Kingpin axis, part 1: KPI, scrub radius, and why your steering fights back
The wheel steers about a line through two joints — not through the middle of the tire. Where that line hits the ground decides steering weight, kickback, and what happens when one front tire finds ice.
Everything so far has treated the wheel as something that goes up and down. Now it turns, and that needs a new line: the steering axis.
It runs through two points — on a wishbone, the upper and lower ball joints; on a strut, the lower ball joint and the top mount. The wheel pivots about that line. It is almost never vertical, and it almost never passes through the middle of the tire, and both of those facts are doing work.
Two numbers, both measured at the ground
Extend the steering axis downward until it hits the road. It arrives at a point, and the contact patch is somewhere else. The gap between them is what matters:
- Scrub radius is that gap measured sideways. Positive means the axis hits the ground inboard of the tire center.
- Mechanical trail is the same gap measured fore and aft — that’s chapter 8.
Kingpin inclination (KPI) is just the angle of the axis from vertical in the front view. It matters mostly because it’s one of the things that decides where the axis lands.
Watch them move together on one layout. The strut layout sits at KPI 16.7° and scrub +1.7 mm — essentially zero scrub. Move the strut top mount inboard by 20 mm, and nothing else:
| KPI | Scrub radius | FVSA | |
|---|---|---|---|
| as designed | 16.7° | +1.7 mm | 3473 mm |
| strut top 20 mm inboard | 18.3° | −4.3 mm | 2867 mm |
Leaning the axis by 1.6° swung the scrub radius through zero and out the other side — from mildly positive to negative. That’s a 6 mm change in where the axis meets the road, from a 20 mm move at the top.
Note the third column. The strut top is also one of the two points defining the instant center, so the swing arm shortened by 600 mm at the same time. You cannot tune the steering axis on a strut without touching the camber curve.
The part that costs people money
In the real world, wheel offset changes your scrub radius too, and nothing about the suspension has to change.
Fit wheels with less offset — more dish, the classic “aggressive fitment” — and the contact patch moves outboard while the steering axis stays exactly where it was. Positive scrub radius grows by roughly the offset change: 20 mm less ET, about 20 mm more scrub.
That’s why a car can develop torque steer, tramlining and kickback after nothing more than a wheel change. The suspension is untouched. The lever the road gets to twist your steering with just got 20 mm longer.
One honesty note about the simulator: its Scrub Radius readout is computed from the steering-axis geometry alone, with the contact patch taken at the wheel’s own centerline. So changing wheel offset in Car Design will not move that number, even though it would on a real car. Use the app to understand how the axis sets scrub; do the offset arithmetic yourself.
What scrub radius actually does
The scrub radius is the lever between a force at the contact patch and your steering wheel.
- Large positive scrub: braking forces pull the wheels toward toe-out, road irregularities feed back as kickback, and drive torque on a FWD car produces torque steer. It also means the tire scrubs — rotating about a point beside itself — when you steer at a standstill, which is where the name comes from.
- Zero scrub: the axis lands in the middle of the contact patch. Minimal kickback, minimal torque steer, and the tire spins in place when parked, which chews tires and loads the steering.
- Negative scrub: the axis lands outboard of the tire center. This is the interesting one, and it’s why most modern front-drive cars use it. If one front brake fails, or one tire lands on ice, the braking force on the working side steers that wheel into the skid and the car pulls straight instead of spinning. It’s a passive safety feature, and it costs nothing.
The Mini’s near-zero scrub is exactly what you’d expect from a modern front-drive hatchback: minimal torque steer, calm steering.
Go look at it
Open the strut layout.
- In GEOMETRY METRICS, read KPI — Kingpin Incl. (about 16.7°) and Scrub Radius (about +1.7 mm — essentially zero).
- Drag the Strut Top Mount inboard by 20 mm. Watch KPI increase as the axis leans further, and watch Scrub Radius move with it.
- Do it precisely: 20 mm inboard should read about KPI 18.3° and Scrub Radius −4.3 mm. You’ve just moved the scrub radius through zero into negative territory.
- Check what it cost. FVSA — Swing Arm dropped from 3473 mm to about 2867 mm, so the camber curve you designed in chapter 3 is now a different curve.
- Use ⟲ Steer and watch Camber as the wheel turns. KPI makes the steered wheel lean — and it makes the car rise slightly as you steer, which is where self-centering partly comes from.
What it costs you
KPI isn’t free either. Because the axis is tilted, steering the wheel lifts the car slightly — which is good (it gives self-centering) until it isn’t (it makes the steering heavy at parking speeds, and it’s why high-KPI cars feel like they’re fighting you in a parking lot).
KPI also produces steer-induced camber in the wrong direction: it leans the outside wheel positive as you turn, working against the negative camber you carefully designed in chapter 3. Caster does the opposite, which is the subject of the next chapter and one reason the two are always discussed together.
The deeper trap: the steering axis is defined by the same ball joints that define your camber curve. Move the upper ball joint inboard to reduce scrub radius and you’ve shortened the swing arm and changed the camber gain. There are still only one set of points.
Rules of thumb
- KPI: 8–15° on most cars; the strut layout above sits at 16.7°. Higher gives more self-centering and more parking effort.
- Scrub radius, front-drive road car: −5 to −20 mm (slightly negative) for brake-failure and split-friction stability.
- Scrub radius, rear-drive road car: 0 to +20 mm is common and unproblematic.
- Scrub radius, race car: small in magnitude, either sign, to keep kickback out of the driver’s hands.
- Beyond about ±50 mm you are building in kickback and torque steer whether you meant to or not, whatever the rest of the geometry says.
- Rally-raid and desert: small scrub in magnitude matters more here than anywhere, because the impacts are enormous. A big scrub radius on a car landing off a dune puts that force straight into the steering rack and the driver’s thumbs. It’s also why these cars run hydraulic assist and steering dampers as a matter of course, not as a luxury.
Try this
- Get the scrub radius to exactly zero. What did you have to move, and what did it do to the camber curve?
- Make the scrub radius clearly negative. Which direction did the steering axis have to lean, and is that packageable around a real brake disc?
- Compare the KPI at the front and rear of the AMG. Why does a non-steered axle have a kingpin angle at all, and what does it mean there?
Next: the other half of the steering axis — caster, trail, and the reason a car with lots of caster gains camber exactly where it needs it.
Preset hardpoints are illustrative sketches, not measured factory specs. The AMG preset’s steering-axis figures in particular sit well outside normal road-car practice.