Lesson 1/Learning Event 1
Fifth wheel steering is commonly used on towed vehicles, such as
semitrailers pulled by tractortrucks. The lower part of the steering pivot
or fifth wheel is mounted over the center, and slightly to the front, of the
rear axle of the tractor. It has a kingpin lock to hold the kingpin or
pivot pin of the semitrailer in the center of the fifth wheel.
Usually, the lower fifth wheel is mounted on the tractor with two pivot
shafts. One shaft is positioned crosswise to the tractor; the other,
lengthwise. This allows the lower fifth wheel to tip at various angles to
the tractor chassis, keeping the bearing surfaces of upper and lower halves
of the fifth wheel in firm contact as the tractor and trailer travel over
unlevel roads.
The upper part of the fifth wheel consists of a pickup plate and kingpin
secured to the bottom front of the semitrailer. A groove around the kingpin
allows engagement of the kingpin lock.
When the semitrailer is connected to the tractor, the bottom of the trailer
is higher than the tractor wheels. This is necessary because, as the truck
and trailer make a turn, the entire rear axle and wheel assembly pivot under
the front of the trailer frame. On a very sharp turn, the wheel on the
inside of the turn will move to about the middle under the trailer chassis.
Clearance must be provided for the wheels.
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