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TM 9-8000
measured in degrees, that the steering knuckle pivots
33-17. Steering Geometry/Toe-Out. The front end
are tilted forward or backward from the vertical axis when
assembly of the modern motor vehicle requires careful
viewed from the side. Caster tends to keep the front
design and adjustment because each front wheel is
wheels pointed straight ahead, making it easier to return
pivoted separately on a steering knuckle. Because of this
the wheels to a straight ahead position after a turn has
construction, the front wheels are not in the same radius
been made. The principle is the same as that used in
line (drawn from the center of rotation (fig. 33-20)) when
tilting the front fork of a bicycle, which makes it possible
a vehicle is making a turn. Because each wheel should
to ride the bicycle without firmly holding the handlebars.
be at right angles to its radius line, it is necessary for the
Part of the effort applied for turning castered wheels out
front wheels to assume a toed-out position when
of the straight ahead position slightly raises the front end
rounding curves. If they do not, the tires slip, which
of the vehicle upward. Consequently, when the steering
causes excessive tire wear. The inner wheel (the one
gear is released, the weight of the vehicle forces the
closer to the center of rotation) turns more than the outer
front end down and straightens the wheels. Caster is
wheel, so it will travel in a smaller radius. This difference
designated as positive for backward tilt and negative for
in the turning ratios of the two wheels is called toe-out. It
forward tilt of the steering knuckle pivots viewed from the
is usually specified as the number of degrees over 20
side. Caster may be obtained on a solid axle suspension
that the inner wheel is turned when the outer wheel is
by inserting a thin wedge or shim between the axle and
turned 20 degrees. Toe-out on turns may be checked but
the spring. The axle can be made so that the supports
there is no provision made for its adjustment. The
for the steering knuckle pivots are tilted vertically. In
steering linkage must be examined carefully for bent or
parallel arm suspension, caster is obtained by mounting
defective  parts  if  this  angle  is  not  within  the
the steering knuckle support in the control arms so that it
manufacturer's specifications.
is tilted to the desired amount. If the axis of the steering
33-18. Caster. Caster is the angle (fig. 33-21),
Figure 33-20. Steering Geometry, Illustrating Toe-Out


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