I would liketo build a new frame for my jeep CJ-7 and want something that will not rust, ever. Would 2" X 4" X 1/4" wall aluminum tubing be strong enough. How would that compare in strength to 2" X 4" X 1/8" wall steel which is what aftermarket frames are made from?
The aluminum would be roughly 2/3 as strong. More importantly to handling, it would be 2/3 as stiff. Whether that's enough, someone else would have to answer.
To get an advantage out of aluminum's low density you would have to use different sectional depths or shapes. Since you're using the same section (2 x 4) with greater thickness, you're facing the specific strength/stiffness/weight relationships of steel vs. aluminum. The rule of thumb is that aluminum weighs 1/3 as much, is 1/3 as stiff, and is 1/3 as strong.
However, the strength varies with the alloy, both of the aluminum and the steel. You can compare weight and stiffness easily, because they're the same, pretty much, for all alloys you're likely to encounter. But as someone will probably point out, you could alter that strength ratio by quite a bit by choosing specific alloys.
All in all, though, I think you'll wind up with a flexi-flyer that doesn't have adequate stiffness. It's a lot cheaper to go all out in protecting the steel. For example, epoxy-based, zinc-loaded primer.
Perhaps you could build the frame from steel in bolt-together sections which you could get galvanized, followed with a contrasting epoxy paint. Thus you can see where the paint has chipped off and touch it up before the onset of rust.
Lacking galvanizing you can flame spray the sand blasted frame with metallic zinc, using a specialized oxy-acetylene flame spray torch. Followed by painting. Check if there is such a service in your area.
I would not use aluminum for the reasons Ed gave, but also, aluminum is not fatigue failure proof as carbon steel is. Depending on the amount of usage, you may find that certain frame joints and attachment points experience pre-mature cracking.
That said some AUDI's have aluminum frames but they spend big $$$ to conduct finite element stress analysis to keep stress concentrations to a rock bottom minimum. Certain heat treated aluminum alloys can carry an amazing amount of stress, 12,000 to 15,000 psi IIRC, with a fatigue life of 50,000,000 cycles,but the stress concentrations negate this.
Can you have an entire Jeep frame hot-dip galvanized? I've never seen anything bigger than about the size of a toaster oven done that way. But maybe the frames come in pieces?
The solution for inside treatments used by homebuilt airframe builders is linseed oil, which seems to work OK. I've also heard of pouring in zinc chromate primer, through a drilled hole, and then letting it out. Then there is another aircraft solution, which is to pull a vacuum in the tubes, after they've been interconnected with drilled holes, and installing a vacuum gage in one of the tubes to keep an eye on it (but that's mostly to detect cracks, although it also detects the integrity of the air seal). That sounds like overkill to me.
The Excalibur car people used to fill the square tubes they used for frames with expand-in-place foam. Supposedly it works quite well. They initially used a urea-formaldehyde foam, but that pulled away from the metal with age. Polyurethane foam-in-place has been recommended.
Considering that the aftermarket frames are mandrel bent in both directions (read LARGE setup cost for the first one!), you would need to cut and weld the aluminum frame. IIRC there are 6 bends per side so 6 cut and weld joints in aluminum. Assuming you use something like
6061-T6, the weld areas will eventually age harden to about 6061-T4. To get a reasonable compromise between cost and strength and stiffness, you would need to go to a 2"x5" or 3"x5" aluminum rail. Other posters have mentioned the fatigue issues, I think I'd be more worried about weld strength, HAZ areas, and the lower yield strength near the welds. THEN you get to worry about fatigue.
The frame dimensi> I would liketo build a new frame for my jeep CJ-7 and want something
It'll break. It's one of the reliable characteristics of aluminum. It flexes, work hardens, and comes apart.
Stick to steel. It welds, flexes, and lasts, far better than aluminum tube will.
The aluminum frames for cars that are out there, definately are not just cobbled together out of tube. Serious stress analyisis, mondo heat treating ovens, carefull process control all the way. Not a home remedy.
many years ago i visited a hot dip place in long island city, near the noguchi museum and mark di suvero's studio. i was amazed they let me walk all throughout the shop. checking out their huge vats of pickle, etc. there was a big vat of molten zinc, it was flush with the floor, no protective barriers around it at all. i was astounded (not just at that vat of molten zinc but that they'd let me walk right up next to it!) the vat was (in my memory) about 5 or 6 feet wide and about 12 or 15 feet long, no idea really how deep, i only guessed it must be about as deep as the acid tanks, 4 or 5 feet deep. that must've been one very HEAVY thing huh?! was way cool. they're out of business now. a shame.
oh, and they had stacks of stuff that they'd already done. was very pretty, that crystaline pattern of the hot dip. the way i remember it they were dipping street corner traffic signal poles, so i bet the tank that i remember was even longer than 15 feet.
Hollow tube frames can be done also if you know what you're doing. A guy near me has Marcos steel tube chassis done inside and out, he said it did take a few goes to get it right and he has the dipper trained to do the job right. Someone else I know had a simple hollow tube chassis done and said he could lift it by himself when it went in but not when it came out as the tubing hadn't been drained properly, costly as you pay by the weight of zinc typically.
Actually, any 6061T6 weldment WILL eventually age to T4. However, aluminum is NOT the material of choice for a jeep frame. ANY deflection in aluminum is a cumulative stress, and the frame WILL eventually fale from flexing.
Go with steel, hot dipped, or perhaps stainless. Acid dipped and then hot dip galvanized will coat inside and outside. I'd sand blast first, particularly the weldments, to be sure the zink will get a good adhesion to the steel.