Well, I was in a bit of a quandry over the top tube on Scott's bike. You see, the design called for a single curved TT, but it really needed to be an 1.25" tube for stiffness and aesthetics. Problem? bending a tube that big in my normal fashion ends up in a kinked tube. I tried to find an alternate source (another builder), but he was too busy to make them for others. I looked at muffler and chopper shops, but they were scared to try or had no clue in how to do it. I didn't really want to do the cerrobend thing (metal that melts at a low temp) too many noxious ingredients to that stuff and who wants to die for a top tube?
Finally, I went back to square one to try again. I made a new mandrel, packed in sand extra tight, and viola! A perfect bend! I realized that if if move the tube in the mandrel after the first kink starts, the tube is supported a lot better and the bend is finished nicely. The only issue with this is that you have to sacrifice the first 8-10" of tube for scrap. For that reason, I recommend starting with a 5' tube so your leftover section will be more useful- I used a 4' section and the remainder was too short to make another top tube. Bend and learn!