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The Included Angle is the sum of Camber and S.A.I. It is
actually this angle that is measured by the S.A.I. vial
during the caster swing.
MEASUREMENT METHOD
S.A.I. is calculated by subtracting the measured camber
from the included angle. See the examples shown below.
CAMBER = 2
3 - 2 = 1 S.A.I.
CAMBER = -1
3 - (-1) = 4 S.A.I.
Thus, the sum of the Camber and S.A.I. angles should equal
the included angle. Camber is measured on the front wheels
individually when each front wheel is swung to zero toe. This
ensures the high accuracy and repeatability of the camber
and S.A.I. readings.
WHAT IA DOES
I.A. is another method of SAI measurement and is often used
to determine damage to the frame, sub-frame, K-member or
suspension arms. I.A. is always a positive angle.
SCRUB RADIUS
Scrub Radius is defined as the distance between the
projected steering axis inclination (SAI) and the projected
Centerline of the wheel at the point where the tire contacts
the road surface. The point where the SAI angle intersects
the road is where the force is applied to the tire. If there is a
forward force of the vehicle (as it would be driving down the
road), and the SAI was projected to the inside of the tire, the
drag on the tire would tend to force the wheel outward. If the
force were on the outside of the tire it would force it inward.
Scrub radius is a design determined specification, and
not mechanically adjustable. However, if the scrub radius
is incorrect, tire wear and hard steering could result.
POSITIVE SCRUB RADIUS
Positive scrub radius is defined as a steering axis inboard
from the Centerline of the wheel (typical of Rear Wheel
Drive vehicles), and negative scrub radius would be a
steering axis outboard the Centerline of the wheel (typi-
INCLUDED ANGLE = 3
INCLUDED ANGLE = 3
cal of Front Wheel Drive Vehicles.) The Scrub radius will
change the effect that vehicle forces impose on the toe.
Positive Scrub Radius will tend to create a force moving
the wheels outward on a rear drive vehicle. Negative Scrub
radius creates a force that forces the wheels out on a front
wheel drive vehicle while under power. This is found on front
wheel drive vehicles using MacPherson Strut suspensions.
NEUTRAL SCRUB RADIUS
A Neutral or near zero scrub radius will reduce the effects of
these forces. For this reason wheel selection can be critical
particularly on a front wheel drive vehicle. The wrong wheel
offset can create a different scrub angle and destroy the
good handling of the vehicle and it cannot be compensated
for in the alignment.
The suspension design for scrub radius can often be seen
in the difference between front wheel drive and rear wheel
drive. On front drive the wheels typically have their mounting
surface toward the outside of the wheel making them more
flat looking from the outside. Rear wheel drive suspension
running a positive scrub radius will typically be dished.
CASTER ANGLE
Caster is the rearward or forward tilt of the steering axis from
true vertical.
This angle is primarily used for directional stability while the
vehicle is moving straight ahead.
Positive caster is defined as a rearward tilt at the top of the
steering axis and a negative caster is a forward tilt.
The point where the caster axis intersects level ground is
where the vehicle forces are projected. Most vehicles utilize
some amount of positive caster. This angle does not nor-
mally contribute to tire wear when the vehicle is within the
manufacturer's specifications.
MEASUREMENT METHOD
Caster is calculated by the change in Camber reading over
a specific turning range of wheel from left to right. Caster
can only be measured on the front wheels.
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