What is the differnce in pipe and round tube

Weighs as little as 35lbs per cubic foot, a cubic foot would fill many linear feet of pipe/tube. So it might add 40-60 lbs total. Pumicecrete would probably also work..

I didn't say use the cheap pipe, I was suggesting that it might make any roll bar system stronger. Hmm, actually it may make the pipe with a seam tend to blow out more than if it was empty so maybe it'd be better to use with the seamless tube.

Try crushing an empty paper towel tube, then take another one, fill it with spray foam and try crushing it. The empty tube is a lot weaker. Filling it (even with a lightweight material), makes the tube stronger.

Sometimes in construction folks use lolly columns (or lally columns), which are typically steel posts filled with concrete to add strength. So that's what gave me the idea. The steel post filled with concrete is typically stronger than the same size steel post that's empty.

Hey, I'm not saying it's a great idea, just interesting and I admit I'm a total laymen in welding, physics, jeep crushing, etc.

My full cage dates back to 1975. I don't think it has a seam, and if I had to make the choice today I'd buy the best damn cage money can buy. A jeep on my head would not be an improvement (in my opinion anyways) R)

Edited to add: it occurs to me that SOMEONE within the racing / roll cage production industry has probably tried this (filling the pipe/tubes with some material to strengthen it). Since it's not in common use I'd guess it didn't work too well.