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10 Things You Didn't Know About Semi Trucks

Popular Mechanics logo Popular Mechanics 2015-06-03 By Phil Berg

© Jon Feingersh/Getty Images

Truckers have a split personality in the public imagination: They're either invisible knights of the highway known to be ever ready to lend a hand to a stranded motorist, or solidarity-seeking union protesters who block highways, traveling in speeding packs. What's indisputable is the degree to which the U.S. economy relies on the semi truck.

The Origin of "Semi"

There are about 5.6 million semi trailers (or tractor trailers) registered for use in the U.S., almost three times the number of semi trucks (also called tractors). It was the trailer that gave rise to the name semi truck. Since the trailer has no front wheels and can be used only when connected to the tractor part of the truck, it's called a semi-trailer. The terms "semi" and "semi truck" evolved from that.

Trailers are typically 53 feet long. They have brakes that are automatically applied when the trailer is standing unattached to the truck. When the truck is connected to the trailer, pressure from the truck's engine-powered air pump releases the brakes so that it can roll.

Trucker Central

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One-third of all of the 1,900,000 semi trucks operating in the U.S. are registered in California, Florida, and Texas. There are 3.2 million truck drivers in the U.S., however. About 90 percent of trucking companies and owner-operators have fewer than six trucks.

Two-Thirds of America

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On average, semi trucks drive about 140 billion miles a year in the U.S. (as of 2006). Sixty-eight percent of all goods in the U.S. are delivered by semi truck. That works out to about 60,000 pounds per American every year, although the two largest commodities by weight are agricultural and building materials.

Long-Distance Operator

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Semi trucks make up just 15 percent of commercial trucks in the U.S., yet they travel 42 percent of all miles covered by commercial trucks. Large commercial trucks covered about a quarter of a trillion miles in 2006, while total vehicles miles were about 3 trillion.

It's Not Exactly a Prius, Folks

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In 1973, the feds estimated that semis got about 5.6 miles per gallon of diesel; today's estimate is 6.5 mpg, although different trucks get fuel economy in a range from 4 to 8. Going up a steep hill, a truck's mileage might drop to about 2.9 mpg, while going down the same hill will raise it to more than 23 mpg. New fuel-economy standards that took effect beginning in 2014 require semi trucks with a sleeper cab to get 7.2 mpg on level roads.

How do you make a huge, heavy truck more fuel efficient? Truckmakers are working heavily on aerodynamics and tire rolling resistance. Fairings that hide the leading edge of the trailer, side skirts that prevent wind turbulence under the trailer, and round caps over the rear trailer doors all combine to improve fuel mileage. The new "super single" wide wheels and tires that replace dual wheels and tires are said to improve fuel mileage by up to 7 per cent. A promising future engine technology will measure the speed at which the engine is most efficient compared to load and environment and automatically maintain that speed. 

Heavy Duty

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The maximum weight for a U.S. semi truck and full trailer is 80,000 spread over 18 conventional wheels. out in the wide-open spaces of Australia, however, "road trains" can have four trailers and weight in excess of 300,000 pounds. 

Torque to Spare

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How much does it take to move a fully loaded semi and trailer? A Detroit Diesel DD15 14.8-liter inline six-cylinder engine weighs 2880 pounds, or 345 pounds more than a Mini Cooper. This colossal powerplant makes up to 560 hp and 1850 lb-ft of torque. Most turbocharged diesel engines put out between 1200 and 2050 lb-ft to keep all that weight moving.

Big Screen

Sam Peckinpah's Convoy grossed about $15 million in 1978, when it came out. Not bad -- though that year's top-grossing movie, "Animal House," made $141 million. In December 1975, the song "Convoy," on which the movie was loosely based, was the No. 1 song in the U.S. for a week, written and sung by award-winning ad man Bill Fries under the name C.W. McCall, a character he created to promote an Iowa bakery. Here's a trucker-to-English translation of one of "Convoy's" verses that inspired the movie:

Lyrics: "It was the dark of the moon on the sixth of June in a Kenworth pullin' logs, a cab-over Pete with a reefer on, and a Jimmy haulin' hogs. We's headin' for bear on I-one-O 'bout a mile outta Shakeytown, I says 'Pigpen, this here's Rubber Duck, and I'm about to put the hammer down."

English translation: One June 6 night, three trucks -- a Kenworth, a Peterbilt with tilt-cab engine access, and a GMC -- were approaching a highway patrol car on Interstate 10 one mile east of Los Angeles, when the Kenworth driver announced on his CB two-way radio to the GMC driver, who had a trailer filled with live hods, that he was getting ready to speed up.

Despite all the CB lingo, the film couldn't touch 1977's trucker-themed Smokey and the Bandit, which grossed more than $100 million. (Maybe it needed a Trans Am.) Plus, don't forget about trucker movies like White Line Fever and Breaker! Breaker!, the latter starring Chuck Norris.

Top Dog

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The top-selling brand of semi truck is Freightliner; it sells about a third of the 190,000 new semis sold annually. Freightliner is owned by Daimler Trucks North America, which also owns the Western Star brand. Second most popular is Navistar International, followed b by PACCAR, which owns the Peterbilt and Kenworth brands. Fourth is Volvo, which also owns the Mack brand.

Stop That Thing!

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Antilock brakes on semi trucks have been required since 1997, which has significantly reduced the number of jackknife crashes, which the reader wheels of the truck lock up and trailer swings around to an acute angle with the truck. Today, the most dangerous semi accidents are rollovers. Federal highway investigators say that these happen when a semi truck goes onto a loose surface and the driver overcorrects while trying to steer it back onto pavement.

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