A “semi” is short for semitrailer, a trailer with no front axle that relies on the tractor to support its front.
Meaning
“Semi” is short for semitrailer. A semitrailer has wheels only at the rear and rests its front on the tractor’s fifth wheel.
Why semi
It is only partly supported by its own wheels. “Semi” here means half.
Parts
- Tractor (cab and engine).
- Fifth wheel hitch.
- Trailer.
Size
A typical trailer is 53 feet long. US gross weight is limited to 80,000 pounds.
Origin of the term
“Semi” is short for semitrailer. A semitrailer has wheels only at the back and rests its front weight on the tractor unit through a coupling called a fifth wheel. A full trailer, in contrast, has axles at both ends and carries its own weight.
Parts
- Tractor: the cab and engine unit.
- Semitrailer: the cargo section.
- Fifth wheel: the horseshoe shaped coupling plate on the tractor.
- Kingpin: the pin under the trailer front that locks into the fifth wheel.
- Landing gear: legs that support the trailer when parked.
Size and weight
In the US, the standard trailer is 53 feet long and 8.5 feet wide, and the legal maximum gross vehicle weight on interstates is 80,000 pounds (40 tons) under federal rules. The overall rig length is commonly around 70 feet.
History
The first semitrailer was built by Alexander Winton of Cleveland in 1898 to haul cars, and the term “semi truck” became common as the design spread.
Engines
Most semis use diesel engines of 12 to 16 liters producing 400 to 600 horsepower and large torque, often 1,500 to 2,000 pound-feet.
Fuel economy
The average is about 6 to 7 miles per gallon, according to the Department of Energy.
Industry
The American Trucking Associations reports that trucks moved about 72% of US freight by weight in recent years, and the industry employs over 8 million people.
Drivers
A commercial driver’s license (CDL) Class A is required to drive a combination vehicle over 26,000 pounds.
Other names
Tractor trailer, big rig, 18-wheeler (though many have fewer than 18 wheels), eighteen wheeler.
Why it is called a semi
The word semi is short for semi-trailer, a trailer that has wheels at the back only, while the front end rests on the tractor’s fifth wheel coupling. Alexander Winton of Cleveland is often credited with developing the first semi-trailer truck around 1898 to deliver cars. The tractor unit pulls the trailer, which can be disconnected. In the US, the federal weight limit on interstate highways is 80,000 pounds gross vehicle weight, and the standard trailer is up to 53 feet long.
Engines and industry facts
Class 8 tractors typically have 12 to 15 liter diesel engines producing 400 to 600 horsepower and get about 6 to 7 miles per gallon. The American Trucking Associations reports that trucks move about 72 percent of US freight by weight. About 2 million heavy and tractor trailer drivers work in the US, according to the Bureau of Labor Statistics, and they need a commercial driver’s license (CDL) and must follow hours of service rules, limited to 11 hours of driving within a 14 hour window. Other names include big rig, 18 wheeler and tractor trailer.
Based on data from the Federal Motor Carrier Safety Administration and the American Trucking Associations
