Originally Posted by 79pmooney

Andy, you are looking for an exact solution. No issues. Just model up the seat tube, lug, TT & seat stays plus post, seat and rider in a FE program, add the load and solve. You now have spent many hours and have one solution to a problem that may never play out just like that. Huge waste of time for any engineer tasked with getting plans and products that work out the door. My simplified approach is, yes, very crude but if you can run across one or two bikes with bulged seat tubes and many without, it probably won't be hard to see that a seatpost sticking down this far and a seat tube this thin bulges but all these more conservative bikes and seatposts are fine. And what is the weight cost of a seatpost that is inserted a cm or two more than needed? 10?, 20? grams. (For an aluminum post built like a tank.)

It comes down to: what is the cost to the ride of too safe a system; in weight, race results, etc. vs the risk and consequences of a post snapping or a seat tube being bulged (metal) or cracked (CF). The answer to that won't come from theory.