Sorry I got a little wrapped around the axles in that last post. Please disregard. (To avoid spreading my confusion maybe one of the administrators will delete it. Please!)
I think I can simplify this. Assuming I use 60 feet of materials for the main rails and about 40 feet of materials for the outriggers and cross-members that's roughly 1000 feet of material.
Using 5" channel, 1.750" flange, .190" web thickness at 6.7 lbs 1000 feet is 670 lbs. This material is probably close to the factory material. However the cross-members and outriggers are probably lighter. 40 feet of this material for outriggers and cross-member is 268 lbs. So the factory material is probably lighter than 268 lbs. The factory frame is probably between 402 and 670 lbs.
Using 5" inch channel with a 1.885 flange and .325 web thickness. 9.0 lbs per foot 1000 feet is 900 lbs.
Using 5x2 rectangular tubing in 3/16 gauge at 8.15 lbs per foot 1000 feet is 815 lbs.
Just for fun let's say the factory frame weighs 550 lbs. Compared to the .325 channel that's a 350 lbs difference. Compared to the 3/16 tubing that's a 265 lbs difference.
Assuming I use the 3/16 tubing what difference will 265 lbs (or even 350 lbs) make? In terms of axles, tv, etc? As Kip is suggesting it seems like worthwhile tradeoff.
