Jackknifing
From Wikipedia, the free encyclopedia
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] 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 camber. This is not the same as jackknifing and is not 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.
[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 not noticeably reduced the number of car accidents but have significantly reduced the number of heavy vehicle accidents.
- The most important device that has prevented jackknifing over the years is the load-sensing brake-pressure regulator. This is a simple device which reduces the brake pressure to the rear wheels of an empty vehicle so that it doesn't spin around and face the other way when the brakes are applied. Since this device is fitted to almost every vehicle it's taken for granted, but without it cars would frequently spin off the road and empty articulated vehicles would be almost impossible to stop in a straight line.
- 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 off of 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.
