The kingpin is the primary structural pivot point that allows a steering knuckle to rotate relative to the axle. On older solid-axle vehicles and heavy trucks, it is a physical steel pin connecting the knuckle to the axle beam. On vehicles with independent front suspension, the kingpin is an imaginary axis drawn between the upper and lower ball joints.
The front end is wandering all over the road, so check the kingpin bushings for play.
Heavy truck mechanics still regularly replace physical kingpins using a heavy press, while light-duty mechanics only measure kingpin inclination (KPI) via the ball joints during an alignment.
Related terms
Ball joint
A pivoting spherical bearing that connects a vehicle's control arms to its steering knuckles.
KPI
Kingpin inclination is the inward tilt of the steering axis when viewed from the front of the vehicle.
Steering column
The structural shaft assembly that connects the steering wheel to the steering rack or gearbox.