building jeep frame

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My original jeep CJ-7 frame has rusted out and I was thinking of building a new one from aluminum. Would 2 X 4 X 1/4 wall (if that is even available) box tubing have the equivalent strength of the stock
1/8" wall steel frame? I would like aluminum because it will last forever, no need of any paints etc..., very easy to work with and cheaper than building a steel one and having it galvanized. My second choice would be stainless 1/8" box tubing.
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I don't think a frame made of that tiny tubing would be very good.
Steve <g>
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mattathayde had written this in response to http://polytechforum.com/metalworking/building-jeep-frame-226620-.htm :
------------------------------------- mark wrote:

http://www.pirate4x4.com/forum/ is going to be your best resource for this info, these guys do very crazy builds.
i dont know my al and steels very well but i think AL might have issues with the welds stressing and with temper changing over time
-matt
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Aluminum is far less stiff than steel, and doubling thickness may yield a too-floppy frame. I would stick with steel unless you reengineer the frame.
Joe Gwinn
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This looks familiar, Mark. Didn't you post this same question here three or four years ago? Someone posted something similar.
Anyway, as Joe Gwinn says, aluminum has 1/3 the stiffness of steel, roughly 1/3 the strength for low alloys of each, and weighs 1/3 as much. A box section tube doesn't take advantage of aluminum's low density, so there is no weight advantage in using aluminum in this way. To get equal strength in the same section (2 x 4), the aluminum tube will have to have walls that are 3X as thick as the steel one.
I don't know Jeep frames but if the frame isn't boxed (in other words, if it's a U-channel or top-hat section rather than a rectangular tube), and if you use box-section aluminum to replace it, it will be a great deal stiffer and stronger. But that's because the tube is stiffer and stronger, not because it's aluminum.
All in all, aluminum sounds like it's a lot more trouble than it's worth. Welding that thick section and producing a *strong* weld with it will be no picnic, unless you're an expert. It will cost a lot more for the material. I can't speak for the galvanizing but I thought that hot-dipping a frame was supposed to be a reasonable cost proposition.
I'm sure you'll get other opinions.
--
Ed Huntress



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During our 8 year tour in the Marshall Islands where you can corrode a glass thermometer, we saw jeeps made of Stainless Steel that were made in the Phillipines. No rust.. People that owned them were not excessively rich. Don't know what they cost, but looking back we should have bought and shipped a couple of them to the US .
Stu Fields
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wrote:

You do NOT ewant an aluminum frame. Perhaps stainless steel. Aluminum frames on a jeep WILL flex. Particularly a 2X4 x1/4 tube. ANY time aluminum flexes it is a stress, and all stresses are cumulative. Frame life would likely be measured in months.
Steel is different. It has an elastic limit, and as long as that loimit is not exceeded, no cumulative stress occurs.
I'd build a stainless steel TUBE frame if I was going to the trouble - but Iron Horse (I believe that is still the name of the company) supplies ready made replacement frames at a very reasonable price.
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snipped-for-privacy@snyder.on.ca wrote:

Oh boy, if you can buy a frame that's the way to go. Unless you're just determined to build your own frame for the sake of it, and have a well equipped shop at your disposal, you'll spend less time and probably less money buying one.
Aluminum works differently than steel, and is not corrosion proof -- particularly when you bold a bunch of steel parts to it.
Stainless is hard to weld, and fatigues far more readily than steel.
Steel can be painted or galvanized to not rust, and works pretty darn well for frames. Very few cars have aluminum frames for a very good reason.
--
Tim Wescott
Control system and signal processing consulting
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The way I understand it: Frame flexibility is a compromise between handling and body distortion and reducing the bending stresses on the connections between frame side rails and cross members.
On my Landcruiser, all cross members (except for one) and attachment points are riveted to the frame rails. This allows them to flex w.r.t each other and not build up bending torque. There is a single tubular cross member near the center of the vehicle welded between the side rails that acts as a big torsion bar. It serves to stiffen the legs of the H and return the frame to a flat configuration after twisting forces are removed. The other cross members (being riveted) do not carry bending torque due to frame twist.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Why is a riveted joint more flexible?
jsw
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This Jeep is not for rock crawling or very uneven ground, 99% of it's life will be on paved roads so there will not be extreme flexing involved. As for buying one, by the time it got to me it on the east coast of Canada, with shipping, exchange rate, tax, galvanizing option, brockerage fees it would be closer to $3000.00.
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mark wrote:

Ooh -- ouch. Everyone assumes (well, at least I did, and I didn't see qualifications to the contrary) that someone who's posting is local.
OK -- build it yourself. But use mild steel. Unless you're _really_ out in the boonies there should be reachable outfits that do galvanizing. If not, mix up some zinc chromate epoxy primer (if you can still get _that_) and roll it around inside all of the frame rails when you're done with the build up.
Be sure it's square and straight, or your doors will _never_ fit.
--
Tim Wescott
Control system and signal processing consulting
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Tim Wescott wrote:

If box section then you need to provide drain holes for the zinc to enter and exit. A guy I knew had a box section chassis galvanised and the company didn't drain it well, they charged by weight so he got a bigger bill than originally estimated and couldn't carry the chassis easily any more. So get a fixed price or make sure it can be drained easily. A company near me gets Marcos chassis galvanised, talking to the owner he said it took them a few goes and a tame galvaniser to get the drain procedure right but the Marcos chassis is somewhat more complicated than a ladder frame Jeep chassis.
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Jim Wilkins wrote:

Probably because a rivet (like a bolt) doesn't support torque loads along its axis. So the joints can be made more flexible.
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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wrote:

Setting aside the question of whether SS is a good choice for the OP's project, the common austenitic stainless steels -- 304 and 316 -- are among the easiest materials to weld. The fatigue limit of both alloys is in the 35 to 40 ksi range, higher than the fatigue limit for common structural steels.
--
Ned Simmons

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Why not aluminum? I have an aluminum boat trailer. Works very well. 3400# boat. The Covette has an aluminum frame as well as the Cadillac bodied Vette. Look at a Corvette and see what they use. Airplanes have aluminum frames. And as long as you design well, the flex should not be a problem.
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Boat trailers are rarely twisted the way that off-road vehicles routinely are.
The same thing applies to Corvettes.
After all, when was the last time that you went rock-crawling with your 'Vette? <Grin>
How about mud-bogging or bouncing around on deeply-rutted roads?
Jeeps are expected to do all of these and more without any ill effects. (Getting dirty/muddy is, for a Jeep, a good thing!)
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Hell, I raced a vette, steel chassis, and it got to rock clrawing a couple times. :>) And boat trailers are regularly towed over uneven ground.
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With only 1 wheel in contact with the ground while the boat was loaded and tied down and the tow vehicle at tilted in the opposite direction?
I doubt it.
"Uneven" is a long way from "rock crawling" and your 3,400# boat is unlikely to be towed over "Jeep" roads on an all-Aluminum trailer with any regularity.
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With three points taking out the loads -- hitch and suspension supports, which generally are paired but close -- there is no significant torsional load on a boat trailer. It's all simple bending. You can deal with that, but if you towed your boat 100% of the time, I think you'd develop fatigue problems in aluminum.
The aluminum Corvette chassis are semi-space-frame with some shear panels. The subframes resolve their loads in three dimensions. There isn't much flexing there.
The same applies to aircraft, which often are near-monocoque. If they flex, you die.
--
Ed Huntress



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