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Figure 12-2.—Caster angle. | Up
Construction Mechanic Advanced - Construction methods and practices | Next
Figure 12-6.—Difference of radii between inner and outer wheels. |
Figure 12-3.—Kingpin inclination.
independent suspension, the upper pivot point (ball
joint) is set to the rear of the lower pivot point.
Caster is a directional control angle, but not a tire
wearing angle. Positive caster causes the vehicle to steer
in the direction in which it is moving. This is called an
automatic steering effect; for instance, the forward
momentum of a vehicle tends to keep wheels with
positive caster in the straight-ahead position. After
rounding a turn, this causes the wheels to return to a
straight-ahead position when the driver releases the
steering wheel. This automatic steering effect is also
called self-righting or self-centering action.
Positive caster makes the turning of the steering
wheel more difficult, whereas negative caster turns
more easily, but will cause the vehicle to wander.
3. KINGPIN INCLINATION. The inward tilt of
the kingpin at the top is known as kingpin inclination
(KPI). KPI (fig. 12-3) is measured in degrees from the
center line of the ball joint or kingpin to true vertical (0).
It is a directional control angle with fixed relationship
to camber settings. It is also nonadjustable. One purpose
of this inclination is to reduce the need for excessive
camber. Figure 12-4 shows a dead axle with fixed KPI.
The angle of the kingpin and spindle is made extreme to
clarify the principles involved.
Timing the wheels to the left or right revolves the
spindles around the kingpin. As the spindle is moved to
the left or right from the position shown in figure 12-4,
B, its end moves down, as shown in figure 12-4, A and
Figure 12-4.—Fixed KPI.
C. Thus, as the front wheels turn, the spindle will attempt
to move down from the high point. Since the wheels and
tires prevent the spindles from moving down, the axle
is raised. This action tends to raise the front of the
vehicle. As the turning force is removed from the
wheels, the weight of the vehicle helps force the wheels
back to the straight-ahead position.
Vehicles with ball-joint suspension have what is
known as steering axis inclination (SAI) which is
defined as the inward tilt of the spindle support arm at
the top. The spindle assembly is supported at the upper
and lower control arms by ball joints. The pivoting axis
of the wheel around the ball joints is the same as the
kingpin axis of vehicles with dead axles.
4. TOE-IN. This is the distance between the front
of the front wheels as compared to the distance at the
12-3
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