18-Wheel Tractor-Trailer Specifications

by Michael Gunderson
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The tractor of an 18-wheel tractor trailer has ten wheels that are dispersed evenly among three axles. The front wheels, also called the steer wheels, are used to guide the tractor. Two rear axles turn the drive wheels, which is what moves the 18-wheeler. The trailer portion has two extra axles in the rear. These axles use eight wheels to support the rear. The front of the trailer is situated on the rear drive wheels and they are attached by what is commonly known as the fifth wheel. Strict regulations govern the manufacturing and operation of 18-wheelers to lessen the chance of death, injury and property damage in accidents.


New regulations require heavy truck tractors to be equipped with anti-lock brakes. When the trailer capacity is at the gross vehicle weight, the driver must be able to achieve a 30% reduction in speed within 250 feet at 60 mph. To comply with these specifications, many tractor trailers also have drum brakes and air disc brakes.


The length of a standard tractor with the trailer attached is 60 feet. A trailer alone cannot exceed 102 inches in length and can be no taller than 13.5 feet. The gross vehicle weight limit is 80,000 pounds for driving on U.S. interstate highways, although special permits may be obtained to increase the weight. These special permits are most often used for trailers that carry international containers.


An underguard is a steel skirt designed to stop cars from going underneath the tractor trailer. In 1996, the National Highway Traffic Safety Administration issued a requirement for the guards. However, safety advocates are concerned because this specification does not require underguards to be installed at the sides or fronts of the rigs. In addition, the regulation does not require the guards on trucks manufactured before 1998.

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