Which material is best for you depends largely on the conditions.
Both aluminum and steel are fine trailer-building materials; which one might be "best" for you depends on a number of factors. In general, steel trailers are cheaper and stronger than aluminum units but are heavier and more prone to damage from oxidation. All of these factors come into play when you consider the cargo and operating conditions, be they horses in the desert or laptops in the arctic.
Weight
Weight is the primary criteria by which most people decide trailer material, and for good reason. Heavier trailers can be more dangerous to tow and difficult to stop, and will ultimately limit your cargo capacity. For instance, a 30 foot aluminum trailer might only weigh 8,500 pounds, compared to 10,000-plus for steel. Although this difference doesn't seem like much on the surface, it makes a huge difference to the bottom line if hauling cargo is your livelihood. In the above example, that's 1,500 pounds more cargo per trip for the same time and fuel, making for a 15 percent difference in profit.
Strength
Aluminum alloy can be nearly as strong as some steels, but most trailers aren't made of those super-strong mixtures. The fact is that steel is far less likely to flex and bend under load, and the welds that join steel are generally superior to those on aluminum. If you have to tow your trailer (especially loaded) through rough terrain like fields and pastures, steel is a safer bet for long term use.
Corrosion
Steel rusts and aluminum doesn't. Or does it? Aluminum can be just as prone to oxidation damage as steel under some conditions, but those conditions are rarely encountered on the road. In the end, aluminum beats most kinds of steel for longevity and corrosion resistance, especially when hauling livestock or over snowy, salted roads. Stainless steel trailers are far less prone to rust than standard steel units, but are also much heavier and are likely to cost far more than even aluminum trailers.
Cost
Steel trailers generally cost half of what aluminum units do at buy-in, but that difference may balance out over time. When used on primarily flat roads, aluminum trailers require little to no maintenance compared to steel. this is especially true for livestock applications; that cow, horse and pig wee contains huge amounts of ammonia and acids that will eat through a steel floor. If you use a steel trailer regularly, expect to have to repair or replace some portion of the floor on a regular basis. Add to that the cost of periodic repainting and rustproofing and steel's total cost will eventually reach that of aluminum.
Combination Units
Some home builders have attempted to build combination trailers with a steel skeleton and aluminum skin only to find themselves with a pile of metal in the barn six months later. Steel and aluminum have vastly different galvanic corrosion rates; in the presence of moisture, the steel will eat any aluminum with which it has direct contact. If you're looking to build or buy a combination-material trailer then make sure it has rubber, plastic or nylon insulators at every point where the steel and aluminum touch.
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