Why are Big Trucks Called Semis?

For those who aren’t in the trucking industry, or maybe those considering going into the trucking industry, the question of why big trucks are called “semis” may have entered your mind. We took this question to the experts, both truckers and non-truckers alike, to find out the answer to why trucks are called “semis”. Here is what they had to say:

I am definitely not in the trucking business.

But ... semis are called such because they pull "Semi-Trailers." A semi-trailer is a trailer that only has one axel. Semi is actually short for "semi-trailer" truck.

Jeremy Shepherd
Founder & CEO

A semi is a trailer box with a rear axle but without a front axle. The truck pulling it is called the tractor.

Jeff Davis

This comes under general knowledge. A semi is correctly a semi-trailer. It doesn't trail behind the rear wheels of the vehicle pulling it. The attachment point is in front of the rear wheels of the tractor. Hence it is not a full trailer, but a semi-trailer. Correctly the rig is known as a tractor/semi-trailer. The reason for this configuration is that a large portion of the load is carried by the tractor and not on the hitch of the trailer attachment.

Ken Bodnar

Semi is short for Semi-Trailer Truck. A semi-trailer is one that does not have front wheels. Instead, the front of the trailer rests on a coupling plate ("fifth wheel") on the tractor unit.

In the U.K., it is called an Articulated Lorry.

John Z Wetmore
Producer of "Perils For Pedestrians" Television

For many years I owned and operated a high rise rigging and heavy hauling trucking company covering the - states of NY, Conn, NJ, Pa, and Va.

Any haulage combination connecting a fifth wheel tractor to a king pin trailer articulating and controlled by the tractor operator managing a quick connect common system of air brakes and running, stop and signaling lights is considered a semi type haulage rig.

Adding to my description the etymology of the word semi meaning half is confusing when associated to the big rigs. There is never a half between a tractor and trailer.

It is at best there could be a one third tractor to two thirds trailer. However, for description purposes the word semi distinguishes a straight single frame type rig from a tractor trailer articulated combo as I described.

Arnold Gendelman
Dallas, Texas
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