A trailer skirt or side skirt is a device affixed to the underside of a semi-trailer, for the purpose of reducing aerodynamic drag caused by air turbulence. Trailer skirts have been recognized by the U.S. Environmental Protection Agency’s SmartWay Transport Partnership as a verified aerodynamic technology, eligible for funding under the Diesel Emissions Reduction Act.
A 2012 investigation by SAE International of nine trailer skirt designs found that three provided fuel savings greater than 5%, and four provided savings between 4% and 5%, compared with an unmodified trailer.
One 2008 Delft University of Technology study found fuel savings of up to 15% for the particular design studied. Sean Graham, president of a major supplier of trailer skirts, estimates that in typical use, drivers will see fuel savings of 5% to 6%.
Trailers with skirts fitted have also demonstrated reduced tire spray making roads safer by improving visibility for other motorists. Semi-trailer drivers report improved stability in crosswinds.
Standard trailer skirts have an estimated payback period of ten to eighteen months, while “advanced” skirts (those which improve fuel efficiency by over 7%) are estimated to pay for themselves in seven to fourteen months.
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