Subframe conectors, what kind of steel?

[Guest iskone]

I'm planning on using 1x3 .085 but I don't know what type of steel to use.

Mild steel, 304ss, chrome moly.

 

Isk

[Guest iskone]

I have a choice between .065 and .120 so no more .085. Both in mild I was told that would be fine.

.065 1.67 lb/ft

.120 3.07 lb/ft

 

Would the .065 be too thin?

 

Isk

[Wagz]

I used .085 mild steel for my connectors. My personal feeling was that if I was going through the trouble to build the SF connectors, I didn't want to take the chance of using something too light. .065 is probably ok, but I would opt for the thicker stuff.

[MTenneZ]

When you say .085 do you mean 14ga? Because when I asked for .085 around here people thought I was crazy. Perhaps they're right? So I asked for 14ga and they said "Oh you want .083" So anyway that is what I went with. And that .120 (11ga) would definitely be stout enough, but perhaps overkill. I'd stay away from .065(16ga) but that may just be me. Others (Pete) have used .095 (13ga) I believe.

[Guest iskone]

I'll go with the thicker stuff.

 

I've got alot to figure out before I go trhough with anything though. Lets just say a long post is coming.

 

Isk

[Wagz]

MTenneZ, I believe you are correct. .083 or 14g was what I used. Pete used the same for his subframe connectors (I followed his plans verbatim). Pete used .093 for the frame rails in the engine compartment. The .120 is the way to go. The weight gain is nominal.

[pparaska]

Yes, 0.095-ish in the engine frame rails. 0.083-ish in the subframe connectors. I'd not go below that and do the .120"

[Guest iskone]

Looks like I'll use either 1" x 3" .120 or 1 1/2" x 3" .080

 

My diff mount is 1 1/2" x 3"

 

Isk

[pparaska]

If you can deal with it fitment-wise (I don't think having the subframe connector extend down 1" below the floor and 1" above is a problem, personally) , I think that 2x3 is a better solution. That's assuming that the we're adding the subframe connectors to add vertical bending stiffness to the floor area, and probably some torsional stiffness too.

 

The stiffness in vertical bending of the subframe connector (with the 3" side being the horizontal side in this application) is a good bit higher than a 1x3 or 1.5x3. It has to do with the mass moment of inertia (I) of a hollow rectangular cross section ( I = width1*height1*height1*height1/12 - width2*height2*height2*height2/12 - where width1 is outer width of tubing, height1 is outer height of tubing, width2 is inner width of tubing, and height2 is inner height of tubing).

 

http://www.engineersedge.com/calculators/section_square_case_10.htm

 

Here are the numbers for the moment of inertia of the 3 tubes, assuming the 3" portion is horizontal, and the wall thickness is 0.120":

1x3: I = 0.15 in^4

1.5x3: I = 0.38 in^4

2x3: I = 0.74 in^4

 

The 2x3 is 5 times as stiff in vertical bending as the 1x3, and twice as stiff as the 1.5x3. I'll take the 2x3, thank you!!!

 

The tubing probably also gets twisted as well, since the chassis in general twists, mostly under suspension loading. Thge torsional stiffness of a structural member is proportional to it's polar mass moment of inertia of the cross section (J). This is a bit messier mathematically, see this interesting Cobra builder's web site for an interesting discussion and the equation for J: http://www.erareplicas.com/factory5.htm

 

Again, assume the wall thickness is 0.120"

 

1x3: J = 1.1 in^4

1.5x3: J = 1.6 in^4

2x3: J = 2.2 in^4

 

The 2x3 is twice as stiff as the 1x3 in torsion, and 39% stiffer as the 1.5x3 in torsion.

 

Now, about weight of the different tubes (all 0.120" in this case):

1x3: 3.1 lb/ft

1.5x3: 3.5 lb/ft

2x3: 3.9 lb/ft

 

No big deal really. The subframes are about 6 ft long each IIRC, so that's a total weight addition of:

1x3: 37 lbs

1.5x3: 42 lbs

2x3: 47 lbs

 

2x3 sounds good to me!

[Guest JAMIE T]

FWIW, I use a similar method as Pete for my front subframe and mid frame. But, I used 2x2 .120 wall for the front, and 1.5x2 .120 wall for under the car. As it is, I can jack one corner of the car and lift 3 wheels off the ground.

[boodlefoof]

I'll give a second to 2x3 being stronger than 1x3. Most people I know of making connectors are using 2x3x.083... those are going into Camaros, and Mustangs.

[BillZ260]

iskone, Pete's the man when it comes to this part of the build, hit up his site for some good drawings he posted, as well as documented step by step how to. I used his info while doing mine and had no problems. Keep in mind each year the chassis may have changed a bit here and there so you'll probably end up changing the over all design a tad for adjustments.

 

Personally, I went 2x3" .120 wall and after I finished sticking them in, like Jamie T, I now have awesome jack pads! The tops of mine are same level as the floor, but either way is fine. Show us your progress!

 

And FWIW, you need about 6' of tube to make each connector. I bought a 24' bar and had it cut in to 6' lengths and was able to make two and have enough for two more, or a small press

[Guest iskone]

Thanks for the ionfo Pete!!!!

I will have to figure out how I'll use the 2x3

 

Isk

[74_5.0L_Z]

I used 2.5" x 0.085 wall square mild steel tubing for my subframe. The cage tubing interior to the car is 1.625 x 0.120 wall 4130 steel, the hoop at the firewall is 1.625 x 0.083 4130, and all of the tubing forward of that is 1.625 x 0.065 4130 steel.