's pretty clear the STA and HTA aren't even close to parallel on my '73 SS. When I zero the angle gauge on the top tube, the HTA measures 73.6° (relative to the top tube) and the STA measures 72.9°. That's a 0.7° difference.
I maintain that the only truly accurate datum is a line through the front and rear dropout centers. Everything else, especially the top tube slope, can vary.