I'm cutting apart a Diamond back that a bicycle shop was unable to remove the seat post from. It is a step through frame and the seat tube went 2 inches past the weld. The tube turned out to be bent 2mm at the joint where the manufacture hammered the tube in place before welding. The tube is seamed with a razor sharp burr of .5mm this elimated any possibility of rotating the seat tube. I have made seamed tubes they come out of the welder looking like a banana with the seam being in tension. There is no way they can be straightened to the tolerance required of a seat post. However Diamond back managed to bend the tube the opposite way. This is valuable information now I would not dream of adding a mid tube to a small tandem. Also I don't see why manufactures don't use a double butted seat tube.