oooooh, so inferior....
of course it was! it was a bleeding racer! much the same as yours except it was a porsche... and its not the only frame ive seen fail in this spot...
do you know what triple butting is? it has little bearing on JOINT STRENGTH. well, ok, it has some, cause a joint using the butted section wall thickness would fail hella quick. its thickened at the ends for a reason. dont go shooting your mouth off without really understanding what it means...
and, as i said, the seat stays on these frames are NOT reinforced, gusseted, lugged, socketed, whatever... no amount of butted tubing is going to cure that!
you will note that (if you look at the pic)...the TOP TUBE is socketed/lugged to the SEAT TUBE and STEERER TUBE. the DOWN tube is lugged to the STEERER TUBE and the BB. the SEAT TUBE is lugged to the BB. the CHAIN STAYS will be invariably lugged to the BB (cant see behind the crankset, but it is) but there the lugs end....
all you have to do is open your eyes.... look! use your head! that is the one critical joint! the seat tube to seat stays! and compare it to most other bike frames... i say most. not all. most.
i wouldnt open my trap if i didnt know about these things. you want a pros help yet cant take the advice they give you that is sensible, and based on practical experience, all im doing is saving you some future frustration and possibly hospitalisation, but if you want to act like a lil kid, go for it
why ask for help if youre minds made up?
the easiest fix is to gusset the frame there(other than using another frame)... gee, if i hadnt thrown out that frame, a pic now would make all the difference...
yours forever and a day, bub