On Feb 17, 6:04 pm, "Ed Huntress" wrote:
"mark" wrote in message

...

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.

--
Ed Huntress


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.

Wolfgang