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Jackknifing means the folding of an articulated vehicle (such as one towing a trailer) such that it resembles the acute angle of a folding pocket knife. If a vehicle towing a trailer skids, the trailer can push from behind until it spins round 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 deliberately jackknife the vehicle in order to halt it following brake failure.

When an articulated vehicle jackknifes, the cab ends up facing in the opposite direction to the trailer. As such it is impossible for the cab (which contains the engine) to move and the vehicle becomes stuck. Since a jackknifed trailer is almost always facing sideways across the lanes of a road, and since it can no longer move, these sorts of accidents can cause major congestion on roads, even if there is relatively little damage to vehicles.

Trailer swing

When a trailer skids to one side, this is known as a trailer swing or trailer slew. This could happen on a slippery road surface, often where there is a cant. This is not the same as "jackknifing" and is not as serious since the trailer moves 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, or 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.

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. Fitted originally to airplanes in the 1950s, anti-lock brakes have significantly reduced the number of heavy vehicle accidents.
Electronic brakeforce distribution varies the pressure to the rear brakes during heavy load or hard braking, enhancing driver control.^[1]
Tractors used to be fitted with a lever in the cab to operate the trailer brakes. The vehicle could be slowed down or stopped using the trailer brakes only. Theoretically this was a sure way to prevent jackknifing, but truck drivers will have noticed the recent disappearance of this lever from their cabs. The reason is that this lever was often the cause of jackknifing in a roundabout way. 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, the driver would go straight for the foot brake and the truck would surely jackknife because the tractor brakes would lock while the trailer brakes would be ineffective due to previous overheating.
An alternative to having a trailer brake lever in the cab is to fit the trailer with an electromagnetic brake.
Shorter car body.^{[clarification needed]}

References

^ Brake Pressure Regulator - Patent 3762776

[1] Brake Pressure Regulator - Patent 3762776

[1]

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 {{About|vehicle accidents |the statistics procedure |Jackknife (statistics)}}
-[[File:Jackknifing truck.gif|thumb|right|Animation of trailer swing. The red car is crushed like a nut in a [[nutcracker]]]]
+[[File:Jackknifing truck.gif|thumb|right|Jackknifing truck crushes the red car like a nut in a [[nutcracker]]. Blue car hits the side of trailer like a wall, and both cars risk [[Semi-trailer truck#Underride guard|underriding the trailer platform]].]]
-'''Jackknifing''' means the folding of an [[articulated vehicle]] (such as one towing a trailer) such that it resembles the acute angle of a folding [[pocket knife]]. If a vehicle towing a trailer skids, the trailer can push it from behind until it spins round 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 deliberately jackknife the vehicle in order to halt it following brake failure.
+'''Jackknifing''' means the folding of an [[articulated vehicle]] (such as one towing a trailer) such that it resembles the acute angle of a folding [[pocket knife]]. If a vehicle towing a trailer skids, the trailer can push from behind until it spins round 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 deliberately jackknife the vehicle in order to halt it following brake failure.
-When an articulated vehicle jackknifes, the cab is facing in the opposite direction to the trailer. As such it is impossible for the cab (which contains the engine) to move and the vehicle becomes stuck. Since a jackknifed truck is almost always facing sideways across the [[lane]]s of a road, and since they can no longer move, these sorts of accidents can cause significant [[Traffic congestion|congestion]] on roads.
+When an articulated vehicle jackknifes, the cab ends up facing in the opposite direction to the trailer. As such it is impossible for the cab (which contains the engine) to move and the vehicle becomes stuck. Since a jackknifed trailer is almost always facing sideways across the [[lane]]s of a road, and since it can no longer move, these sorts of accidents can cause major [[Traffic congestion|congestion]] on roads, even if there is relatively little damage to vehicles.
 == Trailer swing ==
-When a trailer skids to one side, this is known as a '''trailer swing''' or '''trailer slew''' (see animation to the right). This could happen on a slippery road surface, often where there is a [[Cant (road/rail)|cant]]. This is not the same as "jackknifing" and is not as serious since the trailer moves back into line as the vehicle continues forwards. The driver must be aware, however, that the trailer could slide up against parked cars or the wheels could slide into a ditch.
+When a trailer skids to one side, this is known as a '''trailer swing''' or '''trailer slew'''. This could happen on a slippery road surface, often where there is a [[Cant (road/rail)|cant]]. This is not the same as "jackknifing" and is not as serious since the trailer moves 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]], or 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]].
 == Anti-jackknife devices ==
+{{prose|date=June 2014}}
-*One system with limited success was a device that limited the angle which a trailer could swing.
+*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]]. Fitted originally to airplanes in the 1950s, anti-lock brakes have significantly reduced the number of heavy vehicle accidents.
 *[[Electronic brakeforce distribution]] varies the pressure to the rear brakes during heavy load or hard braking, enhancing driver control.<ref>[http://www.freepatentsonline.com/3762776.html Brake Pressure Regulator - Patent 3762776<!-- Bot generated title -->]</ref>

Revision as of 15:23, 3 June 2014

Trailer swing

Anti-jackknife devices

See also

References