Originally Posted by
merlinextraligh
IMHO, it also depends on the frame. If we're talking a conventional tube constructed frame, if the bottom of the post is below the bottom of the top tube, you're likely ok. I rode a steel frame where the minimum insertion mark was just exposed above the frame lug, half the way around ( the lug was curved) for years with no adverse effect.
If the seat tube on your frame sticks up above the top tube, so that you're not getting the seatpost below the top tube, I wouldn't do it. If the seat post still reaches below the tt, you're likely ok.
Realize that the minimum inseertion mark is put on the post to deal with a range of frames. What you actually need for your seatpost, frame interface can varie.
All that said a longer seatpost is cheap.
Merlin, you're right about the amount of post that needs to be gripped by the frame. But that is only part of the safety story. There is also the question of how much post is exposed from a post strength and applied torque point of view. The post could be perfectly securely held in the frame and still snap off somewhere along the exposed length, most likely at the clamp. The longer the exposed length of post, the higher the applied torque on the post at the clamp. Just sayin' that minimum insertion varies both with the minimum amount needed for the seat tube to securely grip the post as well as with the total length of the post. and, of course, the post material, wall thickness, and resulting strength play an important part is this determination.