OK there are a couple different ways the subrame connectors can be done (with "realative ease") on the 69 B body/GTX in the sig pic.

We can go 2x2 and have Todd weld them to the subrames only, or 2x3 and run them to the floor where it drops down into the back seat footwell area. I have seen them done both ways, and either one looks good to me.

Which is the stronger of the two?

Does the 2x3+ welding to the floorpan add very much strength?

Gotta say I am not really worried about originality, even though it is a "real" GTX I want to do it as best I can. If that means cutting the floor, the sawzall is ready

On the other hand, if the extra 1" isn't going to add anything, why bother doing it that way?

Not being an engineer I have no idea which is better so please vote for your preference and feel free to post why you think it's better. Thanks.

Which way to do frame ties: single choice
	2x2 welded to the subframes only
	2x3 welded to both the subrames and the floorpan behind the front seats
Votes accepted starting: 04/07/05 08:14 AM
View Results

For future roll bar installation, wouldn't the 2x3 coming through the floor be better?

That way the main hoop could run right through the frame connector, instead of sitting on a plate. Wouldn't it be stronger that way?

Help?

Hey Zip, I know mine is an ugly A body, but you can check them out Friday night. I bought them from Mancini, but don't remember the cost. Maybe you can get an idea anyway.

Zippy, the section modulus (SM) of 2 x 3 steel tubing will be 50% stronger than 2 x 2 providing the wall is the same thickness. The (RBM)resistant bending moment will be significantly improved with 2 x 3 as well. That deals with length x strength. Hope that helps.

Gene

why do you need to cut the floor with the 2x3? i made connectors for my car with 2x3 and i didn`t cut the floor. mine overlap the rear frame and i had to make an l shaped piece that i welded on to the "open end" of the beam to drop it down 1/2" and it cleared the floor. mine kind of look like this- http://www.bigblockdart.com/connectors/connectors.htm

2x3 tubing is more than 3x as resistant to bending as 2x2 tubing. In fact, you could reduce the wall thickness a bit to drop the weight and still have a much stronger beam out of 2x3. I'd cut the floor and use the big stuff. Besides, 2x3 matches the existing frame stubs pretty well. If you spend a little time on it, you make it look as if there is a full frame under the car.

Rich,
I did the exact same thing rbdodge' did,,,have 2x3 with it cut/sectioned in the back and plates up front,,,no floor cutting required...
You can see it on Friday if we all get together...
Todd Lamothe (bubba's) brother did the welding...wow, what a differene in the car.
DOUG

Zip, the thing to do for the main hoop is to weld in pieces of 2x3 out to the rockers from the frame connectors & then weld your hoop to those.

I would use 3"x2" in 14 gauge (.083"), it is about 2.67 lbs. per foot. To compare weight, 2"x2" 11 gauge (.120") is 3.07 lbs. per foot. In this application, the lighter wall with the extra 1" of depth will more than make up the strength compared to the heavier wall but smaller tube. Welding it in or to the floor ties everything together much more, in that case I would be tempted to use 3”x1 ½” 14 gauge.

This goes against popular opinion, but I’ll say it anyhow. Tying the rear and front frame sections together will do a minimal amount of good to solve twisting, compared to a roll cage or even a 6 point bar with a couple of diagonals thrown in. I’m not saying it isn’t worthwhile, but it just doesn’t provide the torsional strength needed.

If you use a piece of plywood to represent a flimsy unibody, and nailed 2x4’s on it to represent the front and rear cross members and frame rails, it will be quite flexible. If you connect the two ends in the middle it stiffens it better to act as a bridge or a beam, but you would notice the resistance to twisting is not improved a huge amount. Two parallel frame members don’t resist twisting well, a cage or roll bar adds depth in multiple directions, and will add much more strength.

OK mopardad, I got ya. Outriggers. If that's the way to do it then there's no good reason to use 2x2 then. So that's out.

I like the article at http://www.bigblockdart.com/connectors/connectors.htm and it is helpful, but other than overlapping it they don't explain exactly how the 3" wide connector is fit to the roughly 2" wide rear frame rail. What did they do, heat with a torch and pound on it? Smash it with C clamps or a vise? Just weld the heck out of it? Add more pieces to narrow that part? It doesn't say.

From what little I understand about such things, it looks to me like if it were facing the other way (2" horizontal, 3" vertical) the sectioned back end of the connector will fit better over the frame rail, but will be somewhat less convienient to install.

I don't know anything about this stuff, and never claimed anyone has to run them through the floor, I am saying that I don't mind doing it IF it gives a worthwhile improvement in strength.

Looks like the silent majority is voting for 2x3 through the floor in the back and that's the way I'm leaning towards right now. Is it worth doing that way?

Quote:

---

I would use 3"x2" in 14 gauge (.083"), it is about 2.67 lbs. per foot. To compare weight, 2"x2" 11 gauge (.120") is 3.07 lbs. per foot. In this application, the lighter wall with the extra 1" of depth will more than make up the strength compared to the heavier wall but smaller tube. Welding it in or to the floor ties everything together much more, in that case I would be tempted to use 3”x1 ½” 14 gauge.

This goes against popular opinion, but I’ll say it anyhow. Tying the rear and front frame sections together will do a minimal amount of good to solve twisting, compared to a roll cage or even a 6 point bar with a couple of diagonals thrown in. I’m not saying it isn’t worthwhile, but it just doesn’t provide the torsional strength needed.

If you use a piece of plywood to represent a flimsy unibody, and nailed 2x4’s on it to represent the front and rear cross members and frame rails, it will be quite flexible. If you connect the two ends in the middle it stiffens it better to act as a bridge or a beam, but you would notice the resistance to twisting is not improved a huge amount. Two parallel frame members don’t resist twisting well, a cage or roll bar adds depth in multiple directions, and will add much more strength.

---

Thx Mike. the frame rails are about 2" wide, so I was figuring that would fit the easiest over them, so it looks like 14 gauge 2x3 is what I need to get.

I'm definitely planning on some kind of roll bar, just won't be able to swing the dough for it this year. But now that you put it that way, it makes sense that it probably would help even more than the frame ties.

I agree with M D no matter how you install it your not going to stop the twisting. I did mine in steps first the frame connectors them a 6 point roll bar then added 2 more bars down to the frame connectors then braced the front end up to the fire wall. guess what it still twisted. when you get over 500hp your kind of over powering the poor unibody. and traction remained about the same.

Quicktree you aren't saying it's not worth doing are you?

Another thing I found that helps once you have a bar installed is to tie the bar into the roof structure where the bar makes its bend to go horizontal,usually a short piece of tubing a few inches long welded in will make a big difference. Zip, the 3" is the vertical,cut thru the floor & weld solid where necessary

Someone asked ealier about the tubing thickness on 2x2" box subframes, and my reply was:

Here's the way I look at it, and as always, I could be full of it...

Obviously the OEM frame material is a lot less than .120 wall (1/8"), so that begs the question, why use anything THICKER than the existing frame? Well, the frame sections are a lot taller, AND they are tied into the floors/fenders, so they are "borrowing" load-bearing capacity from adjacent structures. That's the beauty of unitized construction. In contrast, the subframe connector is mostly on its own. It's tied into the rear floor for ~1 foot, and then it's just hanging out there. In that case, it helps to have it a little thicker. May want to look into tying it into your seat mounts that come off the rocker panel. Pretty easy to do.

I used 2x3x1/8" on my Charger and I'm currently installing 2x2x1/8" on my 71 RR.

If the car is going to see much track time, I'd go with 2x3" tubing, and with the 14 guage (thinner) material like M D and Andy recommended.

I'm going with 2x2 11 guage on my 71RR because 1) it's a street car and 2) I don't want the connectors to be visible in the front seat area. Too bad my pic hosting service dropped me (moparpages) or I'd post a bunch of pics.

I did the 2x3 through the floop

I say if you don't cut them through the floor pan and weld the pans 100% to the 2x3 you are just about wasting your time. I hang out sometimes in a professional chassis shop and just watch and b.s. with the welded there. They ALWAYS come up through the floor pans and weld them solid. I don't know what class it was for but one of the door slammer type cars that is required to have so much of the stock floor pan left in the car to qualify had tube style connectors put in it from back to front and they came up almost half way through the floor pans and were welded solid. For the structual welds they use tig with the appropriate steel filler but for thr pans to the connectors they use tig with sillicon bronze filler to keep from burning up the sheet metal.

Eric

Sure seems stronger to run them through the floor.

OK guys I'll shop for 2x3 tomorrow then.

Thanks!

Zippy, I got mine from Mancini and they were about $100.
You really don't want to weld them in, if they did flex you could rip the steel, also if welded the car could be crooked on the jack stands and when you weld them in it will be crooked off the jack stands, bolted would be better.

If you bolt them in with any torque on the nuts, you WILL collapse the frame section. Seen that many times. Bolted in frame connecters are absolute junk and serve no purpose. Sorry, but that's a fact. If they're welded in correctly, nothing is going to tear. On that note, it's a really good idea to gusset the butt welds where the connector butts up against the trans x-member.

For the bolt-in fans, I have some MP bolt-ins for sale. $50 for both.