1.6.0 Tracking
When a vehicle is running on the
highway, all of the axles should
be tracking perpendicular to the
vehicle centerline; that is, the rear
wheels should track directly
behind the front wheels when the
vehicle is moving straight ahead.
When this happens, the thrustline
created by the rear wheels is
parallel to the vehicle centerline,
as shown in
Figure 9-12, View A
.
However, if the axles are not
running perpendicular to the
vehicle centerline, the rear
wheels will not track directly
behind the front wheels, and the
thrustline of the rear wheel
deviates from the centerline of
the vehicle, as shown in
Figure 9-
12, View B
. The steering controls
Figure 9-12 - Correct and incorrect
thrustline.
fight the vehicle thrustline, resulting in an uncentered steering wheel and accelerated
front tire wear, which causes the vehicle to oversteer when turning in one direction and
understeer when turning in the other direction. Oversteering is an overresponse to
steering input, which in vehicles yaw or lateral tracking off the intended turning radius.
Understeer is an underresponse to steering input, most often causing steering tire slip
at high speeds. Incorrect directional tracking can occur on single-axle vehicles, tandem-
axle vehicles, and trailers. On a
single-axle vehicle, the rear axle
thrustline can be off if the entire
axle is offset or if only one wheel
has an improper toe angle. On a
tandem axle, there are a number of
different combinations that can
cause incorrect tracking.
Figure 9-
13
illustrates some of these
combinations.
One method of checking a single
axle for misalignment is to clamp a
straightedge across the frame so
that it is square with the frame rails
on each side. Then measure from
the straightedge to the center of
the hub. The distance on each side
should be within Y inch of each
other. If not, the axle must be
aligned.
Figure 9-13 - Typical incorrect tracking of
tandem-axle vehicles.
NAVEDTRA 14050A
9-12