Jack Knife is a term that applied to the dangerous situation when a large 18 wheel truck and its trailer go into a skid and the trailer swings out and stops to form an angle of 90 degrees with each other. This term comes from a description of how the blade of a jack knife forms the angle with its protective handle. When a big rig jackknifes, the driver must have assistance from other vehicles to right itself. Moreover, if the rig jackknifes with any speed, the vehicle is likely to roll over.
Generally, jackknifing occurs when the drive wheels are locked when they reach a slick spot while the front wheels and the trailer keep rolling forward. Slippery roads often cause jackknifing. It can also happen when a driver either takes a curve too fast or swerves to avoid other vehicles.
When a diver encounters an emergency situation he has three basic braking options. He can lock the the steering axle brakes, he can lock the drive axles or he can lock the trailer axles. The potential outcomes depend upon which option the driver chooses.
If the driver chooses to lock the steering axle brakes, the rig will move straight ahead regardless of the wheel angle. This is certainly the most desirable outcome, although the driver will not be able to steer the rig in this situation. If the driver chooses to lock the drive axles a jackknife will occur sooner or later regardless of what the steering axle is doing. If the rig slides long enough, the road crown will cause a jackknife.
If the driver locks the trailer brakes, he loses directional control. He then experiences trailer swing which will further effect the directional stability of the vehicle.
To avoid the jackknife, the driver should attempt to evenly apply brake pressure to within 5% of locking the brakes. This is because truck tires can only offer a limited amount of traction and locking the brakes all the way will cause lateral forces to control the rig. Anti-lock ABS brakes help solve this problem in passenger vehicles. However, most big rigs are not equipped with ABS brakes. The problem is further exacerbated by an uneven distribution of the truck driver's load or unbalanced brakes.
In sum, locking the drive axles should be avoided at all costs. This choice will lead to a dangerous jackknife. Locking the steering axle brakes appears to be the best choice, but has the potential to lead to steering difficulties.
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