Frame Stiffing ERW vs DOM

[pontiacmcl]

Looking for some input. I am working on an A body (GTO) and am wanting to stiffen it up. I am currently welding the seams up and will be boxing them in. I am looking at doing some thing like what HPI customs (and others ) have done and weld some tubing under the floor board and add a cross member. I am by no means an engineer, so I have some probably basic questions. I for this intended purpose should I go DOM or is ERW more than enough (about 5,000 difference in tensil strength 55000 vs 60000) I am looking at 1 1/2 .125 erw (1018) or .120 dom (1020) . I also have a design question. Here is what I am thinking.
https://static1.pt-content.com/images/pt/2013/03/chevelle_frame0002_zps9b0cbec0-1.jpg
The front to rear tube is straight in this picture with a brace from under the frame in the front up to it. I thought this would add strength to this weak area. Then I might step up to 1.750 tube for the cross members.

Thanks in advance.
David

[raustinss]

Dave as a fitter the use of dom is more for applications associated with machined parts and machine building which I do both... regular...erw tubing is certainly more then enough for the frame. also for the costs involved I highly recommend going with a hpi frame brace kit. The exhaust hangers and drive shaft loop is also included and look awesome. Again I'm a fabricator and do this stuff all day and will be buying their kit just because they have already done all the work of engineering and all the necessary measuring as to what fits where. Good luck with your build
Ryan

[pontiacmcl]

I admit this would be much easier. HPI seams like a very reputable company and I think their price is very fair, but coming to east TN I would have close to 800.00 in it. At this point in the game I have more time than money (not much of either). Thanks for reaffirming my knowledge of DOM vs ERW, I can pick up a 20ft stick for 35.00 here then add some 1/8x4.5" flat for the boxing and I think I will go 3/16 for the rear cross member. I am lucky to work at a Vocational training school and have access to much equipment I usually would not. Any opinion on limiting frame flex in the font where it turns in and up?

[raustinss]

Honestly if I had to guess and it would be just that I would say you shouldn't have really any flew in that area...think of a flat piece of sheet metal....flimsy right?.. feed it thru a roller...quite stiff right? so using that logic and looking at the amount of bends and curves there I'm going to say no flex, now while on the subject I do know quite a few chevelle guys like to stiffen up the rear cross member where the upper control arms mount to and also have seen a piece of tubing put across from one side to the other where the rear shocks mount. Hope I've been of some use lol best of luck... don't forget post pictures
Ryan Austin

[raustinss]

Honestly if I had to guess and it would be just that I would say you shouldn't have really any flew in that area...think of a flat piece of sheet metal....flimsy right?.. feed it thru a roller...quite stiff right? so using that logic and looking at the amount of bends and curves there I'm going to say no flex, now while on the subject I do know quite a few chevelle guys like to stiffen up the rear cross member where the upper control arms mount to and also have seen a piece of tubing put across from one side to the other where the rear shocks mount. Hope I've been of some use lol best of luck... don't forget post pictures
Ryan Austin

[Ron Sutton]

You need to be careful assuming that ERW is made from 1018 mild steel. What I do is check with the vendor to be sure. In my experience, most ERW ranges from 1010 to 1014. DOM varies too. From 1018 to 1026. If you have access to 1018 ERW & it is priced at half of the DOM ... that should work well.

[19,69camaro]

Not only is ERW strong enough I would also go with .083 of .095 wall with tha amount of bracing thats going in there. Also I would look at bend strength as well. From what you drew I think you may be better off with 1.75" tube and some sort of boxed brace that goes from the tube to the frame rail.