Joined: Nov 2006
Posts: 19,810
Likes: 597
From: NJ, NYC, LI
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
I don't know why seat tube angles vary so much.
There was a time in the early development of the diamond frame that seat tube angles were very slack (say 68°) but seat posts were 7-shaped so the seat was mounted in front of the post itself. This was kinda clever in that the effective top tube increased if you raised the seat, but it was inherently weak and resulted in a heavy seat post.
By the mid 50's designers had settled on 72° or 73° and that seemed to work. But after the mid 70's head angles got steeper and seat tube angles soon followed. This doesn't make sense to me, since it resulted in seat posts with more and more setback to get the seat into the right position.
Whatever. If you have the seat in the same position relative to the pedals on every bike, you're doing it right. as long as you can do that, the seat tube angle is pretty irrelevant.