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, and drivers have reported 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.