Seems to me the practice is, for a frame that will have a full cage on top of it to share the loads (and is designed to do so), .083 2x3 works. With just a four-point cage or no cage at all, .125 wall thickness frame rail material is better. For heavier hot rods, 2x4 rails at the .125 wall thickness is a good step up. (Stock frames, carefully engineered and stamped like we cannot do in our garages, are often .104.) With thinner material you want to be careful how you design brackets, crossmembers etc. so they don't attach into the middle of flat areas but catch around corners and areas of max strength instead.

Note that if you just go down to whatever steel supply and buy a length of 2x3 rectangular tubing, you will probably get something made from hot-roll steel as opposed to cold-roll. Fine if you're building a trailer but hot-roll really has no place on an automobile where strength vs. weight is important, cold-roll is 1/4th to 1/3rd stronger for a given thickness. If you buy that .083 wall stuff from a race car shop it will likely be cold-roll. 2x3x.125 cold roll can be tough to find, hot roll usually has what's called mill scale on it (which is a thin, brittle, dark skin created as the red-hot steel cools in air) so you know what it is but if "pickled" (cleaned in acid), HRS (guess what that means) looks like CRS. Google for more info.

Certainly when you're going to build a frame, getting good material to start with is vital and worth effort. You might consider shopping for formed rails from hi-perf chassis shops that have the bending equipment, they likely use good stuff and you won't have to have there be welding at the critical kick-up points. 'Way more costly than the scrap yard but if you're having trouble finding steel anyhow, worth looking into.

 

how good is your welding.

60 years ago in a college welding class we had to weld test pieces then cut into 1 inch strips do a pull test and bend test to get certified. . check out welding tips and tricks on how to do your own etch and inspection. what welding process are you going to use. Tig does a smaller heat affected zone. chrome moly tubing is stronger and lighter but is harder to get good welds. abird cageg chassis is a lot lighter and with proper design stronger.

 

Ok say it with me monocoque.

I wrote a big thing explaining this. But I think the link below does a good job at getting the point across.

https://www.buildyourownracecar.com/race-car-chassis-basics-and-design/

 

If your doing this as a back half, with no cage support helping, I would use .125". the thinner .083" is only acceptable to me with at least 6 point roll bar or cage supporting it.

I.ve done back halfs in several street strip cars that have 8+ point cage set-ups with the thicker .125", might not have been needed but it makes for a real stiff assembly. The last i did was a back-half '67 El Camino with a 4-link and coil over shocks with a Dana 60. 2/3 x .125 tube, 1-5/8" mild steel 10 point cage(front support bars in engine bay plus 8 point cage)....you can jack that car up by the drivers rear frame corner at the very rear corner until it is just teetering on the passenger front tire and both the drivers front tire and passenger rear tire or grazing off the ground and you can still open and close the doors with no binding.....so it was pretty much overkill.
The .125" wall back-half tied to the stock frame just behind the front edge of the bed/back of passenger compartment with a heavy 2/4 crossmember with driveshaft loop cut-out and corner triangulation where it met the frame., frame tubing was hand built miter cut in a chop saw, beveled for weld at each section, and :fish-plated" over each weld joining the tube sections together.
We did it making our own rails this way because commercial pre-bent rail kits were going to stick up too high and ruin any bed capacity.

If you use chromoly tubing, remember that it has to be TIG welded to pass a dragstrip inspection...it cannot be MIG welded and be legal.

 

black laquer chassis

A friend used to paint all of his chassis and suspension parts with black laquer. Stress points will crack the laquer and you see potential problems before you start seeing cracks,
fireball tools sells jigs and fixtures adjustable squares and set up squares .
this video shows 2 of the 6 he presently makes

home depot sells a layout tool for carpenters but works great for getting your cut lines around corners of tubing
Husky-Universal-Square
it's T shaped aluminum

 

Pics of chassis, being built from pressbrake-formed CRS (mostly 12ga/.104") in order to get whatever size/shape/thickness is best for a particular portion and be able to have internal bracing where appropriate. Long runs along sides where channel is made into tubing are mig-welded, joints tigged. Body will be aluminum. It's turning into an "art project" I'm afraid, but am making slow progress. At my age it's time to build something I've thought about for awhile, and take a step forward from the Challenger which I felt was a rewarding project. Power this time is 500hp LS2/T56.

Because of using larg-ish box sections where helpful and expecting to get strength from from the 16 & 18-gauge floorpan plus some engine compartment bracing I almost built this from the next lighter gauge of steel (still not .083) but chose to err on the heavy side. Let a few more HP make up for it.

From here, my comment to the Original Post-er: However you do decide to build, just do so! If some part doesn't come out right just scrap whatever you have questions about and start over, and careful thought will keep the scrap pile small.

 

Seems to me that .083 is minimum spec for chrome moly roll cage and space frame material. 1000 series mild steels will need more thickness to match that. Ladder, or parallel rail frames are given to twisting, which makes handling somewhat unpredictable. Production cars make use of K members up front to add strength to engine weight and stiffen the area for improved suspension support. These things are cheap structural solutions but add a lot of weight.

Production vehicle bodies add a lot of rigidity once the cushion of the rubber mounting pucks is used up. A racers trick has been to replace these with aluminum or urethane pucks to remove the crush movement of rubber. Body skins held on with Dzus fasteners not so much. If you’re racing this the sanctioning body will have materials, weld, and design rules. They may require testing of sample parts and coupons including your welds. They might require that the fabricating welder have certifications for the weld technique and materials used, so you need to check this out.

There several books and design manuals on this subject that should be helpful.

If you run this on the street you assume the liabilities resulting from a frame failure. Here in Seattle several years ago we had one of those WW-II duck trucks used to move tourists across our bodies of water and city streets have a frame failure of a modification they made that resulted in a collision with a bus that killed and maimed a lot of people, the law suits against the company and the criminal investigations are into mind numbing size figures.

So I’m not saying don’t construct a frame, but do be aware that there are rules, regulations and liabilities involved.

Bogie

 

hand built race car

on this guys channel he builds a race car from scratch and has excellent explanations of the suspension geometry and how to set it up for various tracks the how and why's
https://www.youtube.com/c/SK8215/videos

 

Back half? Use the thicker stuff for sure. It will be stronger and add traction. You can look up the weight of steel tubing on a chart easily.


.082" is 2.728 lbs/ft, .120" is 3.98 lbs/ft. To me .082" is an oddball thickness, the smallest I've ever used is .095".