Or, just as often, “tractor-trailers”? A tractor trailer is a trailer associated with a tractor. But there is no tractor, there is a truck, and most of the time, there is a trailer attached.
In this article about the bay bridge crash, the washington post can’t get it straight either. In one photo caption they call it a “semi” and in the other two, it’s a “tractor-trailer”.
What gives with these names? What is the correct name for this vehicle?
The truck is the tractor.
And the trailer is called a semi-trailer because it lacks a front axle, instead resting on the tractor.
It’s short for ‘semi-trailer freighter’. A full trailer has wheels under both ends and is pulled by a drawbar. A semi-trailer has wheels under one end; the other end rests on another part of the vehicle.
Okay, but why is the whole assembly called “a semi”? As in, “we were being tailgated by a semi the whole way here”. A trailer can’t tailgate anyone, it can’t even move on its own.
Because, as noted, it’s a natural shorthand for a more descriptive term. Saying “we were being tailgated by a semi-trailer freighter” gets a bit unwieldy.
ETA:
And what you are probably thinking of as a “tractor” is more properly called a “farm tractor”. The thing hauling the semi-trailer is a “road tractor”.
Same reason that you cook food in a microwave, some pianists play uprights, and people fly in jets. American English hates a long phrase if a short phrase works just as well.
A tractor is a device principally used for pulling something else. The most common use of it is as shorthand for “agricultural tractor”, which pulls plows, harrows, reapers, combines, balers, wagons, and other such farm machinery. The large engine-and-cab unit that provides the motive power and steering to a semi is also a tractor, normally only called that in popular parlance in the combination “tractor-trailer”. Captain Kirk of Star Trek was wont to order things to be brought to his ship by tractor beam – the device emitting the tractor beam would no doubt be another form of tractor, an ironic twist when according to the ST backstory, Kirk was from Iowa.
A semitrailer has wheels only beneath the rear – normally two axles bearing two tires at either end each. The front of the semitrailer rests on the large “fifth wheel” device at the back of the tractor, being held there by a giant pin and an assortment of other connections, the whole thing being supported by the two four-tire axles beneath the rear of the tractor unit providing motive power to the whole combination.
Tandem (double) and triple units occasionally seen on major highways consist of, from the front, tractor unit, semitrailer, and one or two full trailers with axles and tires beneath both front and rear.
Thanks for this thread. I always wondered where the tractor part was coming from with discussions of semi’s. It still doesn’t make sense why they’re called tractors, but at least I’ll know what people are talking about.
A tractor is a machine that pulls other equipment. A farm tractor pulls farm equipment. A road tractor pulls trailers or semitrailers. Your car or truck is a tractor unit if it’s pulling a trailer, presumably. I suppose a locomotive could be called a tractor unit, although they are quite capable of pushing as well, since railway cars are guided by the rails in a way that road trailers aren’t.
With the exception of faux-intellectuals who use “at the present time” when “now” would work better. 
And furthermore:
attract - pull/draw toward
retract - pull/draw back
detract - pull/draw apart
contract - pull/draw together
extract - pull/draw out
distract - pull/draw attention
protract - pull/draw out longer
If you don’t mind a bit of a highjack, on a trip a week ago I noticed that not a single semi was a cab-over.
I assume this change was done for fuel efficiency reasons, but had never noticed it before. Saw hundreds of trucks, and not one was cab-over.
Anyone got any info on when this conversion occurred? I apparently was asleep at the wheel…
Perhaps you would find the British “articulated lorry” less confusing? Except that they sometimes abbreviate that to “arctic”, which sounds like they are referring to a “reefer”, which, in turn, has its own problems with connotation.
Nitpick: Quite commonly double and triple trailers use 5th wheel attachments at each point, making each unit a semitrailer. This configuration is known (in these parts, at least) as a Super B. See, for example, this trailer manufacturer’s grain trailers, specifically the 3rd and 4th items down.
I only see ‘artic’ used around here with respect to buses.
“artic”.
“Tractor” is also the term for an airscrew that pulls the plane forward; originally a “propellor” was what we now call a “pusher”, though that usage has been defunct for a long time - possibly since a wartime request for fifty aircrew to be sent to a station that was fully staffed but lacked airscrews. :smack:
There used to be a length limit on the tractor/trailer combo. By shortening the tractor, you could have a longer trailer and haul more cargo. You could accomplish this by putting the cab over the engine instead of behind it.
I believe the laws have been changed to limit the length of the trailer only. Since cab-overs are no longer neccesary, they aren’t gettting built or used much anymore.
ETA
From Wiki:
Well, OK, it’s YOUR turf, but here’s a couple examples:
http://www.manbymotorplex.com/res_website.asp?supplierCode=MBY100&page=artic
Wow. Future generations will have no idea what C.W. McCall was talking about when he spoke of a “cab-over Pete”.