On 2007-04-10, 17:15 Z,
in ,
Harry Bloomfield wrote (paraphrased):
The threaded rod would be in tension, supporting and lifting the
bike. Turn the top (welded-onto-rod) nut and the rod screws into
the lower, captive nut, pulling up the frame.

Harry: Now that you described the design details, if we combine the
torsional shear stress on the threaded rod (due to repeated
torquing) with the tensile stress, and look at fatigue strength of
the threaded rod at the thread root (instead of static strength), my
current calculations in this scenario indicate an M14 x 2.0 mild
steel threaded rod (property class 4.6) can withstand an allowable,
applied tensile load of 2570 N for a long lifespan. An M12 x 1.75
might also work, if well greased, but the M14 gives you extra
margin.

I assume your stainless steel rectangular tubes are perhaps AISI
304L stainless steel, or comparable. Welding these with common mild
steel welding rods isn't recommended. I recommend using AISI 309L
welding rods. And this same welding rod (309L) can be used for
welding stainless steel 304L to mild steel.