I've been thinking for a while of trying to make a suitcase size bike for taking abroad. IT has to be 26 or 700 for touring from foreign jump off points, so it will need to be a pretty big suitcase, but bear with me.
After a lot of thought I took a hacksaw to an old steel frame I had lying around - no value, so nothing lost.
I cut it near to the seat tube on both crossbar and downtube.
Conveniently, a modern stem will fit both tubes neatly. I have a couple that have a longish span (though I would prefer one that had longer), which I slide onto the cut tubes. I can reassemble the frame, slide the stems over the cut, tighten them fully, and hey presto, the frame is solid (from a stretching and pulling point of view, anyway.
I need to put the wheels on and actually try it out, but I'm hoping it'll hold together without any flex.
I'm thinking of putting a length of steel tubing into the downtube (luckily, again, I find that I have some which will fit - this should give extra rigidity and that crucial join, which is where it would come apart under normal weight should the step clamp fail). Having this only on one tube will also mean the frame will actually come apart, as if it were in both disassemble/assembly would be impossible.
I'll try it out on monday and let you know how it rides.