Have you heard different opinions about an aluminum trailer frame? I do. Here is a deeper look at the Engineering to clear up some facts. Maybe we can also debunk a few aluminum trailer misconceptions. We will start with the benefits of an aluminum trailer frame, then wrap up some misnomers. We get questions about material fairly often, so hopefully we can shed some meaningful light on the subject.
Of course, a discussion around the relevance of a material needs a frame of reference, so we will use the format of Aluminum for a trailer frame. Other uses of aluminum, like for CNC parts, are sort of irrelevant in a discussion about trailer frames.
To be complete in our look at aluminum for a trailer frame, there is some repeat information here. Please see the more general comparison of Steel v. Aluminum in this article.
Aluminum Positives (And Caveats)
Lighter Weight
Absolutely. Comparatively, the density of Steel is 7800 kg/m^3 -ish. Aluminum is around 2700 kg/m^3. That means a cube of Steel it is about 3 times heavier than the same size cube of aluminum. That is a lot.
The Caveat? Unfortunately, in most cases you need more aluminum (volume, quantity) than steel to achieve the same performance. That is OK. For trailers, the aluminum version of the trailer frame often ends up roughly half the weight of the steel equivalent. (That is a really rough number, so don’t go to the bank with it. I give it only for perspective.)
The total trailer weight improvement is less, because things like axles, wheels, tires, hitches, and the like do not change. So, it is not a 50% total weight reduction.
Does Not Rust
Again, this is a wonderful characteristic. So, in the context of an Aluminum trailer frame, that means less worry about weather and storage, etc..
To say that aluminum is not affected by the environment is the misnomer. It does corrode, just not as rust. More accurately, it oxidizes. In some ways you can think of aluminum “oxidation” as the material protecting itself. Have you noticed that when you handle aluminum a lot, your fingers end up black? That is aluminum oxide, which is a form of corrosion. Aluminum reacts with Oxygen instantly when you cut it or scrape it. The oxide starts thin, then “grows” over time.
You can accelerate corrosion with chemicals like salts, too. Maybe you have seen a white frosty look on aluminum? That is also a corrosion, but again, it is not rust, and it is not nearly as destructive as rust. The nice thing is the more it “corrodes”, the less it wants too.
The bigger issue with aluminum corrosion comes where it interfaces with other materials – like copper or steel.
Does Not Need A Finish
This one is technically true, but not a good idea. Look above at the discussion about corrosion. Really, it is best to finish the trailer – if for no other reason than to avoid black hands every time you handle it.
Aluminum finishes a little different than Steel. (I am not going to speak to this as thoroughly because I have limited experience finishing large aluminum items.) I have done a ton of anodizing in both Type II and Type III for smaller parts, but I have only painted large parts. The trick to a good finish is the right preparation and primer. Talk to your local automotive paint supply for details.
Has a “Cool” Effect
Some people list this as a benefit of aluminum – and especially for an aluminum trailer frame. Of course, there is no way to argue an opinion, so I will not support or decry. I love aluminum CNC parts, that is for sure. If the look is appealing to you, weigh that into the choice mix, for sure.
The Negatives (And Caveats)
Not as Strong
Actually this is a misnomer. Aluminum is strong; but it is not as stiff. You can read about the effect in our Gantry Crane Analysis which is quite eye-opening for many people.
For pure strength, there are 2 properties to know. First, YIELD strength. Metal will permanently bend (not just flex) if we apply loads in excess of the YIELD strength. Second, TENSILE strength. Parts break when we exceed the TENSILE strength. Most metals will deform (Yield), before they break, so in trailers, Yield strength is the driver. That said, Fatigue is the wildcard and we will talk about that below.
Typical steel for making trailers (A36 for example) has a Yield strength of 36,000 psi (min), and a tensile strength of 58,000-78,000 psi.
On the other hand, Al-6061-T6 extruded (common aluminum for trailers) has a Yield strength around 40,000 psi; and a Tensile strength around 45,000 psi. (These numbers are not absolute, there is variation. That is just the nature of it.) Please note that available alloys differ. For instance, Al-6063 is common, but not as strong: 31,000 psi Yield, and 35,000 psi Tensile.
Comparing these numbers, we see that size for size an Aluminum trailer frame is roughly as “strong”. Yet, that is NOT the whole story.
Deflection
Another important material property is, in engineering terms, the Modulus of Elasticity. Oversimplified, this determines how much something deflects under a load. For example: If you hold a typical plastic spoon by the handle and push on the other end, the spoon deflects without a lot of force. Do the same on a metal spoon, and the result is quite different. The plastic has a much lower Modulus of Elasticity.
Now for trailer materials. Aluminum has a lower Modulus of Elasticity than steel, so it deflects more under load than steel. In DIY, for short beams, brackets or short columns and such, deflection is not much of a concern. However, when we think about a trailer frame and particularly the long main beams that hold everything, deflection matters.
In engineering terms, the Modulus of Elasticity is ~30,000,000 psi for steel, and ~10,000,000 for aluminum. That means 3 times the deflection in an aluminum trailer frame. Again, I invite you to see this effect in our Gantry Crane Analysis.
An interesting point about Modulus of Elasticity: It does not change much with the alloy. Strength numbers change a lot with the alloy, but they all deflect about the same. That means “stronger” aluminum is not significantly stiffer. It will just deflect farther before it permanently bends.
So what is the big deal about a little deflection? In many situations a little more deflection does not mean much. Right? Maybe. And that brings us to the next topic of Fatigue. If the trailer is flexing more, then every bump and each load causes these larger repeat beam movements. This bounce and flapping is really just more deflection, and unfortunately for aluminum, contributes to fatigue.
Fatigue
Cyclic loads. As the trailer rolls down the road it repeatedly deflects a little – kind of like a spring bouncing – with each change in the road. It is only a tiny bit, but continual changes in stress over time cause materials to weaken, mostly locally, and we call this fatigue. If there are oddities in the material, then the fatigue stress concentrates and if the stress is high enough, it may cause a crack.
We like to think of material as consistent throughout. While that is largely the case, it is definitely not true when we interrupt it.
Fatigue issues most often occur near interruptions – like welds or holes. Welding is not nice to metal, because it changes the material properties some. At the molecular level, it changes the structure, and we call it Heat Distress. Unfortunately, aluminum does not handle this as well as steel.
We could talk about % Elongation, and ratios of Yield to Tensile strength. We could look at aluminum deflection which causes a greater localized stress effect at the joints. And, we can talk about localized heat treatment at the weld. That is all in engineering fun, but what you need to know is aluminum fatigues easier, and fails by fatigue faster.
Here is an example of a catastrophic aluminum tongue fracture, due to improper design for welding aluminum. (And the follow-on article about how to prevent fatigue failures in an aluminum trailer frame.)
So, to properly build an aluminum trailer frame, we must design the weld joints properly to avoid fatigue cracking. One way is by using thicker material, and another other way is adding external support – like gusseting and stress plates. This requires a higher level of design understanding, as well as a higher level of welding/fabrication skill. Read more in this article about joining beams.
Costs More
Prices fluctuate constantly in raw material markets, so prices today are likely off tomorrow. So, we can use some swags. In general, Aluminum is something like 1.25 – 1.5 times the cost of material of the same dimensions in steel. When you increase thickness to accommodate for aluminum, it becomes more like 2+ times the cost. The cost per pound is more too – (3ish to 3.5ish times as much), but that is not so relevant in the context of an aluminum trailer frame. Because a 12-ft trailer needs 12-ft material – whether in aluminum or in steel.
Again, as a rough perspective, experience says cost is something over 2 times as much for material to build an aluminum trailer. The previous comparison article a few years ago suggests 3 times the cost for equal performance. This varies, as mentioned, so your experience may be different. If you have recent knowledge, please share it in the comments.
Build Considerations
Finally, there are some build considerations to address. For instance, almost all axle mounting brackets are steel – for leaf spring axles as well as torsion axles. Same for hitches, tongue jacks, safety chains, etc. While these can bolt-on instead of welding, some pieces need reinforcing at the bolt joints also. This is not usually a big deal, but like the axle story above, the way you do it is important.
Some quick ways to handle this include welding the axle brackets to steel, then bolting the steel to the aluminum. For torsion axles which often bolt-on anyway, it means having more reinforcement at the connection to handle the added stresses of torsion axles.
In all bolting situations, drill carefully, and don’t drill into high stress areas of a beam. Drilled holes are another interruption of the material and can be a focus for fatigue cracking if they are in high stress places.
Before bolting, use an anti-seize coating between steel and aluminum on the trailer frame. Dissimilar metals like to create a galvanic corrosion that can ruin bolts and “nearly” freeze connections. Again, not a big deal if you know how to handle it.
Lastly, don’t use an aluminum trailer frame as the electrical ground path for lights. The dissimilar metals issue with connectors, fasteners, electrical path, etc.. It will work fine at first, then after a while, the lights just quit working, and it can be hard to figure out the issue. Personally, I like running a full ground wire to everything, even on steel frames, so this does not become an issue. Read more about trailer wiring in this article.
So, Is An Aluminum Trailer Frame Bad?
I admit, reading some of the above makes it seen that way, but No. Aluminum is an awesome material when done right. And that is the key.
If you are purchasing a trailer, look much more carefully at the trailer frame before you buy. Inspect the joints and look for any sign of cracks. Look at the joints to see if they are reinforced to avoid and handle fatigue. Look at the welds to see if the welder knew what they were doing. If any of these seem out of whack, turn and run. Not all trailer builders do a good job with aluminum.
And Building An Aluminum Trailer?
How about building an aluminum trailer frame? I would ask myself the same questions before building one. Do I have enough aluminum fabrication experience and skill to positively make good, robust aluminum welds? Have I tested my welds to know for sure they are good?
An aluminum trailer frame is the wrong project to learn on. There is too much at stake with too much liability on the road. How about a technique for welding aluminum? Do I know how to reinforce the joints in torsion? In compression? Do I know how to mitigate fatigue for welding aluminum beam joints?
Interestingly, aluminum bicycle frames get a heat treatment – the whole frame – after welding to mitigate the stresses of welding. That helps keep the frame from cracking later. While an oven that size would be super awesome, it would be really hard to do, DIY, with a full trailer frame. So, we need to do other things. Also, in bicycles, they butt the tubes – meaning they are thicker at the weld locations, than for rest of the tube. (Now that would be a fantastic trick to do with a trailer!!) In my opinion, you need to know how, and have significant experience with aluminum, before diving into building an aluminum trailer.
If you have got the skills and experience, then yes. Absolutely! An Aluminum trailer is an awesome project.
The next obvious question is “Why no plans for aluminum trailers?” Read there to find out.
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