You've learned camber, caster, and toe. But there are two more steering geometry parameters that determine whether a car tramlines under braking, whether it fights back through the wheel over bumps, and whether hitting a pothole yanks the wheel out of your hands: kingpin inclination (KPI) and scrub radius.

Kingpin inclination is the angle, viewed from the front of the car, between vertical and the steering axis — the line the wheel pivots around when you turn. On a modern car with MacPherson struts, the steering axis runs from the upper strut mount down through the lower ball joint. That line is almost never vertical; it leans inward at the top, typically 8°–15°.

Scrub radius is the horizontal distance, at the ground, between where the steering axis intersects the pavement and where the tire's centerline contacts the pavement. It can be positive (steering axis intersects inboard of tire center), negative (outboard), or zero.

Why do these matter? Three big reasons:

• Self-centering under load. KPI lifts the front of the car slightly when you turn the wheel (the wheel arcs downward, pushing the chassis up). Gravity pulls the chassis back down, which returns the steering to center. This works alongside caster to give you on-center feel.
• Reduced steering effort at rest. Combined with caster, KPI keeps the contact patch close to the steering axis, so you're not levering the entire tire footprint around a distant pivot when parking.
• Behavior under uneven braking. This is where scrub radius earns its keep. If one front brake grabs harder than the other (worn pads, ice patch, blown caliper), that wheel experiences a rearward force at the contact patch. With positive scrub radius, this force creates a torque that steers the car toward the failed side. With negative scrub radius, the same force steers away from the failed side — self-correcting the yaw before the driver reacts.

Modern front-wheel-drive cars almost universally use slightly negative scrub radius for exactly this reason. It's a passive safety feature: split-mu braking (one wheel on ice, one on asphalt) doesn't rip the wheel out of your hands.

Rule of thumb: Aftermarket wheels with wrong offset shift the tire outboard, moving contact-patch centerline away from the steering axis. A 20mm offset change on a car designed for -5mm scrub can push you to +15mm — reversing the failure mode. This is why "just get spacers" is a bad idea on modern cars with sophisticated brake-torque geometry.

The unloaded wheel also matters: hitting a pothole with positive scrub radius kicks the wheel back through the steering column. Negative scrub radius absorbs it quietly.