What Is King Pin Inclination (KPI)?
On current suspension systems, the kingpin is set at an angle to the vertical plane when viewed from the front or rear of the vehicle. This angle is known as the king pin inclination. The purpose of the KPI is to produce vertical displacement of the vehicle in during steering in an upward direction. The larger the KPI, the larger the effect. This lifting effect produces a self centering torque similar to that of caster. The KPI also generates scrub radius.
King Pin Inclination and Caster
Caster angle, similar to KPI, causes the wheel to rise and fall as well. However, dissimilar to KPI, the effect is opposite from side to side. Therefore, with symmetric geometry, the effect of right steer will roll the car to the left causing a diagonal weight shift.
Positive lifting of the car tends to increase the self-aligning torque whereas negative lifting of the car will reduce the self-aligning torque. This gives an unfamiliar feel to the steering as is the reason that it is used in harmony with KPI.