Ok i been having this doubt for zillions of years and I need somebody to explain it to me ok?
This is the deal...
For example, steel frame with 73 degrees st angle, front tube 73 degrees also. Now, of you make an imaginary line (parallel to the ground and tt) between the rear and front dropouts, the intersection with the seat tube with this imaginary line has a different angle, not 73 degrees, lets say 72.8 or whatever... why is that??? Anybody knows the reason?
By geometry it should be the same angle, opposite angles between parallel lines, but looks like in bikes is not like that, anybody?
with older "horizontal" dropouts, the rear dropout height could vary depending on the wheel position. If the fork was replaced, that could vary the effective ht/st angles. I can't think of any other thing other than some sort of construction error. I'm sure there are plenty of bikes out there with issues along those lines, it's easy to do.
The baseline is a line through the centers of the rear dropout and the front fork dropout. Everything is relative to that baseline (STA, HTA, BB drop). If the rear dropout is "horizontal" similar to the Campy 1010, the wheel axle can be above the baseline if it's positioned at the rear of the dropout, or below the baseline if it's positioned at the forward end of the dropout. Similarly, if the fork is longer or shorter, or has a different headset than the frame designer assumed while designing the frame, the fork dropout could be higher or lower than the designed baseline. Slight changes in where the baseline is (as opposed to where it should be according to the design) will change the angles of the seat tube and head tube relative to the ground since the actual baseline is no longer parallel to the ground.
In your drawing, the top tube is not parallel to the baseline, so the top tube-to-head tube angle and the top tube-to-seat tube angle will not be the same as the head tube-to-baseline angle (HTA) and seat tube-to-baseline angle (STA). The frame in your drawing has parallel head tube and seat tube (both are 74 degrees from the baseline).
The BB drop is the vertical distance from the baseline to the center of the BB shell. The BB height will depend on what size wheels and tires you use.
I would guess it's related to the fork offset. A proper parallelogram exists between the TT, ST, Steerer, and your imaginary line. When you offset the fork perpendicular to the steering axis, you raise up your imaginary line just a bit.
If the tt is not parallel to the base line then the st/tt and tt/ht should be different, not equal, m'i wrong?
as long as the HT/ST are parallel, the st/tt and ht/tt angle have to be equal. As the predominance of slanted top tube bikes shows, there is no significance to the top tube angle to the ground. The steering geometry of the frame is determined by the fork rake and head angle and wheelbase. Everything else about the geometry is there to hold the rider off of the wheels.