Oxy-Acetylene is still widely used for chromoly tubing frames in aviation and motorsports. I doubt this is chromoly, but The extra heat put into the tubing by torch welding allows it to cool more slowly and anneal the weld. you can also use dead soft filler rods. Lots of weld failures in high vibration environments occur because the weld has 2-3 times the tensile strength of the base metal. This creates a stress riser where the edge of the weld meets the base metal (typically where you see failure)