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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 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.
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 lanes of a road, and since they can no longer move, these sorts of accidents can cause significant congestion on roads.
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. 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.
- One system with limited success was a device that 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.
- 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]