| What
is a Jack Knife?
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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