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Are skirts and extensions allowed on tractor trailers?

Question: I have been seeing many tractor trailer rigs where, under the trailer, there is a skirt that I am assuming is for less drag. I have also seen maybe a half a dozen trailers that have an extension off the back of the trailer that I believe is also for increased aerodynamics. Are these extensions allowed even if they cause the overall length of the tractor trailer rig to exceed the state maximum length?

Answer: Very good question and I checked with the Minnesota State Patrol Commercial Vehicle Section and here is what I learned.

The air deflectors under the trailers are allowed; however, Minnesota State Statute 169.81 addresses length of trailers and specifically references non-cargo carrying equipment adjacent to the truck tractor used for safe and efficient operation as being allowed.

In other words, refrigeration units and air deflectors mounted on the front of semi-trailers is allowed without regard to maximum trailer length. Technically, by State Statute, a 53 foot trailer would not be a legal trailer in Minnesota with these rear deflectors mounted. Possibly, a 48 foot trailer could have them installed, as long as it was at or under the 53 foot maximum semi-trailer length law.

A portion of state statutes were used with permission from the Office of the Revisor of Statutes. If you have any questions concerning traffic related laws or issues in Minnesota, send your questions to Trp. Jesse Grabow — Minnesota State Patrol at 1000 Highway 10 West, Detroit Lakes, MN 56501-2205. (You can follow him on Twitter @MSPPIO_NW or reach him at,