Types of trailers
Main article: Semi-trailer
There are many types of semi-trailers in use, designed to haul a wide range of products.

Box, or dry van
Bus
Refrigerator, or “reefer”
Tanker
Dry bulk
Flatbed
Lowboy
Car hauler
Dump

Coupling and uncoupling
The cargo trailer is, by means of a king pin, hooked to a horseshoe-shaped quick-release coupling device called a fifth wheel or a turntable hitch at the rear of the 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”.[19] Jackknifing is a condition where the tractive unit swings round against the trailer, and not vice versa.

Braking

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 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 connectors, 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 cars. 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 ABS lead, all of which are collectively known as suzies or suzie coils.

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 retarders which have an advantage of near silent operation.

Transmission

Traditional manual transmissions 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 semi-automatic transmissions (manual gearboxes with automated gear change), as well as automatic transmissions.

Semi-truck 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 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 (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 constant mesh transmission (non synchronized), such as by the Eaton Roadranger series, not only is 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 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.

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 straight cable would break when the rig went around corners, so a coiled cable is used which retracts these coils when not under tension. 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)
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 dual wheels are the most common, use of two single, wider tires, known as 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.

One of the major disadvantages of the super singles is that they are currently not as widely available as a standard tire. In addition, if a tire should become deflated or be destroyed, there is not another tire attached to the same hub to maintain the dynamic stability of the vehicle, as would be the case with dual wheels. With dual wheels, the remaining tire may be overloaded, but it will typically allow the vehicle to be safely stopped or driven to a repair facility.

Skirted trailers
Main article: 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 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%.[20]

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

Underride guard

Crash test of an underride guard at 30–40 km/h (19–25 mph); the truck platform at head height has been prevented from impacting the windshield
Technically called a Rear Underrun Protection System (RUPS), this is a rigid assembly hanging down from the bottom rear of the trailer, 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 NHTSA recommended requiring a rear underride guard, also known as a Mansfield bar, an ICC bar, or a DOT (Department of Transportation) bumper, but the trucking industry has been slow to upgrade this safety feature.[23]

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. Because of the height mismatch between a passenger car bumper and the much-higher height of the platform of a trailer, the car’s protective crush zone becomes irrelevant and air bags are ineffective in protecting the car passengers, if the underride guard is missing or inadequate.

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. 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 jackknifes across the road. 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 underride guard is that it may be secured to a loading dock with a hook to prevent “trailer creep”, a movement of the trailer away from the dock, which opens up a dangerous gap during loading or unloading operations.

 
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