Pusing tin
Tractors were the leading edge of aerodynamic improvement; now the attention is on the trailers.

By Paul Abelson, senior technical editor

You see them everywhere you go. Skirts. Those flaps that hang below van trailers.

The evolution of aerodynamic trucks and aftermarket parts has been fueled by the industry’s desire to squeeze every last mile out of every single drop of fuel. And the easier the truck and trailer slice through the air, the less fuel it takes to move on down the road.

Tractors were the leading edge of aerodynamic improvement – not only because they cut the rig’s path through the air, but also because they led the way in design since the late 1970s.

Tractors have transitioned from the traditional flat radiator, straight hood, broad bumper trucks with the aerodynamic properties of a brick, to today’s streamlined, airflow-smoothing shapes. The difference in fuel economy between a classic truck and an aerodynamic one is a full mile per gallon or more. I love the looks of the W900s, 379s and Classic XLs, but I hate the thought of money flying into the wind instead of into the bank.

Early thinking was that since they punch a hole in the air, tractors had the greatest potential for aerodynamic improvement. But today we know that 60 to 75 percent of drag is created at the trailer. Of that, 30 percent is at the trailer front face, behind the tractor. About 35 percent occurs below the trailer, around the landing gear, the suspension and axles and even at the cross members. For reefers, the under-trailer shape of drag will be greater because of the size and placement of the fuel tank.

The remaining 35 percent occurs at the rear, from the broad, flat tail to the ICC bar/under-ride guard. Drag is created when air is forced to change direction. The more abruptly and more frequently it has to change, the higher the coefficient of drag. Aircraft have long, smooth noses to help penetrate air with as little drag as possible.

The trend started in the late 1970s with air shields. In 1986, Kenworth introduced the T600. White Trucks, now Volvo, soon followed with the integral sleeper that matched a long, tapered nose with a sleeper from a cabover. It smoothly flowed it all the way to the sleeper without abrupt changes of direction. Today, every truck maker has aerodynamic cabs.

Now the attention is on the trailer. Skirts are the most visible. They keep air from flowing under the trailer and affecting tires, axles and suspensions. But where the fairing ends, the air tries to turbulently fill in any openings. If you have a slider, chances are your skirt’s length allows full fore-and-aft movement. With the slider full aft, you create a gap that allows air to create more turbulence.

While many fixed-length skirts are available, the Aerofficient Side Fairing has two parts. One is fixed below the trailer from before the landing gear rearward. The other slides inside the fixed panel to reach the wheels to ensure there is no gap.

Airman Inc. makes the Wingman, an under-trailer fairing that moves with the slider when tandem location is changed. That keeps it in proper position.

Depending on design and height, skirts and trailer fairings improve fuel consumption from less than 5 percent to more than 11 percent.

ATDynamics pioneered the TrailerTail to extend smooth air flow beyond the trailer rear. The ATS SmartTail is an automatically inflatable device requiring no driver input. ATDynamics also has a TrailerTail for roll-up doors. Its panels deploy automatically just below 50 mph and retract for docking.

Rear fairings for trailers save at least 5 percent more fuel.

Tractors with gap extenders help reduce trailer nose drag, but to minimize turbulence on the trailer nose, Richard Wood with Solus Inc. recommends vortex-blocking slats. With gap closing, they can improve mpg another 5 percent or so. Because most trucks operate in cross winds averaging from 7 degrees in south Florida to as much as 14 degrees in the heartland, trailer nose fairings offer a 3 percent fuel economy improvement even when used behind tractor fairings.

Perhaps the best application of pure aerodynamics in trucking is the highly modified 2003 Kenworth T2000 built by OOIDA Member Bob Sliwa, which was featured in the June issue of Land Line. Bob and industrial designer Jeremy Singley constructed a bullet-like nose, full body and trailer fairings and a 4-foot tail extension. It made the 450hp Cummins engine perform like a 550 and Bob regularly got better than 12 mpg in revenue service. On one run, he reached 13.4 mpg.

While few can afford the time, effort and work to construct another AirFlow “bullet truck,” he demonstrated that attention to aerodynamics can pay off in greatly reduced fuel costs. LL