Any reason not to use round DOM tubing?

From a design standpoint, metal that is farther out from the neutral axis works harder. As a 2x2 square tube has more material farther away from the axis it is stronger in either torsion or bending. This is a Strengths of Materials, learned from Mr. Ron Apanian back in the day. Anyone else reading this take Strengths from Apanian?

As far as stiffness goes, any steel has approximately the same spring constant or modulus of elasticity as any other. So 1018 and 4340 start bending at exactly the same stress. The difference comes when the bend becomes permanent or yields. The 4340 has a much higher yield strength than 1018.

If welding was perfect then welded tubing would be less expensive, but that long seam seems to have the occasional weak spot. Of course, using a safety factor of 4 or so should cut the stress so that the weld will not fail. Just don't ask me to calculate maximum stress or safety factor for a frame connector!


R.

The greatest load in racking (angular twisting of the chassis by application of power, in which the axle planes are no longer parallel) is bending in the vertical plane. More height increases stiffness, which is why rectangular frame connectors are taller than they are wide.
Welding through the floor makes it a structural member, otherwise it's a big tin playing card.
Compare the frame rails of 1/2 ton and 1 ton trucks - the height difference is huge.
Making the tunnel taller is a key element in small sports cars - it's not just to clear the driveshaft. Taller door sills also helps, see my article here:
http://victorylibrary.com/mopar/frame.htm

This may be a dumb question, but you're talking about round tube (going all the way through the torsion bar crossmember), right?

Could this be done with 2x3 1/8-wall box tubing also? The way I usually see it installed is with a 2x6" piece of 1/8" plate welded to the box tubing and then the plate welded to the rear side of the crossmember...

Sounds like a big hunk to cut out though.

Thanks cudaman. I realize it'd be stronger welded to the floor pan everywhere, but there is considerable disagreement (over on FABO, for example) as to just how much stronger...
http://www.forabodiesonly.com/mopar/threads/sub-frame-connectors-please-build-them-correctly.286631/

What I really want to know is, could I cut a 2x3" hole in both the front and rear sides of the t-bar crossmember and slide the box tubing all the way through and weld both sides (as Monte recommended with the round tube)? Again, that would definitely be stronger than a plate connection to only the rear face of the crossmember, but will it be enough stronger to complicate the installation that way.

This cutting the huge percentage wise holes in the TB crossmember seems to ignore the main purpose of the crossmember's function, and effectively weakens it, to gain stiffness of the FC. The TB crossmeber is ALWAYS taking load, except when on jackstands or wheelstand when airborne. The FC does not. I would strongly suggest thinking this solution, at this scale (size of holes), over very carefully, as it is hard to reverse. I mentioned and others have mentioned alternative solutions.

This cutting the huge percentage wise holes in the TB crossmember seems to ignore the main purpose of the crossmember's function, and effectively weakens it, to gain stiffness of the FC. The TB crossmeber is ALWAYS taking load, except when on jackstands or wheelstand when airborne. The FC does not. I would strongly suggest thinking this solution, at this scale (size of holes), over very carefully, as it is hard to reverse. I mentioned and others have mentioned alternative solutions.

You could take a hole saw and cut holes all through the TB crossmember and not weaken it. You could also do the same to your front frame rails and 2x3 subframe connectors and not weaken them in the direction they are designed to support. This is simple engineering principles. The purpose of the TB crossmember is to hold the front of the car up. That's where the springs(torsion bars) mount. The bars are basically trying to twist that crossmember out of the car in a vertical motion. Since they can't twist the crossmember out, they push the car up. Drilling holes will not effect the strength of that crossmember in a vertical plane.

Plus when I say I cut holes, I do it for a purpose. I cut a hole right behind the front frame rail and actually insert my tubing up INTO to the front frame rail and plug weld it, plus plate it inside the TB crossmember, as well as plating the outside where the tubing enters. All this actually ties the frame connector TO the front frame, which is the point of the entire exercise.

Frame ties do exactly THAT. They tie the front and rear frame rails together...........so to properly DO that, the tie needs to actually attach TO the front frame rail, not the backside of a sheetmetal crossmember welded behind it. As I said, I run the tie inside the front rail, but I have seen guys bring the tubing through the TB member and weld it to a plate on the side of front frame rail, where it kicks out

Well then, how about explaining how a few holes in a piece of 2x3 tubing radically weakens it on the vertical plane, when you have it standing up. I am all ears