Ref.: MEL-WA(3D HCV) - OM Ver.1.7
16
5.4.
KINGPIN INCLINATION (Steering axis inclination)
Fig. 8
KINGPIN
Inclination (also referred to as the ball joint angle or Steering Axis Inclination-
SAI) can be a difficult angle to understand. The easiest way to understand Kingpin
Inclination is to first define Steering axis. The steering axis is an imaginary line
intersecting the spindle support. In a conventional steering system, the spindle
supports are the upper and lower ball joints or the Kingpins. With MacPherson strut
systems, Steering axis is the angle beginning at the ball joint and extended through the
strut assembly. Viewed from the front of the vehicle, Kingpin Inclination is the angle
between the Steering axis and a true vertical line established through the tyre. The
Kingpin Inclination is a stability angle and is measured in degrees.
If these imaginary lines were extended to road surface, the area covered between them
would be identified as the point of load or scrub radius. The vehicle body will be
closest to the road surface when the wheels are pointed Straight Ahead as a result of
Kingpin Inclination.
A spindle with Kingpin Inclination will have the outer end of that spindle at the highest
point when the wheels are pointed Straight Ahead. Therefore, as the weight of the
vehicle pushes downward, the spindle will always attempt to move upward to return the
wheels to a Straight Ahead position. After a turn, the Kingpin Inclination helps to return
the tyres to Straight Ahead position. Kingpin Inclination also aids in vehicle directional
stability by resisting road irregularities that attempt to turn the wheels away from the
Straight-ahead position. This angle produces many of the same benefits that improve
steering stability as Positive Caster. Correct engineering of Kingpin Inclination can
reduce the need for high Positive Camber. The effect of Kingpin Inclination on
directional stability is usually greater then that of Caster. Some vehicles with Power
steering require a greater amount of steering wheel returning force than those with
manual steering. Kingpin Inclination is often used with Positive Caster on power
steering equipped vehicles to assist in steering wheel returnability.
