Originally Posted by
Duragrouch
...I would recommend final smoothing of that edge as much as possible, then taking a hard steel dowel or kitchen knife honing "steel" (rod, one that is fairly smooth, not deep striations) and "burnishing" the radius edge (in a circumferential direction, not axial)...
Heh... I did exactly that! I applied black touch-up paint, so it isn't apparent.
FWIW, the seat tubes on both my ill-fated Dahon and the Tern had the slots on the back. There must be *some* tensile stress at that location. I don't know why the stress-breaker doesn't have a larger radius... seems like that could preempt the problem entirely.
EDIT: Here's an ugly yet effective fix for cracks on the Mu's handlepost:
Ugly... but the cracks were stable for several thousands of miles.