Editing Semi-trailer truck (section)

==Construction ==
[[File:Conventional 18-wheeler truck diagram.svg|frame|right|Side view and underside view of a '''conventional 18-wheeler semi-trailer truck''' with an enclosed cargo space. The underside view shows the arrangement of the 18 [[tire]]s (wheels). Shown in blue in the underside view are the [[axle]]s, [[drive shaft]], and [[Differential (mechanical device)|differentials]]. The legend for labeled parts of the truck is as follows:<br />
1. [[tractor unit]]<br />
2. [[semi-trailer]] (detachable)<br />
3. [[engine]] compartment<br />
4. [[Cabin (truck)|cabin]]<br />
5. sleeper (not present in all trucks)<br />
6. air dam (not present in all trucks)<br />
7. [[fuel tank]]s<br />
8. [[fifth wheel coupling]]<br />
9. enclosed [[cargo]] space<br />
10. landing gear – legs for when semi-trailer is detached<br />
11. [[Tandem#Trucks|tandem axles]]]]

===Types of trailers===
{{main|Semi-trailer}}
There are many types of semi-trailers in use, designed to haul a wide range of products.
{{Div col|colwidth=20em}}
* [[Box truck|Box]], or [[dry van]]
* [[Trailer bus|Bus]]
* [[Car carrier trailer|Car hauler]]
* [[Container chassis|Intermodal chassis]]
* [[Bulk cargo|Dry bulk]]
* [[Dump truck|Dump]]
* [[Flatbed truck|Flatbed]]
* [[Grain hopper trailer|Hopper-Bottom]]
* [[Lowboy (trailer)|Lowboy]]
* [[Refrigerator truck|Refrigerator]] 
** [[Refrigerated container|Reefer]]
* [[Tank truck|Tanker]]
{{div col end}}

===Coupling and uncoupling===
The cargo trailer is, by means of a [[kingpin (automotive part)|king pin]], hooked to a horseshoe-shaped quick-release coupling device called a [[Fifth wheel coupling|fifth wheel]] or a turntable hitch at the rear of the [[Tractor unit|towing engine]] that allows easy hook up and release. The [[truck trailer]] cannot move by itself because it only has wheels at the rear end: it requires a forward [[axle]], provided by the towing engine, to carry half the load weight.  When braking hard at high speeds, the vehicle has a tendency to fold at the pivot point between the towing vehicle and the trailer. Such a truck accident is called a "trailer swing", although it is also commonly described as a "jackknife."<ref>{{cite web |last1=Rogers |first1=Brian |title=Truck Jackknife Accidents |url=http://www.frg-law.com/truck-accidents/common-types/jackknife/#what-is-a-jackknife-accident |website=Fried Rogers Goldberg |publisher=Fried Rogers Goldberg LLC |access-date=12 January 2017}}</ref> [[Jackknifing]] is a condition where the tractive unit swings round against the trailer, and not vice versa.

===Braking===
[[File:Back of FMV2Mk2 cabin.JPG|thumb|upright|A pair of semi-trailer "Suzies" at the back of an Australian prime mover, red line for emergency/supply and blue for control]]
Semi trucks use air pressure, rather than [[Hydraulics|hydraulic]] fluid, to actuate the [[brake]]. The use of air hoses allows for ease of coupling and uncoupling of trailers from the tractor unit. The most common failure is ''[[brake fade]]'', usually caused when the drums or discs and the linings of the brakes overheat from excessive use.

The parking brake of the tractor unit and the emergency brake of the trailer are spring brakes that require air pressure in order to be released. They are applied when air pressure is released from the system, and disengaged when air pressure is supplied. This is a [[fail-safe]] design feature which ensures that if air pressure to either unit is lost, the vehicle will stop to a grinding halt, instead of continuing without brakes and becoming uncontrollable. The trailer controls are coupled to the tractor through two ''[[gladhand connector]]s'', which provide air pressure, and an electrical cable, which provides power to the lights and any specialized features of the trailer.

''Glad-hand connectors'' (also known as ''palm couplings'') are air hose connectors, each of which has a flat engaging face and retaining tabs. The faces are placed together, and the units are rotated so that the tabs engage each other to hold the connectors together. This arrangement provides a secure connection but allows the couplers to break away without damaging the equipment if they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines. These connectors are similar in design to the ones used for a similar purpose between [[railroad car]]s. Two air lines typically connect to the trailer unit. An ''emergency'' or ''main'' air supply line pressurizes the trailer's air tank and disengages the emergency brake, and a second ''service'' line controls the brake application during normal operation.

In the UK, male/female quick release connectors (''red line'' or emergency), have a female on the truck and male on the trailer,  but a ''yellow line'' or service has a male on the truck and female on the trailer. This avoids coupling errors (causing no brakes) plus the connections will not come apart if pulled by accident. The three electrical lines will fit one way around a primary black, a secondary green, and an [[Anti-lock braking system|ABS]] lead, all of which are collectively known as ''suzies'' or ''suzie coils''.

In New Zealand all trucks and trailers use a DUOMATIC air coupler which has female receivers mounted on the truck/tractor and on the trailer, and male on both ends of the suzie lines (they can be completely removed and stored in the cab to prevent theft). Connecting the red and blue lines is one operation at each end. The red and blue lines are always on the same side of every fitting so they can never hook up in reverse or the wrong way around. The same system is used in Europe.

Another braking feature of semi-trucks is [[engine braking]], which could be either a compression brake (usually shortened to ''[[Jake brake]]'') or [[exhaust brake]] or combination of both. However, the use of compression brake alone produces a loud and distinctive noise, and to control [[noise pollution]], some local municipalities have prohibited or restricted the use of engine brake systems inside their jurisdictions, particularly in residential areas. The advantage to using engine braking instead of conventional brakes is that a truck can descend a long grade without overheating its wheel brakes. Some vehicles can also be equipped with hydraulic or electric [[Retarder (mechanical engineering)|retarders]] which have an advantage of near silent operation.

===Transmission===
[[File:5x3 Twin stick.JPG|thumb|right|Traditional [[manual transmission]]s have 4–5 ratios on main shift and 3–4 on the auxiliary: pictured is a ''5×3'' with five main ratios and three auxiliaries]]
Because of the wide variety of loads the semi may carry, they usually have a [[manual transmission]] to allow the driver to have as much control as possible. However, all truck manufacturers now offer [[automated manual transmission]]s (manual gearboxes with automated gear change), as well as conventional [[torque converter|hydraulic]] [[automatic transmissions]].

Semi-truck [[transmission (mechanics)|transmissions]] can have as few as three forward speeds or as many as 18 forward speeds (plus 2 reverse speeds). A large number of transmission ratios means the driver can operate the engine more efficiently. Modern on-highway diesel engines are designed to provide maximum torque in a narrow [[revolutions per minute|RPM]] range (usually 1200–1500 RPM); having more gear ratios means the driver can hold the engine in its optimum range regardless of road speed ([[Gear ratio|drive axle]] ratio must also be considered).

A ten-speed manual transmission, for example, is controlled via a six-slot H-box pattern, similar to that in five-speed cars — five forward and one reverse gear. Gears six to ten (and high-speed reverse) are accessed by a Lo/High range splitter; gears one to five are Lo range; gears six to ten are High range using the same shift pattern. A Super-10 transmission, by contrast, has no range splitter; it uses alternating "stick and button" shifting (stick shifts 1-3-5-7-9, button shifts 2-4-6-8-10).  The 13-, 15-, and 18-speed transmissions have the same basic shift pattern but include a splitter button to enable additional ratios found in each range. Some transmissions may have 12 speeds.

Another difference between semi-trucks and cars is the way the clutch is set up. On an [[automobile]], the clutch pedal is depressed full stroke to the floor for every gear shift, to ensure the gearbox is disengaged from the engine. On a semi-truck with [[non-synchronous transmission|constant-mesh transmission]] (non-synchronized), such as by the [[Eaton Corporation|Eaton]] Roadranger series, not only is [[Double clutch (technique)|double-clutching]] required, but a clutch brake is required as well. The clutch brake stops the rotation of the gears and allows the truck to be put into gear without grinding when stationary. The clutch is pressed to the floor only to allow a smooth engagement of low gears when starting from a full stop; when the truck is moving, the clutch pedal is pressed only far enough to break torque for gear changes.

Theoretically, semi-trucks could have [[Diesel–electric transmission|diesel-electric transmission]], as electric motors have better torque at 0 RPM than diesel engines, but this would significantly increase the weight of the truck itself, above the maximum legal weight for road vehicles.<ref>{{Cite web |title=r/Trucks – Why aren't semi trucks diesel-electric like trains and busses? Genuine question |url=https://www.reddit.com/r/Trucks/comments/uuonad/why_arent_semi_trucks_dieselelectric_like_trains/ |access-date=2022-07-20 |website=reddit |date=21 May 2022 |language=en-US}}</ref>

===Lights===
An electrical connection is made between the tractor and the trailer through a cable often referred to as a ''pigtail''. This cable is a bundle of wires in a single casing. Each wire controls one of the electrical circuits on the trailer, such as running lights, brake lights, turn signals, etc. A coiled cable is used which retracts these coils when not under tension, such as when not cornering. It is these coils that cause the cable to look like a pigtail.

In most countries, a trailer or semi-trailer must have minimum
* 2 rear lights (red)
* 2 stop lights (red)
* 2 turning lights; one for right and one for left, flashing (amber; red optional in North America. May be combined with a brake light in North America)
* 2 marking lights behind if wider than certain specifications (red; plus a group of 3 red lights in the middle in North America)
* 2 marking lights front if wider than the truck or wider than certain specifications (white; amber in North America)

===Wheels and tires===
Although [[Glossary of the American trucking industry#Dual wheels|dual wheels]] are the most common, use of two single, wider [[tire]]s, known as ''[[Glossary of the American trucking industry#Super single|super singles]]'', on each axle is becoming popular among bulk cargo carriers and other weight-sensitive operators. With increased efforts to reduce [[greenhouse gas emissions]], the use of the super-single tire is gaining popularity. There are several advantages to this configuration. The first of these is that super singles reduce fuel consumption. In 1999, tests on an oval track showed a 10% fuel savings when super singles were used. These savings are realized because less energy is wasted flexing fewer tire sidewalls. Second, the lighter overall tire weight allows a truck to be loaded with more freight. The third advantage is that the single wheel encloses less of the brake unit, which allows faster cooling and reduces [[brake fade]].

In Europe, super singles became popular when the allowed weight of semitrailer rigs was increased from 38 to 40 tonnes.<ref>{{Cite journal |url=http://www.comt.ca/english/programs/trucking/Pavements/European%20Commission%20COST%20334%20Study%20-%20Effects%20of%20Wide%20Single%20Tyres%20and%20Dual%20Tires%202001.pdf |title=COST 334: Effects of Wide Single Tyres and Dual Tyres |publisher=European Union | journal=European Commission, Directorate General Transport |date=29 November 2001}}</ref> In this reform the trailer industry replaced two {{convert|10|t|lb|adj=on}} axles with dual wheels, with three {{convert|8|t|lb|adj=on}} axles on wide-base single wheels. The significantly lower axle weight on super singles must be considered when comparing road wear from single versus dual wheels. The majority of super singles sold in Europe have a width of {{convert|385|mm|in|abbr=on}}. The standard 385 tires have a legal load limit of {{convert|4500|kg|lb|abbr=on}}. (Note that expensive, specially reinforced 385 tires approved for {{convert|5000|kg|lb|abbr=on}} do exist. Their market share is tiny, except for mounting on the steer axle.)

===Skirted trailers===
{{main|Trailer skirt}}
An innovation rapidly growing in popularity is the skirted trailer. The space between the road and the bottom of the trailer frame was traditionally left open until it was realized that the [[turbulence|turbulent]] air swirling under the trailer is a major source of [[aerodynamic drag]]. Three split skirt concepts were verified by the [[United States Environmental Protection Agency]] (EPA) to provide fuel savings greater than 5%, and four split skirt concepts had EPA-verified fuel savings between 4% and 5%.<ref>{{cite journal |url=http://papers.sae.org/2012-01-2043 |title=EPA Smartway Verification of Trailer Undercarriage Advanced Aerodynamic Drag Reduction Technology |year=2012 |publisher=EPA Smartway |doi=10.4271/2012-01-2043 |access-date=2 October 2012|last1=Wood |first1=Richard |journal=SAE International Journal of Commercial Vehicles |volume=5 |issue=2 |pages=607–615 }}</ref>

Skirted trailers are often combined with Underrun Protection Systems (''underride guards''), greatly improving safety for passenger vehicles sharing the road.

===Underride guard===
{{anchor|Underride guard}}
[[File:Underride guard full-width impact test.JPG|thumb|Crash test of an underride guard at {{convert|30|-|40|kph|mph|abbr=on}}; the truck platform at head height has been prevented from slicing through the windshield]]
Underride protection systems can be installed at the rear, front and sides of a truck and the rear and sides of a trailer. A Rear Underrun Protection System (RUPS) is a rigid assembly hanging down from trailer's chassis, which is intended to provide some protection for passenger cars which collide with the rear of the trailer. Public awareness of this safeguard was increased in the aftermath of the accident that killed actress [[Jayne Mansfield]] on 29 June 1967, when the car she was in hit the rear of a tractor-trailer, causing fatal [[head trauma]]. After her death, the [[National Highway Traffic Safety Administration|NHTSA]] proposed requiring a rear underride guard, also known as a ''Mansfield bar'', an ''[[Interstate Commerce Commission|ICC]] bar'', or a ''[[United States Department of Transportation|DOT]] bumper''.<ref>{{cite web |title=Underride Guard |work=Everything2 |url=http://www.everything2.com/index.pl?node_id=1679241 |access-date=29 November 2007}}</ref><ref>{{cite book |last=United States Congressional Committee on Commerce |title=Reauthorization of the National Highway Traffic Safety Administration |year=1997 |page=39}}</ref> The proposal to mandate rear underride guards was withdrawn in 1971 after strong lobbying and opposition by the trucking industry,<ref>{{Cite news |last=Morris |first=John D. |date=1971-07-19 |title=Agency Drops Safety Plan Opposed by Trucking Men |language=en-US |work=The New York Times |url=https://www.nytimes.com/1971/07/19/archives/agency-drops-safety-plan-opposed-by-trucking-men.html |access-date=2023-06-21 |issn=0362-4331}}</ref> and so they were not federally-mandated until 1996; that mandate did not go into effect until 1998.<ref>{{Cite journal |last=NHTSA |first=USDOT |date=July 10, 2014 |title=Federal Motor Vehicle Safety Standards; Rear Impact Guards, Rear Impact Protection |url=https://www.govinfo.gov/content/pkg/FR-2014-07-10/pdf/2014-16018.pdf |journal=Federal Register |volume=79 |issue=132 |pages=39362–39363 |via=National Archives |url-status=live |archive-url=https://web.archive.org/web/20231215200431/https://www.govinfo.gov/content/pkg/FR-2014-07-10/pdf/2014-16018.pdf |archive-date= Dec 15, 2023 }}</ref>

The bottom rear of the trailer is near head level for an adult seated in a car, and without the underride guard, the only protection for such an adult's head in a [[rear-end collision]] would be the car's [[windshield]] and A pillars. The front of the car goes under the platform of the trailer rather than making contact via the passenger [[car bumper]], so the car's protective [[crush zone]] becomes irrelevant and [[air bag]]s are ineffective in protecting the passengers. If installed, the underride guard can provide a rigid area for the car to contact that is lower than the lip of the bonnet/hood, thus preventing the vehicle from squatting and running under the truck, instead ensuring that the vehicle's crush zones and engine block can absorb the force of the collision.

In addition to rear underride guards, truck tractor cabs may be equipped with a Front Underrun Protection System (FUPS) at the front bumper of the truck, if the front end is not low enough for the bumper to provide the adequate protection on its own.  The safest tractor-trailers are also equipped with side underride guards, also called Side Underrun Protection System (SUPS). These additional barriers prevent passenger cars from skidding underneath the trailer from the side, such as in an oblique or [[side collision]], or if the trailer [[jackknifing|jackknifes]] across the road, and this also helps protect cyclists, pedestrians, and other vulnerable road users.<ref>{{Cite web|url=https://www.driverknowledgetests.com/resources/what-does-side-underrun-protection-do-on-trucks-and-trailers/|title=What does side underrun protection do on trucks and trailers? |first1= Darren |last1=Cottingham |website=Driver Knowledge Tests |date=2 December 2019 |url-status=live |archive-url=https://web.archive.org/web/20231215134340/https://www.driverknowledgetests.com/resources/what-does-side-underrun-protection-do-on-trucks-and-trailers/ |archive-date= Dec 15, 2023 }}</ref> In the 1969 proposal for rear underride guards, the Federal Highway Administration indicated that, "It is anticipated that the proposed [rear underride guard] standard will be amended, after technical studies have been completed, to extend the requirement for underride protection to the sides of large vehicles".<ref>{{Cite journal |last=NHTSA |first=USDOT |date=March 19, 1969 |title=Motor Vehicle Safety Standards; Rear Underride Protection; Trailers and Trucks with Gross Vehicle Weight Rating Over 10,000 Pounds |url=https://www.govinfo.gov/content/pkg/FR-1969-03-19/pdf/FR-1969-03-19.pdf |journal=Federal Register |volume=53 |issue=34 |pages=5383–5384 |via=National Archives |url-status=live |archive-url=https://web.archive.org/web/20230806135800/https://www.govinfo.gov/content/pkg/FR-1969-03-19/pdf/FR-1969-03-19.pdf |archive-date= Aug 6, 2023 }}</ref> However, to date, a side underride guard mandate has yet to ever be proposed by the USDOT or NHTSA. In fact, for side underride guards, NHTSA has disregarded successful crash tests that stop a passenger vehicle from underriding a semitrailer,<ref>{{Cite web |date=August 29, 2017 |title=Side guard on semitrailer prevents underride in 40 mph test |url=https://www.iihs.org/news/detail/side-guard-on-semitrailer-prevents-underride-in-40-mph-test |access-date=2023-06-21 |website=IIHS-HLDI |language=en}}</ref><ref>{{Cite web |title=Wabash prototype: Side underride guard with aero skirt |url=https://www.trailer-bodybuilders.com/trailers/article/21742324/wabash-prototype-side-underride-guard-with-aero-skirt |access-date=2023-06-21 |website=Trailer Body Builders |first1=Charles |last1=Wilson |date=29 September 2017 }}</ref> ignored recommendations,<ref>{{Cite news |last=National Transportation Safety Board |date=April 3, 2014 |title=Safety Recommendations |url=https://www.ntsb.gov/safety/safety-recs/RecLetters/H-14-001-007.pdf |access-date=June 21, 2023}}</ref> disregarded administrative petitions,<ref>{{Cite journal |last=NHTSA |first=USDOT |date=July 10, 2014 |title=Grant of petition for rulemaking. |url=https://www.federalregister.gov/documents/2014/07/10/2014-16018/federal-motor-vehicle-safety-standards-rear-impact-guards-rear-impact-protection |journal=Federal Register |volume=79 |issue=132 |pages=39362–39363 |via=National Archives}}</ref> and denied petitions.<ref>{{Cite journal |last=NHTSA |first=USDOT |date=September 24, 1979 |title=Denial of a petition for rulemaking |url=https://www.govinfo.gov/content/pkg/FR-1979-09-24/pdf/FR-1979-09-24.pdf |journal=Federal Register |volume=44 |issue=1 |pages=55077–55078 |via=National Archives}}</ref><ref>{{Cite journal |last=NHTSA |first=USDOT |date=July 5, 2022 |title=Denial of petition for a defect investigation |url=https://www.federalregister.gov/documents/2022/07/05/2022-14165/denial-of-motor-vehicle-defect-petition-dp21-004 |journal=Federal Register |volume=87 |issue=1 |pages=39899–39901 |via=National Archives}}</ref>  In fact, for decades NHTSA has ignored credible scientific research on side underride guards and failed to take simple steps to stop these crashes.<ref>{{Cite web |last1=Thompson |first1=A. C. |last2=Mehrotra |first2=Kartikay |last3=Ingram |first3=Julia |date=2023-06-13 |title=Trapped Under Trucks: The Inside Story of the Government's Failure to Prevent Underride Crashes |url=https://www.propublica.org/article/underride-crashes-nhtsa-dot-iihs-safety-cars-trucks |access-date=2023-06-21 |website=ProPublica |language=en}}</ref>

In Europe, side and rear underrun protection are mandated on all lorries and trailers with a gross weight of {{convert|3500|kg}} or more.<ref>{{cite web |title=Heavy goods vehicles |url=https://ec.europa.eu/transport/road_safety/specialist/knowledge/vehicle/safety_design_needs/heavy_goods_vehicles_en |website=MOBILITY AND TRANSPORT, Road Safety |publisher=European Commission |access-date=2 November 2017}}</ref> Several US states and cities have adopted or are in the process of adopting truck side guards, including New York City, Philadelphia, and Washington DC. The NTSB has recommended that the National Highway Traffic Safety Administration (NHTSA) develop standards for side underride protection systems for trucks, and for newly manufactured trucks to be equipped with technology meeting the standards.<ref>{{cite web|title=U.S. Slow to Require Side Underride Guards on Trucks|author=The National Law Review|date=17 August 2019|access-date=20 September 2019|url=https://www.natlawreview.com/article/us-slow-to-require-side-underride-guards-trucks }}</ref>

In addition to safety benefits, these underride guards may improve [[fuel mileage]] by reducing air [[turbulence]] under the trailer at highway speeds. Another benefit of having a sturdy rear underride guard is that it may be secured to a [[loading dock]] with a hook to prevent "[[Loading dock#Dangers|trailer creep]]", a movement of the trailer away from the dock, which opens up a dangerous gap during loading or unloading operations.<ref>[http://www.everything2.com/index.pl?node_id=1679241 Underride Guard on Everything2]; Retrieved: 29 November 2007.</ref>