What size square tubing for building a frame?

[brrymnvette]

I'm going to be starting on my frame for my 88 Mustang Notch. I am basically back-halving it sort of.

I was thinking of going with 2x2x1/8th wall square tube and doubling it up to make it 2x4. Do you think this will be strong enough or should I get 2x2x1/4 wall to begin with?



Car will have at least 700 to the wheels and a cage.

Either size of tube won't be too much money, I have a hook-up of sorts. Which makes me wonder why I'm even asking. I should get the thicker anyway if it's cheap.

Thoughts? Would 1/4" thick be enough? Would 1/8" thick be enough?

[Mathius]

I'm going to be starting on my frame for my 88 Mustang Notch. I am basically back-halving it sort of.

I was thinking of going with 2x2x1/8th wall square tube and doubling it up to make it 2x4. Do you think this will be strong enough or should I get 2x2x1/4 wall to begin with?

Car will have at least 700 to the wheels and a cage.

Either size of tube won't be too much money, I have a hook-up of sorts. Which makes me wonder why I'm even asking. I should get the thicker anyway if it's cheap.

Thoughts? Would 1/4" thick be enough? Would 1/8" thick be enough?

What would be the point or advantage of buying 2x2x1/8" and "doubling it up" ?

I would not use anything less than 3/16" for a frame unless you're an engineer. Particularly with 700hp to the rear wheels.

2x4x3/16" is a common size for frames, and heavy enough if you sufficiently gusset the proper areas and put your crossmembers where they should be. Going up to 1/4" is probably just adding more weight.

Mathius

[parsonsj]

Morrison's 2x4 frame rails are .120 thick. 2x2x.120 times two would easily be sufficient, but why not use 2x4x.120 ?

p

[brrymnvette]

2x4x3/16th" is ordered. 20ft cost me $80.

[highschool70ss]

Where did you get your steel from? Is it in the STL area.

[brrymnvette]

Yes in the STL area. The place I used to work for sells/stocks square, round, solid pipe and tube.

National Sales Company
4201 Duncan Ave
St Louis, MO 63110
314-531-3200

[highschool70ss]

Thanks, I will have to check them out.

[JMarsa]

What would be the point or advantage of buying 2x2x1/8" and "doubling it up" ?

I would not use anything less than 3/16" for a frame unless you're an engineer. Particularly with 700hp to the rear wheels.

2x4x3/16" is a common size for frames, and heavy enough if you sufficiently gusset the proper areas and put your crossmembers where they should be. Going up to 1/4" is probably just adding more weight.

Mathius

I'm not an engineer although I play one at home :-)

I've read somewhere that "Doubling Up" has a huge factor increase in strength because of the added corners.

--JMarsa

[Mathius]

I'm not an engineer although I play one at home :-)

I've read somewhere that "Doubling Up" has a huge factor increase in strength because of the added corners.

--JMarsa

The idea you're suggesting is that by stacking the tubing and welding them together is that the horizontal bracing will provide more inherent strength. I believe you're putting twice the amount of potential stress into your welds. It's hard enough to properly engineer a frame capable of racing where you can get good welds, but you're talking about a structural weld basically the length of the frame.

A lot of non welders or home taught welders don't realize that in order to lay a good weld down on tubing you should either sleeve the weld, or purge the inside, otherwise you risk impurities on the inside of your weld.

And again I would like to present the example that you can take a piece of 1/8" steel and bend it with your bare hands. If you get the design wrong, there's a pretty good chance that frame will fail and you could very well die from the experience. Is it worth taking the chance?

I've been welding for something like 8 years now, but I'm not an engineer. That's why I overbuild everything or follow the prints. I'm not willing to put my own safety or someone elses at risk based an idea I think _might_ work.

Mathius

[LateNight72]

I'm going to be using 3x6 for the Chevelle. But I over-engineer stuff.

[parsonsj]

A lot of non welders or home taught welders don't realize that in order to lay a good weld down on tubing you should either sleeve the weld, or purge the inside, otherwise you risk impurities on the inside of your weld.

I disagree. I've read and read and read about all kinds of welding: the vast consensus is that the only common tube material that needs to be back purged is stainless steel.

Here's just one example from Miller Education:

http://www.millerwelds.com/education...rticles70.html

There is not a single mention of back purging.

Back purging your non-ss tubes isn't harmful. Just unnecessary.

jp

[gto406]

For those not familiar with 'back-purging' is there any good sites that discuss the process?

I found the following for TIG welding, but I am not sure this is what you guys are talking about:

http://books.google.com/books?id=RKP...esult&resnum=8

With respect to the first post in this thread...
I would recommend to anyone building a frame (or any structure like a roll-cage, etc.) to examine the rule-book of an auto-racing sanctioning body that runs a class (or type) of vehicle _similar_ to what is planned. For example, if the plan is to do solo events or the odd track-days, the SCCA would be a good place to start (formal rules can be found here - http://cms.scca.com/documents/Club%2...egulations.pdf). These sanctioning bodies will provide (in there rule books) alot of good details on race car construction. I am sure you could also contact one of their stewards to clarify the information (if required). The nice thing about going this route, is that the sanctioning body has the benefit of input from race car constructors, stewards, standards bodies (like SFI - http://www.sfifoundation.com/), all providing input. Also note that these bodies are typically liable (if something happens at an event) so you know the emphasis will be on the safety aspect.

Thanks,
Brian.

[parsonsj]

Here's one of my favorite articles from Lincoln:

http://www.lincolnelectric.com/knowl...hrome-moly.asp

One of the questions addresses back-purging in 4130 welding.

As for me, I back-purge when I weld SS exhaust tube for headers and such. I have a separate tank (100% argon) and regulator. I have it connected to a ball valve assembly with 1/4" pipe thread on one side. From there I make adapters to the diameter of tube I'm welding. I cap one end of the tube and make some small holes in it (aluminum foil and wire works well). Connect the other end of the tube to the regulator, flow as low as your regulator supports (10 cfh or so) through the tube, give it a minute or two to purge the oxygen and replace with argon, and weld as normal. I use the ball valve to stop the flow of argon when I'm not welding.

Back-purging is a real pain in the ass, actually.

jp

[parsonsj]

I believe 3/16 wall thickness in tube is overkill. .120 is more than adequate, and is what is commonly used in race car fabrication.

If it gives you peace of mind, then fine. It certainly won't make the resulting product less safe.

jp

[gto406]

It sounds like back-purging shields the 'inaccessible' portion of the weld area by basically surrounding it with shielding gas (which is why I assume you need to create an air-tight cavity of the tubing you are welding on).

Yes, the process does sound like a major PITA!

Thanks for the clarification JP,
Brian.

[parsonsj]

I dug up some data about tubes. I remembered that, in general, the sectional area of a tube determines the strength of it. Wall thickness has a negligible effect, but I didn't remember exactly how to calculate that.

The basic way to compare is calculate the difference in section modulus. Section modulus is the measure of the resistance to bending, loosely speaking.

S = [x * y (raised to the 3rd power) - x1 * y1 (raised to the 3rd power)] / 6 * y

where
x is the short side of the tube (2 in this case)
y is the long side (4)
x1 is the remaining short side minus wall thickness (1.75 or 1.625)
y1 is the remaining long side minus wall thickness (3.75 or 3.625)

For a 2x4 x .125 tube, the section modulus is 5.17 square inches. For a 2x4 x .375 tube, the section modulus is 5.19 square inches.

So, in general, it means that the heavier wall tube will resist bending through its long wall side by about .05 percent more than the thinwall tube. Note the decimal point. (somebody check my math, please!)

Given that the heavier wall tube weighs nearly 2 lbs per foot more, it doesn't seem the higher cost and weight for .05 percent more "strength" is worth it.

jp

[parsonsj]

t sounds like back-purging shields the 'inaccessible' portion of the weld area by basically surrounding it with shielding gas (which is why I assume you need to create an air-tight cavity of the tubing you are welding on).

Yes, the process does sound like a major PITA!

That's right. The idea is to remove the oxygen from the back side of the weld.

jp

[brrymnvette]

Lots of good info in this thread. Thanks for the tips/help/suggestions. I went with the 3/16th thickness b/c I like to over do some things too.

[Mathius]

I disagree. I've read and read and read about all kinds of welding: the vast consensus is that the only common tube material that needs to be back purged is stainless steel.

Here's just one example from Miller Education:

http://www.millerwelds.com/education...rticles70.html

There is not a single mention of back purging.

Back purging your non-ss tubes isn't harmful. Just unnecessary.

jp

John, you can disagree if you want to, but experience tells me otherwise. Even with back purging, I've had tubes fail to pass a visual inspection inside during a cert practice.

Depends on the process of course, stick welding provides its own flux, but think about what you're dealing with... The whole idea of a shielding gas is to "shield" the weld from impurities in the air while welding. I believe in some cases like argon, the gas actually removes impurities. IIRC argon removes oxygen from the weld... but I digress, it's not import to understand how the gas works to understand what I'm getting at.. If you're going for 100% penetration on a tube, you're technically welding top and bottom. What's removing the impurities from the weld on the inside of the tube?

Also, you don't know what might have happened on the inside of that tube. What chemicals were run through to clean it... was it stored somewhere where it got wet inside? Was it just bad steel?

Personally I prefer plugs. They're much easier to deal with. But there's a whole extra science involved to using them as well. Solid plugs should be used when going from a large to small diameter pipe. Otherwise hollow plugs should be used. You should put holes in the adjoining tubes and use rosette welds. Some will argue that a diagonal weld along the tube is better, but I've never seen a convincing argument that that is necessary.

Mathius

[parsonsj]

Depends on the process of course, stick welding provides its own flux, but think about what you're dealing with... The whole idea of a shielding gas is to "shield" the weld from impurities in the air while welding. I believe in some cases like argon, the gas actually removes impurities. IIRC argon removes oxygen from the weld... but I digress, it's not import to understand how the gas works to understand what I'm getting at.. If you're going for 100% penetration on a tube, you're technically welding top and bottom. What's removing the impurities from the weld on the inside of the tube?

Argon or any other shielding gas does not remove impurities. Shielding gas is to shield the puddle from oxygen so that the molten material doesn't oxidize while it's liquid.

As far as reading your links from the Chopper Underground site, I have to be a member to see them... so I can't read them. My only comment since I can't read them is that my links were from Lincoln and Miller, the two biggest domestic suppliers of welders. Between message board comments and welder companies, my money would be on the welding companies.

Back purging structural welds will certainly do no harm, and I do understand why some people would go to such lengths. My take is that it is unnecessary, and I'm backed up by a wealth of professional welding educational material.

jp