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What is an Eighteen Wheeler?

By J.Gunsch
Updated May 23, 2024
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An eighteen wheeler is a vehicle technically known as a semi tractor trailer. Other common names include big rig and lorry. The purpose of an eighteen wheeler is to tow its cargo that is contained in the trailer which is detachable from the engine. These vehicles are mainly used in transporting commercial freight such as fuel, construction materials, food and other consumer products.

The semi tractor trailer itself does not have eighteen wheels but the addition of the trailer provides the extra wheels which give it the name eighteen wheeler. When an eighteen wheeler is traveling without a trailer it is referred to as bobtailing.

The tractor portion of an eighteen wheeler normally has ten wheels which are dispersed throughout its three axles. The front wheels are called the steer wheels which maneuver the vehicle and the two rear axles turn the drive wheels which give the eighteen wheeler its power. The vehicle’s trailer has an additional two rear axles which use eight wheels to support the rear. The front portion of the trailer sits on top of the rear drive wheels of the tractor and is coupled by what is known as the fifth wheel.

An eighteen wheeler is a powerful machine and as such is constructed very differently than smaller trucks and passenger vehicles. First, the average length of an eighteen wheeler towing one trailer is about 80ft (24.4m) long. It typically has 10 gears, as opposed to five in cars with standard transmissions, two of which are used to reverse. An eighteen wheeler stops with a combination ten hydraulic and air brakes rather than the four brakes in passenger vehicles.

Additionally, the average eighteen wheeler weighs around 40 tons (36 287.389 6 kg) compared with an average 2.5 tons (2 267.961 85 kg ) and so it takes almost twice as long to stop than an average car or pick up truck. Stopping a tractor trailer can be even more difficult when it is transporting heavier loads or when there are inclement weather conditions.

Driving an eighteen wheeler requires special training and skill due to the vehicles immense size, weight, and difference in mechanics than those of passenger vehicles. In the United States, a person must hold a class A commercial driver’s license to operate an eighteen wheeler. Additional regulations apply depending on the type of cargo to be transported such as hazardous materials. In Europe drivers are required to obtain an EC driver’s license and in Australia a class HR or HC license. Most countries in the world have regulations and requirements for the operation of an eighteen wheeler.

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Discussion Comments

By Rotergirl — On Jan 21, 2015

The men and women who drive the big rigs have my respect. It's not an easy job, and it's often a lonely one.

I never understood how an eighteen wheeler got the name "semi." That implies partial, and I never could see how a big rig was a "partial" anything.

I also like trucking songs like "Roll on, Big Mama" and "Six Days on the Road." They're great songs, and such a part of American roots music, because they really are uniquely a part of American music. I respect the role they have in freight and transportation, and know they do a great job. I remember the trucking strikes. Things really ground to a halt for a while. We need the truckers!

By Pippinwhite — On Jan 20, 2015

I think the hardest thing I had to learn to do when I learned to drive was to pass eighteen wheelers, and to not freak out when they passed me.

My favorite tactic in town was to wait until the truck was stopped at a traffic light. I'd get up to the light then, and make sure I got through the intersection before the truck even had time to start accelerating. I always felt better about getting by them when they were stopped. They really did scare me. I know the drivers can't help it, but it still irritates me when one passes me when it's raining and throws up a bunch of spray. Makes it hard to see.

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