Tractor-Trailer = Semi: Semi-what?

[Jinx]

Regarding tractor-trailers, to what does the term "semi" refer?
- Jinx

[scr4]

Ever see a full-sized truck pulling a trailer as big as the truck itself? That's a full trailer. Semi-trailer means the tractor carries no load, just the trailer.

[everton]

As I understand it the semi is the back part of the vehicle and the tractor is the front part, so "semi" is the "back half". I could be wrong, though - we don't call them that over here.

[David Simmons]

My understanding is a that a trailer is a towed vehicle having wheels at or near all four corners. A semi-trailer has wheels only at the back end with the front end supported by the towing vehicle. The Merriam-Webster Collegiate Dictionary agrees with this definition.

[enipla]

Yep,

As David says, a full trailer would have most of its weight supported on it's own wheels.

A Semi trailer has a great deal of the weight supported by the tractor.

[Anachronism]

Just to clarify what David and enipla said. A Semi trailer needs the towing vehicle to support a large portion of the load, in the case of a 'Tractor Semi-trailer combination vehicle' (see why it is shortened to 'Semi') usually about half. A semi trailer can be converted to a full trailer by adding a dolly under the front.This is what you see when you see a truck hauling 'doubles', the first one is a semitrailer and the second is a full trailer.

A full trailer does not need to have wheels at all four corners, it can have one axel in the middle that supports the majority of the weight. A full trailer with an axle in the middle is supposed to be loaded so the hitch only supports a small amount of the weight. So a small trailer that is pulled behind a car is a 'full trailer' but the big trailer pulled by an 18 wheeler is only a 'semi-trailer'.

[buckgully]

I may be completely wrong here as I currently have a cold and I just took some NyQuil, but isn't "semi" short for "semi-articulated"?

[Anachronism]

Originally posted by buckgully
I may be completely wrong here as I currently have a cold and I just took some NyQuil, but isn't "semi" short for "semi-articulated"?
As it refers to trucks 'semi' is short for 'tractor semi-trailer' or more accurately 'tractor semi-trailer combination vehicle'.

[bibliophage]

Staff Report: Why are tractor-trailer rigs called "semis"? (http://www.straightdope.com/mailbag/msemi.html)

[Monty]

From Merriam-Webster's online dictionary:
Main Entry: semi·trail·er
Pronunciation: 'se-"mI-"trA-l&r, 'se-mE-, -mi-
Function: noun
Date: 1919
1 : a freight trailer that when attached is supported at its forward end by the fifth wheel device of the truck tractor
2 : a trucking rig made up of a tractor and a semitrailer