Feature Article: The Dynamics of Aero

Max Kdvera | December 01, 2009

Chassis side fairings can cost up to $2,400 and yield a 2 percent fuel saving.

Rick Mihelic, Peterbilt Motors’ engineering systems manager, cautions that if you start adding aerodynamic items one at a time, you may or may not get the performance gain advertised, because the individual pieces are designed to work in concert with each other.

Peterbilt recently introduced a fuel efficiency package with features such as a rooftop air shield and sleeper side extenders to improve aerodynamics by up to 20 percent, which translates to 10 percent better fuel economy.

“That saving can be $4,000 and over annually, depending on fuel prices,” says Landon Sproull, chief engineer. Mihelic says, “Our design goal is to keep payback within one to two years.”

A critical area is the real estate between cab and trailer. “What you’re doing is putting an aerodynamic vehicle in front of a brick,” Mihalic says. “You can completely negate the benefit of the tractor if you run too long a gap.”

Frank Bio, product marketing manager for Volvo Trucks North America, says you can gain 1 percentage point of fuel savings for each 10 inches you decrease the tractor-trailer gap. For many operators, this will require little monetary investment, if any. Sproull suggests a distance of 46 inches or less, as long as trailer movement doesn’t interfere with the cab.

Trailer devices

Turbulence at the rear of the trailer is diminished with the installation of a TrailerTail in this computational fluid dynamics diagram.
Turbulence at the rear of the trailer is diminished with the installation of a TrailerTail in this computational fluid dynamics diagram.

Warmkessel points to the trailer as the “next area of opportunity” for aerodynamic advances. Many fleets and owner-operators are already chasing that opportunity, though the weak economy is blunting business in the aftermarket.

The product that presents the potential for the greatest fuel savings, while typically being the most expensive, is trailer skirting. The idea is to deflect side winds, control airflow beneath the trailer and limit the air turmoil that exists at the back of the trailer on the road. “Without skirts, air gets under the trailer, it creates all kinds of frictional drag,” says Brian Layfield, president of Laydon Composites, a Canadian maker of truck and trailer aero components.

Advertised fuel savings vary from 4 percent to 7 percent. Skirting costs from around $1,500 to $4,000 and up. The more you use your trailer and the higher fuel costs rise, the faster your payback will be. Many of the manufacturers provide calculators on their websites for figuring the savings.

Aero-chartFreight Wing, which has offered its Belly Fairing tractor skirt for years, has introduced AeroFlex fairing. Under standardized SAE testing, AeroFlex achieved a 5.8 percent fuel reduction, compared with 4 percent for the Belly Fairing. Freight Wing President Sean Graham says test results varied from 5.2 percent to 8.45 percent fuel reduction. “We tend to get much better results when it’s windy versus when it’s calm,” he says.

Larry Olson of Camp Douglas, Wis., running under his own authority, began using the AeroFlex last summer. After 20,000 miles, “I was averaging 7 mpg, which is pretty good, but you always want more,” he says. “Since I put on the unit, I’m averaging 7.5 mpg. I’m saving $300 a month in fuel, so payback will be in nine to ten months.”

Windyne’s three-panel Flex-Fairing trailer skirting can be extended from 24 to 35 feet and one of the panels can be flipped up for access to chains, belly boxes, ramp and landing gear. It’s also one of the lowest off the ground, at 6 inches, which increases the surface area for deflecting air. “Extensive testing has been going on because many fleets are dubious of advertised test results,” Tichelman says. “Next year will be a banner year for us because we have concluded our tests with big fleets.”

  • Bill J

    I have a very unique problem possibly air flow on my 2006 Ford E350 Super Duty with a Rockport 12 box. I get a fluttering noise that is so annoying. I had the front end done tires even the engine is of while moving you still the noise. Sounds exactly like someone if you car opens up a window but not the other side and you get the pressure sound your ears are screaming for you to open up the other window to balance the pressure. That is the sound but opening my windows on my truck does nothing. Can you help? The sound can be heard from 7MPH and up. 19,000 miles great condition. Would mud flaps help?

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