To a lot of people, tractor trailer trucks, or semis, are speeding behemoths that you don't want to be driving next to. To the kids in your back seat, they are a source of entertainment on long car rides—pump your elbow and the driver might honk his horn. But to all of us, they are the lifeblood of our existence, the source of the products on supermarket shelves, Amazon deliveries, and the gas in our cars. You might fly by them on the highway without thinking much about all that, but there's a lot worth knowing about them.
Why Are They Called Semis?
There are about 2.8 million semitrucks (also called tractors) registered in the U.S. as of 2016. Their trailers—known as tractor-trailers—have no front wheels and can be used only when connected to the tractor part of the truck. (Their brakes are automatically applied when the trailer is standing unattached.) Only when a truck is connected, and pressure from the truck’s engine-powered air pump releases the brakes, can the trailer roll. Thus: “semi-trailer.” The terms "semi" and "semitruck" evolved from there.
How Much Weight Can They Haul?
The maximum weight for a U.S. semitruck and full trailer is 80,000 pounds spread over 18 conventional wheels. Out in the wide-open spaces of Australia, however, "road trains" can have four trailers and weigh in excess of 300,000 pounds.
How Much Do We Rely On Them, Exactly?
In 2016, semitrucks drove 175 billion miles. Recent estimates suggest 68 percent of all goods in the U.S. are delivered by semitruck, which works out to about 60,000 pounds per American every year. The two largest commodities by weight are agricultural and building materials.
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How Much Power Does It Take To Move a Loaded Semi?
A Detroit Diesel DD15 14.8-liter inline six-cylinder engine weighs 2,880 pounds, or 345 pounds more than a Mini Cooper. This colossal power plant makes up to 560 hp and 1,850 lb-ft of torque, and at just 1,200 rpm the engine produces more than 1,500 lb-ft of torque. Most turbocharged diesel engines put out between 1,200 and 2,050 lb-ft to keep all that weight moving.
We Hate To Ask But...How About Fuel Efficiency?
In 1973, the feds estimated that semis got about 5.6 miles per gallon of diesel, and today’s estimates are still abysmal. 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. This year Shell’s Starship concept truck used advanced aerodynamics and hybrid motors to attain… 8.9 mpg. (That said, it was carrying a heavy load and achieved 178.4 ton-miles per gallon, more than double the national average in the measure that takes payload into account.)
How do you make a huge, heavy truck more fuel-efficient? 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. “Super single" wide wheels that replace dual wheels are said to improve fuel mileage by up to 7 percent. Trucks are also beginning to incorporate more composite materials—the same kind of transformation that in aeronautics produced the current set of super-efficient airliners, like the Boeing Dreamliner.
What Makes Them Crash?
Antilock brakes on semitrucks have been required since 1997, which has significantly reduced the number of jackknife crashes, in which the rear wheels of the truck lock up and the 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 semitruck travels over a loose surface and the driver overcorrects while trying to steer it back onto pavement.
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Who Drives Them?
There are 3.2 million long-haul truck drivers in the U.S. They hold Class A driver’s licenses, which means they can pilot vehicles with a gross vehicle weight rating of more than 26,000 pounds. Overachievers can get additional endorsem*nts to drive double- and triple-trailers, tankers, and hazardous materials.
Do They Really Sleep In the Cab?
When you’re driving day and night, a guaranteed place to sleep comes in handy. Sleeper cab trucks have space behind the driving cabin equipped for overnight stays. The amount of luxury varies, but even basic sleeper cabs incorporate tables, closets, refrigerators, flatscreen TV mounts, and, of course, beds.
Tell Me the Best Trucker Movies
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 the song'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 hogs, 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 White Line Fever and Breaker! Breaker!, the latter starring Chuck Norris.
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Will Fully Electric Semis Really Happen?
We hope so. Elon Musk’s entry into the heavy-truck industry, the Tesla Semi, is due out in 2019. It’s electric—as expected—but also features some less-expected challenges to the status quo: The driver sits directly in the center of the cab (there’s a jumpseat behind the driver’s seat for passengers), and the dash is just as sparse as that of the Model 3. And like the Model 3, it’s utilitarian but super high-performing: It can get from 0 to 60 in only 20 seconds—while towing 80,000 lbs. The food and beverage industry is particularly excited about electric semis, as Popular Mechanics has previously reported.
