Figure  12-3.Kingpin  inclination.

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Figure  12-3.—Kingpin  inclination. independent suspension, the upper pivot point (ball joint) is set to the rear of the lower pivot point. Caster is a directional control angle, but not a tire wearing angle. Positive caster causes the vehicle to steer in the direction in which it is moving. This is called an automatic  steering  effect;  for  instance,  the  forward momentum  of  a  vehicle  tends  to  keep  wheels  with positive caster in the straight-ahead position. After rounding a turn, this causes the wheels to return to a straight-ahead position when the driver releases the steering wheel. This automatic steering effect is also called  self-righting  or  self-centering  action. Positive caster makes the turning of the steering wheel more difficult, whereas negative caster turns more easily, but will cause the vehicle to wander. 3. KINGPIN INCLINATION. The inward tilt of the kingpin at the top is known as kingpin inclination (KPI). KPI (fig. 12-3) is measured in degrees from the center line of the ball joint or kingpin to true vertical (0). It  is  a  directional  control  angle  with  fixed  relationship to camber settings. It is also nonadjustable. One purpose of this inclination is to reduce the need for excessive camber. Figure 12-4 shows a dead axle with fixed KPI. The angle of the kingpin and spindle is made extreme to clarify  the  principles  involved. Timing the wheels to the left or right revolves the spindles around the kingpin. As the spindle is moved to the left or right from the position shown in figure 12-4, B, its end moves down, as shown in figure 12-4, A and Figure 12-4.—Fixed KPI. C. Thus, as the front wheels turn, the spindle will attempt to move down from the high point. Since the wheels and tires prevent the spindles from moving down, the axle is raised. This action tends to raise the front of the vehicle.  As  the  turning  force  is  removed  from  the wheels, the weight of the vehicle helps force the wheels back  to  the  straight-ahead  position. Vehicles with ball-joint suspension have what is known  as  steering  axis  inclination  (SAI)  which  is defined as the inward tilt of the spindle support arm at the  top.  The  spindle  assembly  is  supported  at  the  upper and lower control arms by ball joints. The pivoting axis of the wheel around the ball joints is the same as the kingpin axis of vehicles with dead axles. 4. TOE-IN. This is the distance between the front of the front wheels as compared to the distance at the 12-3

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