KPI / SAI is the inward tilt of the king pin, or ball-joint
/ bearing plate - at the top. The KPI / SAI angle and camber are related. The
principle purpose of KPI / SAI is for proper component load distribution and to
position the tire so that the KPI / SAI and camber meet at a common point where
the center of the tire contacts the road.
Most front suspensions carry a king pin / ball joint /
bearing plate that is inclined inward at the top. This angle has the effect of placing the
turning point at the center of the tire instead of toward the inside of the
wheel. The result is more stable
steering.
Combining camber angle and KPI / SAI angles is referred to
as included angle. Camber is built into the wheel by forming the spindle with a
downward tilt. KPI / SAI angle is built into the end of the axle. While the two
angles are interrelated, it is best to measure them separately. (This is
necessary in order to determine a bent spindle, king pin or damaged suspension
connection components.)
When the KPI / SAI relationship is correct, the weight of
the vehicle causes the front wheels to automatically straighten out after a
turn. As the wheel turns, the end of the spindle rises and falls. One side of
the vehicle is raised as the front wheels are turned to the right or the left and
is lowered when the wheels are in the straight ahead position.
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