Kingpin Inclination

Kingpin inclination and steering axis inclination are often used interchangeably, since both describe the inward tilt of the steering axis in the front view. Some legacy literature reserves kingpin inclination for systems with a physical kingpin, and uses steering axis inclination for ball joint or strut layouts. Included angle is different. It equals camber plus kingpin inclination on the same wheel side, measured during an alignment sweep.
You use included angle to diagnose bent knuckles or mislocated strut towers, since camber can be adjustable while kingpin inclination is usually fixed by hard geometry. Stable included angle with changing camber hints at correct knuckle geometry.

Kingpin inclination moves the tire contact patch closer to the steering axis, which reduces scrub radius. A small scrub radius helps the car track straight, resists brake pull from split mu conditions, and limits steering kickback over potholes. The inward tilt also creates a lever arm that works with caster to generate self-aligning torque, so the wheel returns to center smoothly.
If the angle is too small, scrub radius grows, steering becomes nervous, and you may feel pull during hard braking. If the angle is too large, you can get excessive jacking and camber change in steer, which makes the vehicle feel darty on rough surfaces.

Knuckle design defines the line between the upper and lower joints, and that line sets kingpin inclination. In a MacPherson strut, the upper point is the strut mount or bearing plate, and the lower point is the ball joint. In a double wishbone, the points are the upper and lower ball joints.
Manufacturers control the angle by fixing tower locations, arm pivot coordinates, and knuckle machining datums. Robust welding fixtures, precise bore angles, and consistent bushing crush help hold tolerance through paint and cure. Process controls such as coordinate measurement of knuckles and towers confirm the angle before final alignment on the production line.

Shops measure kingpin inclination with an alignment rack using a caster and SAI sweep. The system reads camber and steering axis inclination, then derives included angle. Because kingpin inclination is not directly adjustable on most vehicles, out-of-spec readings usually point to bent knuckles, shifted strut towers, worn or collapsed bushings, or incorrect subframe position.
A loose wheel bearing or damaged ball joint can also corrupt the measurement. Before calling parts, verify tire pressures, ride height, and that the rack is level. If included angle is off on one side while camber is correct, suspect a deformed knuckle or mislocated mounting holes.

You tune kingpin inclination with wheel offset and hub face location to target scrub radius. A near-zero scrub radius improves stability under split traction braking and reduces torque steer on front-drive cars. Caster mixes with kingpin inclination to create mechanical trail and camber gain in steer, which shapes turn-in and mid-corner grip. Static camber must be set so that combined camber in steer stays within the tire’s optimal range.
If you chase big wheels with aggressive offset, scrub radius can grow, which forces you to revisit kingpin inclination or knuckle offset. Balanced values reduce steering effort, protect tires, and keep electronic assists predictable.