Compare: Steel vs. Aluminum Trailer Frames & Beams

Aluminum is such a great material.  It’s light, strong, and looks sweet to boot.  So, why is there a debate about Aluminum Trailer Frames and other beams?

Comparing Aluminum and Steel

There is obviously a lot to compare in Steel vs. Aluminum, but we’ll stick to the 6 big items for long beams like cranes or trailer frames.

Weight

The obvious big advantage of Aluminum over Steel is weight — or more accurately, the density.  Aluminum is in the 2700 kg/m^3 range.  Steel is in the 7800 kg/m^3 range.  Those numbers may be meaningless by themselves, but if you think about it in general terms, Steel is almost 3 times heavier for the same volume.  So, a trailer frame that weights 600 lbs in steel will weigh 210 lbs if made just the same in Aluminum.  If it’s more than just the frame, the weight advantage goes up.  That’s the first and most common reason people state for an Aluminum trailer.

Strength

Is steel stronger?  Well, that depends on the alloys being compared (Steel and Aluminum).  While it’s true that the strongest Steel alloys (with proper heat treatment) are much stronger than the strongest Aluminum alloys, in common terms of materials you might purchase to build a trailer (or crane or press), the aluminums have comparable strength numbers. (Granted, there are a lot of alloys, so compare specs before accepting this and assuming all are the same.)

Examples:  Al-6061T6 (a common Aluminum alloy) has a yield strength around 40,000 psi, and tensile strength around 45,000 psi.  A36, a very common Steel alloy for angle iron, channel, etc., has (by definition) a yield strength greater than 36,000 psi, and tensile strength around 58,000 psi.  A500, a common steel tube material has a yield around 46,000 psi.  Yield is the operative condition for structures, because that’s the point where it bends permanently.  You don’t want that happening to your trailer frame or any other beam!

Corrosion

Finally, another big benefit of Aluminum (especially with Aluminum trailers), it does not rust.  Contrary to common belief, aluminum does “Corrode”, or more accurately it oxidizes.  Have you ever slid your hand along a piece of Aluminum and got black on your hand?  That’s from oxidation on the surface.

Aluminum is so reactive in oxygen, it’s effectively impossible to have aluminum present in air without oxidization.  It happens immediately when you cut it, then grows thicker as time goes on.

So, if Aluminum is lighter and just as strong, then why don’t we all have Aluminum trailer frames?  Why not use it for all the cranes and presses too?

Why Is Steel So Much More Common?

There are 3 advantages for steel, 2 are areas where Steel seriously out-shines Aluminum.

Stiffness

Deflection is a big deal in the comparison of Aluminum vs. Steel.  DON’T confuse Stiffness with Strength — they are very different properties.  Nylon rope, for instance, is quite strong, but not stiff at all.

We use the word “stiffness” frequently, yet the descriptive engineering term is “Modulus of Elasticity”.  It does vary slightly by alloy, but not much.  In general, Aluminum is around 10,000,000 psi and Steel is around 30,000,000 psi.  Again, the actual numbers are not so important, because the issue is how much a beam will deflect under load.  In general, for equal beams, the aluminum beam will deflect roughly 3 times as much as steel for the same load.  This effect is illustrated with a Gantry Crane example in this 2nd Post on Deflection.

Cost

Aluminum may be a lot lighter, but it’s definitely way more expensive too.  Unfortunately, this is one of the main reasons you don’t see more Aluminum trailer.  How much more?  Well, that depends on the market of the moment.  Steel and Aluminum prices fluctuate because they are both a commodity.

As of writing, a generally available (though slightly expensive) website Metals Depot, shows a pretty standard Steel 6″ I-Beam (S6 x 12.5 lb — 6.00″ x 3.332″ x .232″) is $13.5 per foot (assuming you buy a full 20′ piece).  It weighs 250 lbs.  A similar I-Beam in Aluminum (6″ X 3.33″ X .230″) costs $23.32 per foot (again, for a full 25′ piece).  For comparison, 20′ of that weights 86.5 lbs.

From above, and from the Posts on Deflection, when we factor in stiffness, an Aluminum beam that is reasonably equivalent to the 5″ steel beam is the 8″ I-Beam (8″ X 4.00″ X .270″).  That cost is $34.28 per foot — almost 3 times the cost of the steel beam — but, it weighs roughly half as much (127 lbs).

So, an Aluminum trailer frame may cost 3 times as much, and weigh about half that of a similar steel trailer frame.  Interesting.

Practical Durability

A third advantage of steel is something we call Practical Durability.  This is not a measured value.  Instead, practical durability is the pragmatic side of building, using, abusing, and owning the equipment (crane, trailer, press, whatever).  This includes things like it’s easier to get good welds when building with Steel, and it has an greater resistance to damage and abuse.

Steel, in general, is also much better for fatigue.  One issue with Aluminum is it’s propensity to propagate cracks — especially in the weld areas.  Welding Aluminum takes a lot more skill.  And, we always recommend more gussets and other joint support with aluminum because the material is not as robust to fatigue and abuse.  That’s the practical side of using Aluminum — especially with Aluminum trailers.

Of course, this does not mean you shouldn’t use it.  Far from it.  It just means it needs a little more care and a few more accommodations in design and construction.

Steel vs. Aluminum Trailer

Trailers, especially utility trailers, are a special design case for any material.  There is a lot that happens when rolling down the road — including bumps, potholes, washboard, curbs, wind, etc..  All of these things change the loading of a trailer in ways we can’t usually calculate.  Situations like frame twisting caused by uneven loading when hitting a highway pothole.  These odd cases are addressed with safety factors.  Because of practical durability, the required safety factor for an Aluminum trailer is a little higher than for steel.

Think about the added fatigue that occurs when driving down a road with washboard (breaking bumps).  The immediate and repeated bounce.  Of course, the suspension and tires take up some of that, but the trailer frame still must handle the additional flexing as it bounces up and down.  See the stiffness section above and consider the context of Steel vs Aluminum trailer.

When you consider the weight of a normal trailer frame (think utility trailers), you may save 150 to 300 pounds with an Aluminum Trailer frame.  Most people say that’s not enough weight savings to justify the added cost, the complexity of the build and the sacrifice in practical durability.  For Utility Trailers, we agree.  On the other hand, on a 4-horse trailer with living quarters, the Aluminum trailer weight advantage is huge.

Steel vs. Aluminum Gantry Crane

The application for a Gantry Crane is similar to a trailer in that the beams are usually quite long compared to their length.  On the other hand, Cranes are quite different from trailers because the dynamics are (typically) much less abrupt.  (Not so many potholes for cranes.)  That said, the above discussion still applies, just in a different priority.

For a Crane deflection is really important perhaps more than for an Aluminum Trailer.  Part 2 of this Comparison of Steel and Aluminum uses a Gantry Crane beam as the example to discuss deflection differences.  For relatively small cranes like the ones we sell plans for here, the weight of the top beam can be a huge driver in the design simply because someone has to lift and carry the beam.  A 5″ Steel beam is 100 lbs, where a similar 8″ Aluminum Beam is half.  And that can mean the difference of a one person job or one that requires 2.  If headspace is an issue, then Steel has a distinct advantage.  If the crane is really long, Aluminum may not be an option, but you get the idea.  Choose the right material for your specific application.

For Gantry Crane legs, I don’t usually endorse aluminum.  The factors of leg failure are more complicated for buckling with an implied bending at the top.  Steel just handles those kinds of things better.  Yes, it can work, but unless it’s really important, Steel is usually a better choice.

Summary

Aluminum does have a lot of really great qualities, as does Steel.  The material you choose may well focus on your wallet and DIY skills rather than weight or function.  Building with Aluminum can cost 3+ times as much, yet weigh only about half of an equivalent steel project.  Why?  Because larger pieces of Aluminum are required to get the same performance as with Steel.  Also, more care is needed for good welds, and more support (like gussets) around the joints to avoid fatigue issues.

Just remember that weight is only part of the story when choosing between Aluminum and Steel.  They are both awesome materials — each with advantages.  Consider the whole picture as you make your choices.

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