We like to build trailers," says Charlie Fetz, vice-president of research and development for Great Dane Trailers in Savannah, "It's wonderful when someone comes along and says, 'I'm interested in R&a...
We like to build trailers,” says Charlie Fetz, vice-president of research and development for Great Dane Trailers in Savannah, “It’s wonderful when someone comes along and says, ‘I’m interested in R&D’, and wants to pay for it.”
That “someone” is Wal-Mart Corp., owner of one of the world’s largest private trucking fleets, and “wonderful” is the retail giant’s pledge to double fuel efficiency on its trucks within 10 years (from 6.5 to 13 mpg US)
Wal-Mart approached Fetz a few years ago and discussions began about building them an aero-trailer. Great Dane’s designers and engineers came up with four designs, ranging from passively aerodynamic to more aggressive and experimental. To their surprise, Wal-Mart chose an aggressive design: a drop-frame 53-ft. trailer with a rounded nose and underskirts that dropped down automatically when the vehicle exceeded 35 mph. The rear section (sides and top) was tapered inwards 13 degrees and the roll-up rear door was recessed inside the trailer about two feet (for an aerodynamic “boattail” effect without any added length).
The trailer also used light-weight components like aluminum posts and composite plastic scuff bands. It sat only 12’6″ high, but the 16-inch drop frame compensated for any loss of cube volume.
Great Dane delivered the trailer to Wal-Mart in June 2006, which in turn had it road tested by International Truck and Engine Corp. Trailer aerodynamics has been likened to trying to move a brick through the air, so shaving any small percentage off the resistance is a victory. In this light, Great Dane’s results could be considered astounding: about a 20% reduction in drag and double digit fuel savings (achieved without super-single tires).
Fetz was pleased with the overall design but troubled by the extraneous devices like the pneumatically powered drop-down skirt, and the lifting suspension required to bring the trailer up to dock level.
“So in the latter part of 2006 we decided to build another trailer,” says Fetz, “something a little less aero but more mainstream, sitting at normal dock height.”
The result was Aero 2 which was delivered to Wal-Mart in March of this year. This one sits 13’6″ tall with a full length fixed skirt supplied by Laydon Composites of Oakville, Ont. The prototype 53-footer rolls on Bridgestone Greatec wide-based single tires.
“We also wanted to learn how to build it,” adds Fetz. “The first one was built entirely in our lab, the second one was built at the plant with the exception of the tail section.”
Wal-Mart again contracted International to test Aero 2, and the preliminary results have been comparable to Aero 1. Fetz is again surprised and happy. “You take a big aero hit when you go from 12’6″ to 13’6″.”
The almost full-length, flexible skirt keeps the slipstream and crosswinds from dragging along the trailer cross-members and rear-wheel boggies. A removable panel fitted over the wheel-well allows service access to the tires and axles for servicing. The parabolic-shaped nose lessens resistance by shortening the gap between the tractor and trailer.
Most interesting is the boat tail effect achieved by tapering the sides and top of the trailer, and recessing the back door inward two feet. This provides an air cavity which increases the air pressure behind the trailer and significantly reduces drag. The tapered rear end channels the airstream into a narrower air-corridor behind the trailer.
“You can think of drag like a parachute sucking you backwards,” says Jason Leuschen, aerodynamics laboratory facilities manager for the National Research Council (NRC) in Ottawa, Ont. The NRC’s eleven wind tunnels (including a full-size 40 ft. tunnel on a rotating platform located near the international airport) have tested everything from Olympic athletes to NASCAR racing cars. (The day I called they were testing mailboxes.)
In 2006, Leuschen supervised the testing of a wide variety of aerodynamic devices on a full-scale tractor-trailer combination, including skirts, caps, filler panels and boattail assemblies. “The boat tail is the winner,” he says. “Ideally it should taper to a pinpoint behind the trailer, but you can still gain a lot.”
After-market hardware like Transtex Composite’s folding rear door trailer deflector, and Aerovolution’s inflatable rear door fairing, smooth out the trailer’s wake effectively acting like boat tails, while the Great Dane design does so without extending the length of the trailer.
Fetz of Great Dane admits some cube capacity has been lost from the parabolic front end, tapered rear section, and recessed door. Likely, the trailer will hold 24 regular pallets on the floor rather than 26. Although still being tested, the increased fuel economy should more than compensate for the slightly decreased volume. Like most advanced engineering projects, a cost-benefit analysis can’t be fully explored until the trials are completed.
And Fetz is also not sure whether the flexible skirt will get hung up easier in snow or be more prone to damages. “That’s why it’s an experimental trailer,” he says.
But Great Dane’s aero trailer will go a long way to help Wal-Mart meet its ambitious fuel economy targets. It already meets and exceeds US EPA Smartway trailer criteria by having low resistance tires, side skirts, weight-saving technologies, and a gap-reduced front end and rear boat tails.
Aero 2 is a holistically inspired creation, but many of its drag reducing features are available as components and kits from specialty manufacturers.
Research from tests at the NRC’s full scale wind tunnel indicates that a tractor-trailer combination equipped with tractor-mounted gap-sealing, along with the largest available side extenders, trailer side skirts and rear boattails, will save 6,667 litres per year when operating at 100 kph over 130,000 km. At $0.79 per litre, that’s a savings of $5,283 yearly. The cost of installing the devices is around US$2,200, so that would mean they’d pay for themselves in five months of operation.
Wal-Mart is currently in the process of upgrading its powered units and has already achieved 15% savings on those trucks (7 mpg up from an average of 6 mpg in 2005). The corporation estimates that saving one gallon of fuel per unit results in an overall saving from $35 to $50 million annually. Taken further, a 25% reduction in fuel consumption is the equivalent to taking 67,744 cars off the road (in terms of CO2 emissions).
Having an industry giant at the forefront of environmental trucking initiatives helps to popularize and accelerate the commercialization of green products. With Wal-Mart picking up some of the tab for R&D in these fields, the cost of developing fuel-saving devices and getting them on the market is greatly lessened. The products become available sooner to the trucking industry at a fair price. As Wal-Mart goes, so goes the continent?
After the US Army, Wal-Mart leads the pack of private trucking fleets providing funding for research on projects from alternative fuels to hybrid and hydrogen vehicles. In many ways, it’s setting the bar for North American trucking operations. But to realize its goal of 50% better fuel economy in 10 years means there’s still lots of work to be done. And no doubt about it, Great Dane’s aerotrailers will be around as part of the solution, delivering cargo for years to come.
Harry Rudolfs is the rare combination of a journalist and trucker. He has logged more than a million miles, and this insight allows him to tackle the issues truckers love to talk about-and, occasionally, a few they don’t.