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Jackknifing is the folding of an
articulated vehicle An articulated vehicle is a vehicle which has a permanent or semi-permanent pivot joint in its construction, allowing it to turn more sharply. There are many kinds, from heavy equipment to buses, trams and trains. Steam locomotives were someti ...
so that it resembles the acute angle of a folding
pocket knife A pocketknife is a knife with one or more blades that fold into the handle. They are also known as jackknives (jack-knife), folding knives, or may be referred to as a penknife, though a penknife may also be a specific kind of pocketknife. A ty ...
. If a vehicle towing a trailer skids, the trailer can push the towing vehicle from behind until it spins the vehicle around and faces backwards. This may be caused by equipment failure, improper braking, or adverse road conditions such as an icy road surface. In extreme circumstances, a driver may attempt to jackknife the vehicle deliberately to halt it following brake failure.


Trailer swing

When a trailer skids to one side, this is known as a trailer swing or trailer slew. This can occur on a slippery road surface, often where there is a
cant Cant, CANT, canting, or canted may refer to: Language * Cant (language), a secret language * Beurla Reagaird, a language of the Scottish Highland Travellers * Scottish Cant, a language of the Scottish Lowland Travellers * Shelta or the Cant, a la ...
. This is not the same as jackknifing and is not as serious, as the trailer will move back into line as the vehicle continues forwards. The driver must be aware, however, that the trailer could slide up against parked cars or a
guard rail Guard rail, guardrails, or protective guarding, in general, are a boundary feature and may be a means to prevent or deter access to dangerous or off-limits areas while allowing light and visibility in a greater way than a fence. Common shapes ...
, or that the wheels could slide into a ditch. This situation can occur especially when the trailer is empty or lightly loaded, and weather conditions cause violent gusts of
crosswind A crosswind is any wind that has a perpendicular component to the line or direction of travel. This affects the aerodynamics of many forms of transport. Moving non-parallel to the wind's direction creates a crosswind component on the object and th ...
.


Anti-jackknife devices

*One system with limited success was a device that mechanically limited the angle which a trailer could swing. *A much more successful system was to fit the tractor with
anti-lock brakes An anti-lock braking system (ABS) is a safety anti- skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintai ...
. Fitted originally to airplanes in the 1950s, anti-lock brakes have significantly reduced the number of heavy-vehicle accidents. *
Electronic brakeforce distribution Electronic brakeforce distribution (EBD or EBFD) or electronic brakeforce limitation (EBL) is an automobile brake technology that automatically varies the amount of force applied to each of a vehicle's wheels, based on road conditions, speed, load ...
varies the pressure to the rear brakes during heavy load or hard braking, enhancing driver control.Brake Pressure Regulator – Patent 3762776
/ref> *
Tractors A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a trailer or machinery such as that used in agriculture, mining or construction. Most comm ...
were once commonly fitted with a lever in the cab to operate the trailer brakes (a trolley brake). The vehicle could be slowed or stopped using the trailer brakes only. Theoretically, this was a guaranteed way to prevent jackknifing; however, frequent use of the trailer brakes alone caused them to overheat and fade while the tractor brakes remained fresh. In the event of an
emergency stop A kill switch, also known as an emergency stop (E-stop), emergency off (EMO) and as an emergency power off (EPO), is a safety mechanism used to shut off machinery in an emergency, when it cannot be shut down in the usual manner. Unlike a normal ...
, the driver, applying the foot brake, could cause the trailer to jackknife because the tractor brakes lock while the trailer brakes are ineffective due to previous overheating. Trolley brakes largely disappeared from tractor units in North America in the 1980s and 1990s *An alternative to having a trailer brake lever in the cab is to fit the trailer with an
electromagnetic brake Electromagnetic brakes (also called electro-mechanical brakes or EM brakes) slow or stop motion using electromagnetic force to apply mechanical resistance (friction). They were originally called "electro-mechanical brakes," but over the years the ...
.


See also

*
Aquaplaning Aquaplaning or hydroplaning by the tires of a road vehicle, aircraft or other wheeled vehicle occurs when a layer of water builds between the wheels of the vehicle and the road surface, leading to a loss of traction that prevents the vehicle fr ...
*
Black ice Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on streets. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low leve ...
*
Fishtailing Fishtailing is a vehicle handling problem which occurs when the rear wheels lose traction, resulting in oversteer. This can be caused by low friction surfaces (sand, gravel, rain, snow, ice, etc.). Rear-drive vehicles with sufficient power can ...
* Underride guard


References

{{Road accidents Trucks Road accidents and incidents