Kingpin Inclination (KPI / SAI) Explained
Kingpin inclination (KPI) is the inward lean of the steering axis when you look at the car from the front. It is also called steering axis inclination (SAI) — the two names mean the same angle. Old trucks had a physical kingpin; modern cars have a line through the upper and lower ball joints, or through the strut top mount and the lower ball joint. That line leans in at the top, typically by somewhere between about 7° and 15° depending on the design.
Why the axis leans at all
If the steering axis were vertical, it would meet the road well inboard of the tire, because the brake, hub and bearing all have to fit between the ball joints and the wheel centerline. Every bump and every brake application would then have a long lever to twist the steering with. Leaning the axis inward at the top aims it back toward the middle of the contact patch, shrinking that lever — the scrub radius.
What it does when you steer
- Lifts the car — because the axis is tilted, turning the wheel either way pushes the tire down relative to the body, raising the front of the car slightly. The weight of the car then tries to push the steering back to straight. This is a second source of self-centering, separate from caster, and it works even at walking pace.
- Adds positive camber with lock — both front wheels gain positive camber as they steer away from center. On the outside wheel that is the wrong direction for grip.
With 13° of KPI and 25° of steering, that is about +1.2° on each wheel. Caster works the other way on the outside wheel, so the two are designed together: a car with a lot of KPI needs more caster to end up with useful camber in a corner. At small steering angles the KPI effect is close to nothing, since it grows with the square of the angle.
Included angle
KPI is built into the knuckle and the mounting points; on most cars it is not an adjustment. Alignment technicians use it as a diagnostic through the included angle.
Because the knuckle is one rigid part, moving the top of a strut in or out changes KPI and camber by equal and opposite amounts, and the included angle stays the same. If the included angle on one side differs from the other, something rigid is bent — usually the knuckle or the strut. If the included angle matches but KPI and camber are both off, the mounting points have moved: a shifted subframe or a damaged strut tower.
What modifications change
- Camber plates and camber bolts — on a strut car, adding negative camber at the top mount increases KPI by the same amount. Camber bolts at the knuckle change camber without changing KPI, which changes the included angle on purpose.
- Longer lower arms — pushing the lower ball joint outward adds negative camber and increases KPI on a strut car, and moves the whole wheel outward at the same time.
- Wheel offset — does not change KPI. It changes where the tire sits relative to the axis, which is scrub radius.