Seat tube crack: Common on Dahons, usually smaller 20" folders with a much longer seat tube and thus greater moment for the same rider weight. Improvements have been an aluminum (replacing plastic) sleeve ("shim") between seatpost and seat tube, and on aluminum frames a gusset on top between the top tube and seat tube. Your crack originated at the edge of the weld, one of the reasons that premium aluminum frames like Cannondales "dress" (strip-sand) the welds to a smoothly blended radius, this distributes the localized strain and stress on the tubing better. Post-weld heat-treating 6061-T6 aluminum frames also helps, but modern 7005 aluminum developed specifically for bike frames, obviates the need for that.
I had one of their 4130 steel frames crack there, didn't see it until the crack was too far gone, so I frequently inspect there to catch any crack in its early stages, so would drill crack end or grind out, smooth edge well and burnish well (similar to shot-peening) as this improves fatigue life.
Last edited by Duragrouch; 03-25-24 at 12:38 AM.