Overcoming the ‘gap’: A big obstacle still remains in truck-trailer aerodynamics

| June 17, 2014
FlowBelow's Tractor AeroKit system of aerodynamic devices includes wheel covers and is designed to work with many different wheel and tire configurations, including super singles.

FlowBelow’s Tractor AeroKit system of aerodynamic devices includes wheel covers and is designed to work with many different wheel and tire configurations, including super singles.


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Any vehicle moving at highway speeds faces the impediments of drag, which is created in two ways.  

One is when anything – rearview mirrors and bug screens, for example – impedes the smooth flow of air around the vehicle. Drag also is created by a vacuum – a low-pressure area that sucks in air that ideally would flow smoothly past the vehicle. The result is turbulence.

While bumper-mounted air dams, wheel coverings and trailer side skirts and tails have improved airflow underneath and around tractor-trailers dramatically, the trailer gap remains a stubborn aerodynamic challenge for engineers.

VorBlade’s small vortex generator blades are wishbone-shaped airfoils that smooth airflow over a truck’s trailing edges like the back of a cab or trailer.

VorBlade’s small vortex generator blades are wishbone-shaped airfoils that smooth airflow over a truck’s trailing edges like the back of a cab or trailer.

“The trailer needs to articulate,” says Mario Bravo, director of marketing for FlowBelow, which makes the AeroKit package of tractor-trailer aerodynamic devices. “Because there are many trailer and tractor configurations, having to design and build many variations of any solution makes it difficult for a manufacturer to reach economies of scale and offer their product at an attractive price.”

While the gap area is not the largest component of tractor-trailer drag, it is made more challenging by other issues, says Mike Henderson, chief scientist for SmartTruck, which makes the UT6-Plus aerodynamic package for trailers. Computation fluid dynamics and testing have shown that gap drag results from low pressure in the gap reacting on the rear of the tractor and its fairing and the front of the trailer, Henderson says.

The solution, he says, is to close the gap or place fairings on the front of the trailer’s top and sides. The larger the gap, the larger the fairings needed.


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Andrew Smith, chief executive officer of ATDynamics, favors a tractor-related solution. “Because of the variation between trucks and trailers, the winning technology to address this area of drag will be as universal as possible and will be tractor-mounted, given the high ratio of trailers to trucks in most fleets,” Smith says.

Small vortex generator blades – wishbone-shaped airfoils that smooth airflow over a truck’s trailing edges like the back of a cab or trailer – also can help, says VorBlade’s Natalie Melomed. “It might be counterintuitive, but small-scale vortex generators destroy harmful turbulence by intensifying the mix between different pressure zones and compensating the pressure jump,” Melomed says. “VorBlades actually reduce drag in the gap and increase vehicle stability.”

  • haller

    52 miles per hr. will give you 14.625 miles per gal. but then the price of fuel will go up to around 6 bucks per gal so the investors will continue to get their due…. Oh, I don’t know what to say or do,, just like the FMCSA, maybe we should only have truck drivers that DON’T speak english, yea thats a good idea because people don’t like trucks and the general 4 wheelen public hate truck drivers because as they know all truckers are stupid and trucks just get in the way when the 4 wheelers are on the phone or getting new apps, or talking to their wife or mama. So the black box on the dash will speak any language you want it to speak and the foreigner will obey the box for $4.25 per hr plus a free carton of food that will last 30 days, and the foreign truck driver’s family will shop at the company store and that will be deducted from the wages and at the end of the 3 year contract the truck driver and his family will owe $8,020.25 for not performing to company standards. BUT the owners of the company, the business people, the politicians, the wealthy and rich will say, “job well done”!!!!!! And by the way, the dot and troopers will keep those boys in line!!!!!!!!!!!!!!!!

  • peterveg

    I have added trailer side skirts to my trailer and see no difference in my fuel mileage, there is only one cure to good fuel mileage and that is SLOW DOWN. I drive the I80 corridor between Milwaukee and San Francisco, and with the high winds along that route and 90% of the time are all crosswinds of 25+ mph, the only thing that helped with fuel was slowing down. I went from driving the 75mph zones to driving 60 and have saved on the avg of 160 gallons per trip.

  • jojo

    Slowing down will always be more economical. Question is, is your customer paying for time sensitive service or can you load and offload at your convenience.
    I make money by saving money, driving 65mph when possible. I also make money running 75 and 80mph at 5.6mpg.
    There are many variables to consider. Your operation and my operation probably only have one thing in common. We both operate trucks as the tool of our trade.
    I want the best fuel economy I can get while completing the job I was hired to do under the prescribed time requirements. These new HOS rules make my job more difficult and way more dangerous.
    I’m forced to take a break when I’m ready to roll and I’m forced to roll when I’m ready to take a break.
    I couldn’t run a marathon tomorrow. But if I worked at it I could run a mile in a month.
    I’ve worked at it for many years to be able to safely run miles when needed. A lot of workouts and training has now been flushed down the toilet because I Don’t Know HOW To Do My Job SAFELY!!! F___IN A, I’ve been trying to Kill Myself and Others for YEARS and HAVEN’T Figured Out Why I’m UNSUCCESSFUL!!!Give me a break.
    Aerodynamics, road conditions, weather, temperature, fuel quality, HP, torque, gearing, weight are only a few of the variables that affect fuel economy. The right combination of equipment and accessories suited for your operation is crucial for your success. It’s up to you to outfit your operation to optimize your fuel economy so that it makes/safes you money.
    My 2.5yr average is 7.1mpg primarily operating at or slightly above the posted limit. I specced my truck to run efficiently at the speeds my freight demands. Time is Money and Time must also be managed.
    One shoe does not fit all. I commend you for doing the hard work and for striving to increase your profits! As they say”there is more than one way to skin a cat”.
    Thanks for letting me blow off steam against this “only one way to make money” ideology that is demanding that we all comply with what they want us to do. Remember, they only want us to do as they wish because it makes them money. They could care less about you or me!

  • Steve

    That’s the beauty of being an OO, I run a triple digit truck, but unless they add a couple hundred dollars to the rate, it will stay closer to 60 than 80.

    But I added the full smart truck UT-6 system
    & picked up 3/4 of a mpg. It paid for itself in just months.

  • nheaf09

    You complain about people not being able to speak English yet you can barely speak it yourself. Your rambling nonsense and ignorance is why people don’t respect truckers.

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