Kingpin inclination (KPI) is the inward tilt of the front steering axis (through the upper and lower ball joints or kingpin) toward the center of the car when viewed from the front.
Spindles on full-size stock cars, modifieds, late models and sprint cars are machined with specific kingpin inclination angles, commonly 7 to 12 degrees depending on the rulebook and front clip. Higher KPI works together with caster to lift the front corner slightly as the wheels turn and reduces scrub radius without tucking the brakes outside the wheel hoop. Because KPI tilts the steering axis inward, changing spindle KPI changes camber gain as the wheels steer: oval teams match left-front and right-front spindle KPI specs carefully when replacing a bent spindle after wall contact.
Installing a replacement spindle with the wrong KPI degree spec throws off static camber, scrub radius and dynamic steering weight jacking.
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