5.0.0 NONADJUSTABLE SUSPENSION ANGLES
5.1.0 Turning Radius
When a vehicle turns a corner, the inner wheel must turn on a smaller circle than the
outer wheel. The inner wheel is said to turn in a shorter radius. On the rear axle, the
wheels are not connected by linkage, and the difference in turning radius does not affect
vehicle operation. In the front, however, the linkage holds the wheels parallel and one
tire has to break loose from the pavement every time the vehicle makes a turn. To
prevent this, the front wheels must automatically toe out on turns to allow the inner
wheel to turn in a shorter radius. The steering arms are designed to angle slightly
toward the center of the vehicle. When the wheels are turned, the inside steering arm
swivels at a greater angle. This causes the inner wheel to turn more sharply than the
outer, increasing toe out. The bend in the steering arm does not affect toe when the
vehicle is traveling straight
ahead.
5.2.0 Steering Axis
Inclination {SAI)
Like caster, steering axis
inclination is a line formed by the
relative positions of the top ball
joint or strut mounting in relation
to the bottom ball joint. While
caster is formed by the back-
and-forth positions of the upper
and lower ball joints (for the
lower ball joint and the strut
mounting), steering axis
inclination is formed by the in-
and-out positions of these parts
in relation to the centerline of the
vehicle. SAI is always slanted
inward at the top (
Figure 9-25
).
The imaginary lines formed by
SAI, camber, and true vertical
always contact the road very
closely to each other. This
Figure 9-25 - Steering axis inclination
shown on a front-wheel drive, independent
suspension.
causes road shocks to be absorbed by the steering knuckle instead of being transmitted
to the steering linkage. The sideways angle of the SAI centerline also causes the force
of the road shocks to enter the frame at a sideways angle, compensating somewhat for
the upward shock of the wheel assembly. The SAI angle also keeps the vehicle weight
on the inner wheel bearing, eliminating the need for excessive camber, which would
wear the tire.
NAVEDTRA 14050A
9-32