fabricating tubular car; metals?

I'm starting to go down the rabbit hole and learn me some metal stuff. 

I see a lot of cages made out of DOM or DOCOL R8 but I see a lot of round tube only for some these applications. I am looking at adding some structure back into this rotted mess and before I really commit I wanted to see where I can learn about what metals and wall thickness to use for the right application. I'm not worried about the cage part as I can farm that out but I need a baseline before I can connect a cage to it, this will be a long project surely. 

I would ask at work but the way we build cars today is totally another scale and method, either various stamped pieces spot welded together like a puzzle or gigacasting stuff and when I look at anything built in the 60s and 70s, all of these cars can benefit from structure. 

there are too many metals out there and I don't want to get pigeon holed and frankly I almost don't know what to ask for other than where to begin? 

DOCOL R8 is only sold by one supplier, here in the United States, as I understand it. I talked to them about it before I built my cage, and it sounds great, but shipping was going to make it very expensive. If you're close to them, then the price per linear foot of DOCOL R8 is comparable enough to Mild Steel.

DOM is just the fabrication technique, I believe. Drawn Over Mandrel. It's how they form it into a tube and weld it. But you're probably more interested in comparing Mild Steel to DOCOL R8? I think both are supposed to be equally-easy to work with. I found Mild Steel to be very easy to cut, weld, and work with. 10/10, would use again.

Round tube is stronger than square. I've heard square tube referred to as pre-crushed round tube (in jest, of course). But there are plenty of applications for square tubing, and if you're just adding structure back to the body, it may be suitable.

Your sanctioning body will probably specify tubing diameter requirements for a roll cage. If you're building a structure that your cage builder will be attaching a cage to, you may want to read through your sanctioning body rules to see what mounting plate thickness needs to be. 

Yep, confuZion3 nails it.  DOM is really just as opposed to ERW (Electric Resistance Welded) tubing.  DOM doesn't have a seam when it's pulled over the mandrel (hence the Drawn Over Mandrel), whereas ERW will be rolled into a tube and welded.   Technically, the seam of ERW gives it a different property & potential failure point in a crash. 

Back in the dark ages, ERW was a lot cheaper than DOM, so a lot of people used that to build cages and stuff.  Pragmatically, I wouldn't hesitate to use it for reinforcing any sort of vehicle structure, but I wouldn't want to use to for the main safety cage. I don't think the difference is worth enough to mess with it, though you'll have to check with your supplier.

ERW/DOM/etc. has nothing to do with the type of steel, though.  You can get DOM in A36 (mild steel) which is what people normally use for cages and other structure work.  Chrome-moly (P22) is the high-end version where you can get higher strength therefore using lighter/thinner tubing, but it's more sensitive to heat fatigue and other issues if it's welded incorrectly, and it's also more brittle which makes designing a cage for a bit more challenging.  Raw material cost was !~20% higher for P22 vs. A36 last I checked, which was years ago.

I don't know anything about DOCOL R8, though, sorry!

I think DOCOL R8 comes from Indianapolis. A.E.D. Metal Products (I'm not affiliated). Give them a call if you're thinking about it. They gave me a lot of good info on the stuff.

confuZion3 said:

Round tube is stronger than square.

It's not that simple. As I recall:

For the same advertised dimension and wall thickness, 1x.065 square and round, square has more material that is also further from the center, causing it to generally be stronger but also heavier.

For the same weight and wall thickness, 1x.065 square vs 1.25x.065 round, round is now generally stronger.

Square is stronger in bending along the flat sides than it is on the diagonal. Rectangular is stronger yet in bending along the longer side. Round (DOM and seamless) is equally strong in bending in all directions, with the exception of the seam in plain ERW, as well as being more efficient in torsion.

Square and rectangular are generally easier to cut and mate for right and single axis angles. Round is easier to have structural bends and to cut and mate for two axis angles.

Pick the shape and size with the properties to suit the application.

confuZion3 said:

... Your sanctioning body will probably specify tubing diameter requirements for a roll cage. If you're building a structure that your cage builder will be attaching a cage to, you may want to read through your sanctioning body rules to see what mounting plate thickness needs to be. 

This. There is no decision making if you're building to a set of rules - they dictate what's used. Even if this is just a track day car, I still strongly recommend still following something like the SCCA rules. They know what works and what saves drivers' lives. Even for Kimini and Midlana, weekend toys, I still built them per the SCCA rule book. Oh, and beware of going with odd-sized tubing unless you already have a line on bending dies. The rule book said that 1.375" was permitted for my build, but no shop could bend it, so I went with 1.5" that most any shop can deal with.

Driven5 said:

confuZion3 said:

Round tube is stronger than square.

It's not that simple. As I recall:

For the same advertised dimension and wall thickness, 1x.065 square and round, square has more material that is also further from the center, causing it to generally be stronger but also heavier.

For the same weight and wall thickness, 1x.065 square vs 1.25x.065 round, round is now generally stronger.

Square is stronger in bending along the flat sides than it is on the diagonal. Rectangular is stronger yet in bending along the longer side. Round (DOM and seamless) is equally strong in bending in all directions, with the exception of the seam in plain ERW, as well as being more efficient in torsion.

Square and rectangular are generally easier to cut and mate for right and single axis angles. Round is easier to have structural bends and to cut and mate for two axis angles.

Pick the shape and size with the properties to suit the application.

This is good information. I stand corrected. 

confuZion3 said:

Driven5 said:

confuZion3 said:

Round tube is stronger than square.

It's not that simple. As I recall:

For the same advertised dimension and wall thickness, 1x.065 square and round, square has more material that is also further from the center, causing it to generally be stronger but also heavier.

For the same weight and wall thickness, 1x.065 square vs 1.25x.065 round, round is now generally stronger.

Square is stronger in bending along the flat sides than it is on the diagonal. Rectangular is stronger yet in bending along the longer side. Round (DOM and seamless) is equally strong in bending in all directions, with the exception of the seam in plain ERW, as well as being more efficient in torsion.

Square and rectangular are generally easier to cut and mate for right and single axis angles. Round is easier to have structural bends and to cut and mate for two axis angles.

Pick the shape and size with the properties to suit the application.

This is good information. I stand corrected. 

this is great information! I appreciate the feedback from everyone. 

I should specify since the rockers on the car are toast. rebuilding them would be more complex than just using square or rectangle tube here so for front frame rail area I was thinking square or rectangle tube to replace the rotted stamped/spot welded sections of the car and making beefier rockers.The rules allow for modifications to the floor to accommodate "other devices" which also is interpreted as seats and safety gear. Ultimately the floor will be replaced but I wanted to get the off its wheels and use pin stands in the body corners once it has some strength back in it.

from there the bulk of the framework would be round tube intersecting the firewall and then more jungle gym.