Originally Posted by
dnomel
.. It is interesting though to try to imagine what normal forces are acting along the steerer tube with a quill stem and threadless. I'd like to understand better how and where they break.
For "normal" use, most bicycles have an ample safe margin in their design.
The steerer tube failures that I've seen have all been fatigue related, in which case it doesn't take that much for something to act as a stress riser and eventually prove fatal to the structure.