Originally Posted by
Fentuz
as the seatpost is on a ~73deg angle, it is load towards the back consequently, the load distribution of the tube will be compressive on the back and tensile on the front. The oval hole believe as a stress raiser.
if you cut a small notch on a piece of branch perpendicularly the branch axis, if you apply a bending load setting the notch side in tension, it will eventually break.
do the same but putting the notch side in compression, it may break but if/when it does, it will be later
Your conclusion and your analysis wrong. Actually it is partly correct but still wrong. Imagine that force applied is the rider's weight sitting on the saddle. So the force is vertical. The seat tube is not vertical, but it is mostly vertical. Therefor, most of the load is a compressive force. However, to the extent some of the span is horizontal, some force will be carried in bending. That part of force will act like you show, except that it is not being applied at the center of the beam, but at the top end. The bending force would put the outer part in tension as you suggest, but since most of the weight is compressing from this beam being a column, or beam-column, that won't happen. All of this calculable since we know the weight, the angle, and we could ask the dimensions of the seat tube. We would start by resolving the vertical and horizontal forces..So I contend the the seat tube stays in compression, with one side less compressed than the other, but compression.
The part about the notch is wrong too, When you create a notch, you change the properties of the piece. The beam has less depth, so the stresses go up there.