The weight savings comes from the wall thickness difference. Typical ERW 1 5/8" mild steel tubing (JUNK IMO) is .134 wall and legal chromoly wall thickness is .083" for 1 5/8".
.134 wall ERW mild steel weighs 2.134 lbs/ft
.083 wall 4130 chromoly weighs 1.367 lbs/ft
If you measure out how many feet of tubing you would have in your cage you can determine pretty close the weight difference between the two.
For example, my current .134 wall M/S cage has 64' of tubing which calculates out to 137 lbs. The same cage in 4130 chromoly weighs 87.5 lbs. That's a 50 lb weight savings which is pretty substantial when your trying to save weight when you can.
I Tig weld at work alot and I do not like to tig weld mild steel. It welds like crap, dirty base material and the welds never look as clean and neat compared to chromoly tubing.
We have not had any come backs on cracked welds with chromoly. If your welding process is correct and you limit heat input into the tubing, the brittleness of the welds is mimimized.
Granted a custom 4130 cage is not for everyone, but IMO the weight savings are worth the effort and expense. But then again, we don't really do any M/S cages, nor does our shop stock any m/s tubing.
