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Why Is A Semi Called A Semi

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How did trucks get the name “semi”? Weighing in at up to 80,000 pounds and typically 70-plus feet long, there certainly doesn’t seem to be anything “partial” or “incomplete” about the massive vehicles.

If you’re wondering, “why is it called a semi-truck?” keep reading to find out how it got its name and what it really means.

Why is it called a semi-truck?

Despite what you may think, the “semi” in “semi-truck” actually refers to the trailer, not the truck itself. In fact, the name “semi-truck” is just a shortened version of “semi-trailer truck.”

This longer, lesser-used name more directly refers to the two parts that make up these large vehicles:

  • A towing engine (more commonly known as the tractor, or truck), where the driver sits and controls the vehicle.
  • One or more semi-trailers, which carry the freight.

What about all of the other names?

Why are 18-wheelers called semis, then? And why are tractor-trailers called semis? The answer is simply because “semi-truck,” or just “semi,” has become a catch-all term to describe the vehicles.

The name “semi-truck” has been used since the early 1900s and remains the preferred term mainly because it’s more convenient to say than the other, longer options.

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Though “semi” is the most popular, some other common terms include:

  • 18-wheeler. Mostly used in the South.
  • Tractor-trailer. Mostly used in the Northeast.
  • Semi-truck-trailer.
  • Big rig.

If you still find this a little confusing, you’re not alone. For example, what is the difference between a semi-truck and a semi-tractor, then?

While both “semi-truck” and “semi-tractor” describe the towing engine portion of the vehicle, you can also use “semi-truck” to describe the entire vehicle, and others will understand what you mean.

What is a semi-trailer?

Some of the key differences between a full trailer vs a semi-trailer are:

Axle position

  • Semi-trailers only have rear axles, so half of the weight is supported by the back of the tractor.
  • Full trailers have front and rear axles, so they can fully support themselves.

Connection type

  • Semi-trailers connect to a tractor using a coupling mechanism known as a fifth wheel, which helps to distribute the load between the tractor and the trailer.
  • Full trailers use a drawbar with a hook to attach to the tractor’s hitch.

Capacity and use

  • Semi-trailers are generally 48 to 53 feet long. This, combined with the fact that only half of a semi-trailer’s weight is supported by its own axles, means they can transport heavier and larger loads.
  • Full trailers can range from 20 to 40 feet long. They generally carry lighter loads and are used for short-distance hauls, commonly to factories, ports and warehouses.

You’re more likely to see a semi-trailer than a full trailer on the highway, as it is the preferred method of long-haul transportation. Semi-trailers are preferred since they allow for:

  • A tighter turning radius.
  • Easier backing ability.
  • Trailers to be swapped/dropped off quicker and easier.
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That being said, if you’ve ever come across a tractor hauling multiple trailers (which requires a doubles and triples endorsement) the rear trailer is often a full trailer.

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