Side skirt

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.

Trailer skirts comprise a pair of panels affixed to the lower side edges of a trailer, running most of the length of the trailer and filling the gap between the forward and rear axles. Trailer skirts are typically constructed of aluminum, plastic, or fiberglass, with plastic the most resistant to damage from side or bottom impacts. Skirts may have a modular design, allowing installation on a variety of trailer lengths. Skirts may weigh between 70 and 160 kg (150 and 350 lb). Installation typically requires three to five person-hours.

As of 2009^[update], a set of trailer skirts cost between C$1500 and C$3000 (US$1300 to $2700). Standard trailer skirts have an estimated payback period of ten to eighteen months, while "advanced" skirts (those that improve fuel efficiency by over 7%) are estimated to pay for themselves in seven to fourteen months.

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. Skirts with reduced ground clearance offer greater fuel savings; in one instance, reducing ground clearance from 16 in (41 cm) to 8 in (20 cm) resulted in an improvement in fuel savings from 4% to 7%. 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 see fuel savings of 5% to 6%.

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