When freight is tall, trailers that carry it must be short. That’s the reason for the drop deck, a variant of the flatbed or platform trailer. A “single drop” or “step deck” has a forward down-step and a main deck that provides another 20 or more inches of vertical space to accommodate large coils of steel and aluminum, outsized mechanical or electrical gear, farm implements, and aircraft engines, among other things. A “double drop” has an even lower deck and an up-step ahead of the rear tandem.

The low main deck lowers the center of gravity, which helps with top-heavy loads, noted Bill Wallace at East Manufacturing, one of the trailer makers HDT visited during the recent Mid-America Trucking Show in Louisville, Kentucky.

The depth of a flatbed’s understructure tapers upward under its nose, partly to match the height of the tractor’s fifth wheel and because less strength is needed at the trailer’s front. Likewise, a drop deck’s underframe also tapers upward at the rear, which allows the deck to be closer to the ground. This also is because less strength is needed, and it adds space for tall cargo.

Because of a single drop’s low main deck, the wheels and tires are typically smaller than the usual 22.5-inch size, 19.5s or even 17.5s. But with a double drop’s high rear deck, the 22.5s, with their longer-lasting tires and brakes, can be used. 

Drop decks are not needed for most freight. And drop decks cannot carry items like long pipes, except when fitted with risers along the main deck. Use of wood or metal risers adds versatility, allowing the trailer to be used in more than one way. And short, dense cargo, like bundles of lumber and shingles, can be carried on both the forward and main decks and sometimes on the rear deck of a double drop. 

About the Drop Deck's 'Step'

A drop deck’s characteristic step is called the “transition” area, explained Bill McKenzie of Mac Trailers. This takes careful engineering because stresses migrate to a 90-degree bend, and that can cause metal fatigue that leads to cracking and breaking. Details vary among the different trailer builders, but a major difference in transition design is the materials used to make the trailer.

McKenzie pointed out that steel is a hard metal and with proper gusseting can take stresses at a 90-degree bend. Aluminum, being softer, needs help with a right angle, so engineers use a “radius” structure underneath. Somewhat like an arch, the radius evenly distributes weight to the structures of the upper and lower decks, avoiding concentrated stresses and cracking.  

Aluminum, of course, reduces a trailer’s weight, but more of it is needed to achieve the same strength as a steel member. So the aluminum member is thicker and taller. The main frame of an aluminum drop deck is 1 inch taller than a steel frame, so the lower deck it supports is an inch higher, explained Kelly Zecha of Doonan Specialized. On a regular flatbed, there’s room under it for the taller frame, so deck height is unaffected.

Choosing Dropdeck Trailer Materials

Whether to choose aluminum, steel, or a combination of both depends on operating requirements and financial considerations. An aluminum trailer will weigh 1,200 to 1,500 pounds less than steel, and much of that goes right to payload capacity and potential increases in revenue.

Then again, not all shipments are so heavy as to demand lightweight trailers, and steel or combo trailers might be entirely adequate to carry a fleet’s usual loads.

Steel can take a pounding, so is the favored material for flats, drop decks and other types of trailers used in oil and gas fields and construction work, said Jeff Ingels of XL Specialized. Hauling of wind turbine blades, towers and generator nacelles is usually done with purpose-built steel trailers, an XL specialty. Here, light weight is not an issue because such loads usually go under special permits anyway. 

Preventing Corrosion When Spec'ing Dropdeck Trailers

Geography also affects choice of materials, said Zecha. In northern areas where road salt is encountered, Doonan’s customers prefer aluminum for its corrosion resistance. In the South, steel is more prevalent and will last a good amount of time if it stays in salt-free southern or southwestern states. In the southern Midwest, combo trailers, which use steel main beams and certain other parts along with aluminum decks and crossmembers, are popular.

Special coatings applied to steel constitute a whole other subject, but in general, they resist corrosion much better than paint. And galvanized steel is a trend in drop decks and flatbeds as it is in van-type trailers.

Rust jacking — a well-known affliction affecting brake shoes, where rust pushes lining away from steel tables — also occurs with spring hangers on steel trailers, said McKenzie at Mac, which makes both aluminum and steel products. Extra protection for steel avoids general deterioration and weakening of a trailer and can add to its life and resale value.

Cost ascends from steel to combo to all-aluminum, but the price difference between steel and aluminum has lessened, primarily because steel is more expensive than it used to be. That makes an aluminum trailer more affordable, but the upcharge over a combo or steel will still be in the thousands of dollars, McKenzie said. That’s offset by an aluminum trailer’s longer life and its higher residual value. It can retain 70% of its original value when sold by the original owner vs. perhaps 25% for a combo and even less for a steel trailer. That and other advantages of aluminum are boosting its share of the trailer market. It now accounts for about 30% of flats and drops, and McKenzie thinks it will be 40% five years from now.