TRUCKING: Tall Load Meets Short Deck
The Drop Deck
When freight is tall, trailers that carry it must be proportionately lower to the ground. 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, oversized 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 of East Manufacturing, an Ohio-based company building durable trailers for nearly 50 years. 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 tapers upward at the rear, which allows closer proximity to the ground, providing more space for tall cargo and requiring less strength.
Because of a single drop’s low main deck, the wheels and tires are generally smaller than the usual 22.5-inch size, 19.5s or even the 17.5s. But with a double drop’s high rear deck, the 22.5s — typically longer-lasting tires (and brakes) — can be used.
Drop decks aren’t needed for most freight; they can’t carry items like long pipes unless specially 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.
A drop deck’s characteristic step is called the ‘transition’ area, explained Bill McKenzie of Mac Trailers. The transition area takes careful engineering because stresses migrate to a 90-degree bend — potentially causing metal fatigue that may lead to cracking and breaking. Details vary among the different trailer builders, but the materials used to make the trailer account for the major differences in transition design.
McKenzie pointed out that steel is such a hard metal that with proper gusseting it can withstand stresses at a
90-degree bend. Aluminum, on the other hand, is a softer metal that needs support with a right angle; so engineers use a ‘radius’ structure for reinforcement. Configured 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 required to achieve the same strength as steel — the reason the aluminum member is thicker and taller. The main frame of an aluminum drop deck is one 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 clearance underneath for the taller frame, so deck height is unaffected.
Whether to choose aluminum, steel or a combination of both depends on operating requirements and finances. An aluminum trailer will weigh 1,200 to 1,500 lbs less than steel; much of the associated savings contributes directly to payload capacity and, therefore, to potential increases in revenue. Then again, not all shipments are so heavy as to demand lightweight trailers, and steel or combo trailers may be entirely adequate to carry a fleet’s usual loads.
Steel can take a beating, making it 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. In these cases, however, weight is a non-issue because such loads usually travel under special permit anyway.
Geography also affects choice of materials, said Zecha. In northern areas where road salt is inevitable, Doonan’s customers prefer aluminum for its corrosion resistance. Steel is more prevalent and will last a long time in salt-free southern or southwestern states. In the southern Midwest, combo trailers, using steel main beams and certain other parts along with aluminum decks and cross members, are popular.
Special coatings applied to steel represent a category all their own but, in general, they resist corrosion much better than paint. And galvanized steel is as much a trend in drop decks and flatbeds as it is in van-type trailers. Rust jacking — a common affliction affecting brake shoes (where rust pushes lining away from steel tables) — also occurs with spring hangers on steel trailers, said McKenzie of 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 goes up from steel to combo to all-aluminum, but the price gap between steel and aluminum has narrowed, primarily because the cost of steel has increased — making an aluminum trailer somewhat more affordable. But the upcharge over a combo or steel will still be in the thousands of dollars, McKenzie said. The good news is that the initial expense will be largely offset by an aluminum trailer’s longer life and higher residual value; it can retain up to 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 — which now accounts for about 30% of flats and drops; McKenzie projects it will be as much as 40% five years from now.
Alkane thanks Tom Berg, Senior Contributing Editor of Trucking Info, for the content of this article.
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