mikeinatlanta

One of those situations where everyone is right, and wrong. It is absolutely correct that the aluminum trailers out there are poorly engineered, poorly constructed, and with the wrong materials. It's true that just saying aluminum means nothing while what type of aluminum means everything. Comparing the strength of aluminum and steel without stating the alloy of aluminum is pure nonsense, as are most of the anti-aluminum ads from steel trailer makers.

The sad truth is that trailer manufacturers fabricate with aluminum as if they are making steel and seemingly have no idea how to properly use the material. The sad truth is also that if they were to properly build using the material as it should be nobody would buy it due to cost.

There is no clearer indication of this that the enclosed trailer category. Most enclosed trailers can't haul much more than what they weigh. My 7k 20' enclosed weighs 3,200 for a payload of 3,800. Considering that it's just and empty box on wheels with no driveline it's truly sad. They are all a bunch of poorly engineered crap. A properly engineered 20' enclosed could easily be built at 2,500lb or less in aluminum.

5.0GN tow

What you say is very true in many cases. I work with both materials a lot in my work. We use a lot of 6061, 7075, 7029, and similar 6000 series alloys. We also use a lot of steel ranging from 1018 and 12L14 to 4140, 4340, and stainless in 15-5, 17-4, and the 400 series.

All materials have their niche, but to build a trailer using 6000 or 7000 series aluminum would be insanely expensive. At between $2 and $4 a pound for raw stock depending on the alloy that gets you to a range of 10k to 20k just in raw weight of material aluminum before any fab costs for a big gooseneck. Figuring that half the weight of the 10000lb empty shell before LQ is frame, floor and shell framing is where I arrived at that number. To be comparable in strength to steel these are the usual alloys chosen. They might be able to go lighter than now common practice to save money as these alloys are much stronger than most.

5.0GN tow

Also just wanted to add, the basic aluminum 3H GN I used to have was only 4550 pounds empty, it had a GVWR of 16800lbs. So some trailer manufacturers are or were building very strong lightweight trailers. It was a 2004 Sooner built before they were bought out by a conglomerate. It also commanded a high price when I traded it in as many guys are looking for the older Sooners.

5.0GN tow

Double post sorry

Järki

Mild steel tensile strength = ~ 78,000 lb.
6061 Al = ~ 42,000 lb.

or about 54%.

acdii

Can you say with 100% certainty that the aluminum referred to, and used in their trailers is 6061 and not some other grade that is 1/3rd the strength of steel, or just tossing it out as an example?

Flamingtaco

There's not really anything that has had a cost effectiveness over 6061 other than 6063, and the strength numbers are no where near the mythical claim of 1/3 that has been tossed about since the 70's. 6063 is now more expensive than 6061, so anyone not using 6061 for structural aluminum members is a real moron. Not saying there aren't morons!

A36 is what you will find as structural. Expensive trailers with serious weight requirements will use HSLA, and some backyard bargain builders use discarded street sign posts. Almost everyone in between is on A36 due to it's bang for the buck.

6061 is aluminum equivalent to A36. Everything else costs more, or is too weak or brittle for these purposes.

Yield
A36: 36300 psi
6061: 40000 psi
6063: 31000 psi

Yes, you read that right, ubiquitous A36 has a lower yield limit than the also common 6061, and not much higher than the sometimes used 6063. But wait, there's more...

Density
A36: 7.8 g/cc
6061: 2.7 g/cc
6063: 2.7 g/cc

That's right, steel is 2.9 times more dense than the two most common aluminum alloys in use. Even if 6063 is used, it's 6/7ths the strength at 1/3rd the weight. Anyone producing "cost effective" trailers that says using aluminum is expensive either hasn't checked market prices in over a decade, or is using a cheaper metal than A36. Cost for A36/6061 tubing is about the same.

Step up to HSLA (80,000 psi yield), 6061 comes in at 1/2 the strength, 1/3 the weight. You could still build a lighter trailer, but an HSLA chassis will be cheaper than a 6061 (or 7xxx). I've only heard of HSLA in use with very large trailers, though, 15 tons and above.

acdii

I do know that quite a few flatbed trailers I hauled back in the 80's were all aluminum and they loaded and rode nice. They were the pre-stressed kind that were curved upward in the middle and when loaded flattened out. Don't know what they were made of, but they were a lot lighter by several thousand pounds.

5.0GN tow

Yes tube 6061 is not super expensive, but let's do a little thinking here. Dont know if you have worked on horse trailers or not, but the main rails of the frame and its cross members are not tubing. They are solid beams. The ones on my Sooner were about like a 4x4 in overall size and solid, I know I drilled into them to frame a LQ. The cross members also were solid I beam type. All vertical structure was heavy channel with horizontal top frames also solid. 6061 in extruded form right now in lots of 10k pounds or more is over $2 per pound. That would be a pricey trailer compared to steel. A 20 foot section of frame channel A36 is going to be under $400 depending on thickness.

One last thing most welded joints, the Achilles heel of aluminum should be considered to be more along the lines of T4 temper 6061, which is yields at 16k psi. Unless the whole structure is reheat treate after welding.

acdii

When we have parts heat treated for the B-17 they have to be restamped immediately after cooling to retain the shape as they get all warped, and has to be done before the molecule alignment sets or they are trash. Can't imagine what heat treating an entire frame would be like.