What Is Scrub Radius?
When viewing a wheel from front view, the scrub radius is the distance between where the extrapolated line of kingpin axis touches the road and the tyre contact patch centre point. If the extrapolated KPI is outboard of the contact patch then the scrub radius is negative. If it is inboard then the scrub radius is positive.
A positive radius allows the wheel to roll over on lock making parking situations easier. A negative scrub radius gives a harder feel to the steering wheel as it requires more input to turn. Having a small scrub radius is beneficial as it allows the wheel to react less to braking inputs due to the smaller moment arm and increases steering stability under braking conditions.