Some tube questions.
 

Assuming all the numbers are equal whats stronger. A tube with a big diameter and thin walls or a tube with a small diameter and thick walls? And would this be the same for square tubing as well?

Thick walls always stronger regardless of diameter... but you could go chro-mo and get a lot more strength with thin wall. That's why the racers do it, same diameter as mild steel but a lot thinner gauge, so less weight.

chromo is not for amature welders, it has a very tightly grained structure, inproper welding will cause it to crack, brazing it will make it shatter as it cools.

Correct! but if it was a small part, he could farm it out to a pro like I did. Just tryin' to throw out some options. :D

true

but do remember, chromo breaks where mild steel bends. if that works for you then do it.

Sorry, but the bigger the diameter, the strength increases, so weight for weight a big diameter tube would be stronger.
Also, a round tube is stronger than a square tube. I have the equations if you want...

Rich

I believe you, I'm not sure I understand why though. Is it just because of the larger diameter having more surface area to dissipate/spread the load?

I figured the larger diameter tube was stronger. I dont plan to do much messing around with chromo until I can get a TIG welder. I like the weight benifits of a chromo cage but in the end I think a mild steel cage will be more protective cause I know havin a big metal tube stickin through you isnt very safe.

It basically comes down to the fact that it has more cross sectional area further away from the Neutral axis (this is the part of the tube that is neither stretched or compressed).

Rich

4130 chromemoly can be mig welded effectively, the biggest problem is cooling it too fast-that's where the brittleness comes from. If you require a very light weight car and can afford the chromemoly it's good stuff and welded correctly will not "shatter". Mild steel is considerabley cheaper, easier to get good welds on and with the correct dia. and wall thichness is more than strong enough for anything you might like to fab.

What do you plan to build?

ken