Here at Jalopnik, we sometimes wonder why things are called what they are. Why do we park in a driveway and drive on a parkway? Where did Ferrari come up with a name like Ferrari the Ferrari? Why do they call it a flatbed trailer if it’s curved in the middle? We may never know the answers to some of these questions, but there is a reasonable explanation for why some so-called flatbed trailers are not really flat — at least when unloaded.
A typical flatbed trailer can carry a maximum load of 46,000 to 48,000 pounds. This weight causes the bend in the trailer frame to flatten out under the load. If the trailer was truly flat while unloaded, that same load could cause it to sag underneath the weight. Designing a bend into the trailer frame while unloaded enables it to absorb this effect and make the bed flat under load, which is when it matters most. This bend is called camber in the trailer industry, which is not to be confused with camber in wheel alignments.
Some trailers bend more than others
Flatbed trailers can be categorized into three types: aluminum, steel, and combination. Aluminum trailers are lighter than steel, don’t rust (though they are prone to corrosion and cracking), and are more customizable. The trade-off is that aluminum bends more than steel under load. As a result, aluminum trailers have the most pronounced bend in the middle, which can make loading and unloading some cargo more difficult than on a flat floor.
Steel trailers don’t bend nearly as much, and often look flat even when unloaded. Interestingly, this flatness also makes them more suitable to become temporary replacement bridges. However, they are heavier than aluminum trailers. This extra weight reduces how heavy a payload they are allowed to carry while keeping the entire semi under the 80,000-pound federal gross weight limit. Floors made of wood rather than steel help offset this weight penalty, but they wear out faster than the frame and will eventually need replacement.
The final type of flatbed trailer is a combination, made of both steel and aluminum. They combine the advantages and disadvantages of steel and aluminum trailers. The aluminum floor is lighter and more durable than the typical wood floor of an all-steel trailer, while retaining the strong steel frame with less of a bend than a pure aluminum trailer.