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Discussion Starter · #1 ·
I'm looking to make a custom subframe and would like some advice from someone more knowledgeable than myself on the matter.


Would a 304 Stainless round tubing in say .75" with a .0625 wall be sufficient for a max of about 200lbs?

I could use .125" tubing and that should definitely be sufficient (I believe. Someone correct me if I'm wrong please), but it's also significantly more expensive. I don't want to skimp on safety, and won't if I have to, but if the thinner wall tubing will work well, then why spend more?

The reason I would like to use 304 Stainless is its corrosion resistance, bendability, and weldability. I would rather not use regular steel because of the likeliness of corrosion, and I'd rather not use aluminum because it's not as strong as steel, plus I'm pretty certain the guy that will do the welding does not have the capabilities to do aluminum welding. However, if you have a suggestion for a better material, don't hesitate to put it out there.
 

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I had a race subframe made out of 3/4" square tube aluminum 1/16" thick. Don't know what alloy. I weighed 200+ when I used it, no issues with strength.

My current subframe uses stock lower rails with 3/4" square tube 1/16" thick 1018 upper. The design is not ideal from a strength perspective, but it's still plenty strong.


So yes, I think you should be perfectly fine with the subframe you're proposing, presuming it's a reasonable design and put together well.
 

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Discussion Starter · #3 ·
Another question.

Should I modify my tube selection based on the fact that I would like to bend it at about a 6" diameter curve?

Theoretically it shouldn't matter because the U-bend will be receiving downward stress instead of lateral stress. Either way, I'd like to see if it might matter before I buy my tubing.
 

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I'd use 4130 Crome-moly tubing as it is very strong, commonly available and easy to properly weld. Very Old School.:) You ARE planning on painting it, yes? I'd think about stainless only if you planned to polish with no finish, and realize that you might be sacrificing strength vs the Cro-mo. Of course, how you build it will determine the strength and you could use spaghetti if you did it right.

If you are worried about internal corrosion you can drill a small hole in each run and fog in some preservative oil. They do this routinely with small aircraft. You are going to TIG weld this..yes? If not..you really should.:)
 

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Discussion Starter · #5 ·
Good thought about the preservative oil. I will wind up painting it I'm sure, but I'm just paranoid about doing the work and then having it corrode, which is why I was thinking stainless. My bike tends to spend some time exposed to the elements and I don't really want to deal with it rusting.

As for welding, I was going to have a shop do it because I have no experience (or a welder), but I'll make sure to have them do TIG instead of MIG. I know TIG is generally considered to be better than MIG, but why do you say so?
 

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TIG makes prettier welds.
If you're going to have a shop do it, why not use aluminum and go with less weight?
 

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I know TIG is generally considered to be better than MIG, but why do you say so?
MIG seems to splatter a bit, and the small welds on the tubing will look a lot better with TIG...plus you can be sure to get good penetration. Saw a fellow welding up a new rear end for his street rod a while back. Using MIG...and he had the current turned WAY down. When asked why he was so low...he replied that it splattered all over the place if he turned it up any higher. I expressed a concern about adequate penetration at so low a heat level and left.

Couple weeks later I passed this same fellow in his new street rod on its' shake-down run. Rear end was laying behind it and it looked like it came out at some speed as there was other damage on the car and the driver looked pretty shaken up. It DOES matter when you stick metal together!

TIG gives you the control to get things incorporated well and not cook the crap out of the rest of the tubing too....so you don't need to heat treat it afterwards, at least with 4130 chro-mo. But...I am not a welder, so please take my recommendations with a large grain of salt and confer with those who run the torches for the final word on this. The only tubing I've personally welded was with oxygen-acetylene torch...but you don't want to do it that way..unless you are trying to duplicate the 'Old Ways'.:)
 
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