Talk:Semi-trailer

Definition
"A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, by a detachable front axle assembly known as a dolly, or by the tail of another trailer. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled."

I work for a trailer manufacturer in the UK and this definition is not correct. The frontal weight is not supported by a dolly (a dolly is, as the definition states, detachable...) but on the "fifth wheel" of the tractor unit itself.

Perhaps I can suggest an alternative and expand on this a bit further...

A semi-trailer is an unpowered, primarily goods-carrying vehicle, which commonly has between one and three axles (although this can extend to several) and is connected to a powered tractive unit, such as a road tractor. The trailer has a downward facing "Kingpin" situated in a fixed position under the front chassis area known as a "Bed-plate" or "Skid-Plate". The tractive unit has a fixed, upward facing "Fifth Wheel", which contacts and slides in a rearward direction along the trailer bedplate until it reaches the Kingpin and locks in that position by means of an automated jaw. This procedure is known as "Coupling". When coupled, the trailer can then be drawn by the tractive unit and detatched again when required. As the Kinpin can revolve freely within the Fifth Wheel on a horizontal plane (and partially vertical), the trailer "articulates" in relation to the tractive unit; hence the commonly used term "articulated semi-trailer", or shortened term "artic' ".

Some of the frontal trailer weight is supported by the tractive unit, via the Fifth Wheel. The bulk remainder of the weight is supported by the rear-mounted trailer axles.

As the trailer uses both pressurised air and electric for braking and lights, there are also several "Suzie" lines/hoses spanning the distance between the tractor and trailer.

"Landing Legs" are commonly situated at the front of a trailer chassis, enabling it to be supported in a level state without the need for a tractive unit. These landing legs are retracted during transit.

"A" Doubles: A term which refers to a trailer unit that has a detatchable "Dolly" axle attached, via an articulated arm, to its' rear. This dolly has a seperate fifth wheel which enables another semi-trailer to be coupled to it.

"B" Doubles: A term which refers to a specialist trailer unit with a fixed fifth wheel attached to the rear of an extended chassis. This additional fifth wheel can be used to couple to another standard semi-trailer.

Both "A" Double and "B" Double systems can be used to couple numerous trailers together in a train formation, sometimes refered to as a "Road Train". Road Trains are an extremely efficient way to move road based cargo and are a common sight in countries such as Australia.

Publically, semi-trailers connected to tractive units are known as "Lorrys", although this term alters depending on the country. "Trucks" refer to Cab-chassis with containing bodywork where all supporting axles are common to one chassis.

"Compared with a rigid vehicle, a semi-trailer truck has a turning circle smaller than its overall length making it more maneuverable.": This is not entirely correct. Although trailers do have a turning circle less than their overall length, this is hardly a good argument. A Semi trailer combination in the UK commonly has an inner and outer turning circle defined in law by 5.3m and 12.5m respectively. Common Rigid Vehicles are therefore more maneuverable (although this too a generic statement refering to standard length 7.5T - 18T rigid trucks).

It may also be worth mentioning the difference between a straight-frame and step-frame chassis. A straight-frame chassis will commonly run on 22.5" Wheels with a rear deck height of circa 1350mm. A step-frame however has a stepped chassis, where the rear 2/3rds are lower than the front "Swan-neck".  This lower chassis area will normally run on either 17.5" or 19.5" wheels and provides far greater cube than its' straightframe counterpart.  Double Deck trailers are primarily of step-frame design.

Reference www.donbur.co.uk —Preceding unsigned comment added by 217.41.22.24 (talk) 09:58, 27 October 2010 (UTC)

Here in the United States, most tractors do have a fixed fifth wheel. Some don't. The fifth wheel is adjustable. Ujiyo Himura (talk) 05:37, 17 February 2022 (UTC)

Advantages
Below is a cut and paste of much of that list, but it is big. Didn’t know how to make that weather stuff work with just trailers. Someone from the EU has to stand up for truck-full trailers. If anyone can use anything...Sammy D III (talk) 02:10, 30 July 2013 (UTC)

In road haulage, semi-trailers predominate over full-trailers because of their flexibility. The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trunked between depots. If a power unit fails, another tractor can replace it without disturbing the cargo.

Compared with a full trailer, a semi-trailer attached to a tractor unit is easier to reverse, since it has only one turning point (the coupling), whereas a full trailer has two turning points (the coupling and the drawbar attachment). Special tractors known as Shunt trucks (also as tugs, yard dogs, switchers, jockey horses, yard trucks or just shunts) can maneuver semi-trailers at a depot or loading and unloading ferries. These tractors may lift the coupling so that the trailer legs clear the ground.

Compared with a rigid vehicle, a semi-trailer truck has a turning circle smaller than its overall length making it more maneuverable. And since the drawbar adds to the overall length of the composition, a semi-trailer can have a longer cargo bed in the same overall length. Because of this a semi-trailer can haul longer objects (logs, pipe, beams, railway track) than a full trailer. (This depends on the legislation. In some European countries, at least in Finland and Sweden, the full trailer can be as long as the semi trailer.)


 * I was thinking of some input, maybe a consensus, but nobody ever comes here, so I’m just going to do it. This section, both before and after, is pretty lame, with no refs at all. This makes it conclusions based on observation and reasoning, seems pretty close to O.R. Anyway, if anyone cares, revert, fix, or delete it (the last sounds good to me).Sammy D III (talk) 16:34, 31 July 2013 (UTC)

Reefer and Tanker questions.
Why is there an entry for “Refrigerator truck”? The article is about trailers. If you must link to another article, why not do trailers and then link them to the (not very strong) “Refrigerator truck” article? There isn’t (anymore?) a trailer article. Also, “Refrigerator tank trailer” links to “Tank chassis” (see below), shouldn’t it link to “Refrigerator truck”?

Tank trucks are very common around here. Gasoline is transported from the refinery or depot to individual stations by tanker, there are plenty of other cargoes, too. There is even a picture of a Japanese Shell truck in the article. This article only has “Tank chassis” with a “Frac” sub-heading. Neither are common here. Chassis (from it’s article) appears to be a sort of lowboy to put shipping tank containers on. “Frac” appears to be some kind of equipment, not intended for cargo. It is a semi-trailer, though. Still, should it be considered a tank trailer?

The “Tank chassis” article does have some good US axle load stuff, general info, not specific to tank chassis. Maybe it should be here somewhere. Maybe also “Semi-trailer truck” (why are these two separate articles?) and/or “Truck”?

Just a couple of thoughts. Sammy D III (talk) 17:13, 13 May 2014 (UTC)

What happened
What happened to this reference?

Ref
Peter Horn User talk 13:05, 4 August 2017 (UTC)

Trailer types
So what exactly is the difference between a 1 or 2 axle trailer with a gooseneck hitch and a semi-trailer? Neither one is a true trailer, with two axles that can roll around without any additional support. And for that matter, if a "true trailer" has two axles and can roll around unsupported, then what is a normal trailer with a bumper hitch that can be pulled behind a pickup, but requires the support of the hitch? Also, what is the difference between a "true trailer" and a semi with a dolly attached? I cannot see one, except for the fact that a dolly can be detached. AnnaGoFast (talk) 15:13, 3 November 2017 (UTC)
 * The number of axles doesn't matter. A semi-trailer can not roll on it's own, the front of the load is carried by the tractor (or dolly). Your gooseneck sounds like semi-trailer. The trailer behind the pickup balances most of it's load on its own axles, the weight on the PU is a small part. (EDIT): On "tag" trailers that balance on their axles 10% (?) of the weight goes on the hitch. This is for drivibility, keep the hitch loaded so the trailer doesn't pick the back of the truck up. Think in profile and push down on the rear of the trailer. There are pickups with a fifth-wheel in the bed, it's trailer would be a semi. A semi-trailer with a dolly is a full trailer, the dolly holds the front end up. Thank you. Sammy D III (talk) 15:28, 3 November 2017 (UTC)