Jackknifing
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Jackknifing means the accidental folding of an articulated vehicle (i.e. 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.
Jackknifing is not very common and usually only happens to an empty vehicle. Most truck drivers are skillful enough to correct a skid before it becomes a jackknife. It would be an exaggeration to claim that jackknifing accounts for a large number of tractor-semitrailer accidents since in many cases it is the collision that would have caused the vehicle to jackknife and not vice versa. Radio stations often report jackknifed trucks because people phone to tell them, but more often than not, the truck has not technically jackknifed; it may be stuck in the snow or damaged in a crash.
[edit] Anti jackknife devices
Over the years systems to combat jackknifing have been developed.
- 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 planes in the 1950s, anti-lock brakes have significantly reduced the number of heavy vehicle accidents.
- Another important device that has prevented jackknifing over the years is the load-sensing brake-pressure regulator. This is a simple device varies the pressure to the rear brakes during heavy load or hard braking.[1]
- Prime movers 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 being that this lever was often the cause of jackknifing in a round about 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.
- An alternative to having a trailer brake lever in the cab is to fit the trailer with an electromagnetic brake.
- A new device called the "Atlas AJSS" has recently been invented that automatically engages at a speed of 45 mph and stays engaged unless manually released. It is actuated electronically and operated pneumatically from the truck's accumulator air supply. The device prevents the trailer and the truck from becoming over 15° out of alignment with each other. Under normal operating (highway) speeds, this 15° swing does not interfere with driving even on the most winding roads; of course, the system disengages under 45mph to allow for the hair-pin curves of winding mountain roads which are traversed at speeds under 45mph.
