Jackknifing
Encyclopedia
Jackknifing means the folding of an articulated vehicle
Articulated vehicle
An articulated vehicle is a vehicle which has a permanent or semi-permanent pivoting joint in its construction, allowing the vehicle to turn more sharply. There are many kinds of articulated vehicles, from heavy equipment to buses, trams and trains...

 (such as one towing a trailer) such that it resembles the acute angle of a folding pocket knife
Pocket knife
A pocket knife is a folding knife with one or more blades that fit inside the handle that can still fit in a pocket. It is also known as a jackknife or jack-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 jack knife 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.

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
Camber
Camber may refer to a variety of curvatures and angles:* Camber angle, the angle made by the wheels of a vehicle* Camber thrust in bike technology* In the steel industry, the concavity of rolls...

. 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.

Anti-jackknife devices

  • 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.
  • Electronic brakeforce distribution
    Electronic brakeforce distribution
    Electronic brakeforce distribution ', Electronic brakeforce limitation ' is an automobile brake technology that automatically varies the amount of force applied to each of a vehicle's brakes, based on road conditions, speed, loading, etc...

     varies the pressure to the rear brakes during heavy load or hard braking, enhancing driver control.
  • 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
    Brake fade
    Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions...

     while the tractor brakes remained fresh. In the event of an emergency stop
    Emergency stop
    -United Kingdom driving test:For the practical section of the driving test, an examiner will often request the candidate to perform an emergency stop. This is done in order to evaluate how well a candidate can react to a dangerous situation and bring their motor vehicle to a safe stop. This is to...

    , 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
    Electromagnetic brake
    An eddy current brake, like a conventional friction brake, is responsible for slowing an object, such as a train or a roller coaster. However, unlike electro-mechanical brakes, which apply mechanical pressure on two separate objects, eddy current brakes slow an object by creating eddy currents...

    .
  • Shorter car body length.
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