Every time you turn your steering wheel, your front tires pivot. However, they don't pivot completely in place on an exact center axis; instead, they swing in a tiny arc. The distance between the exact center of your tire's contact patch and the invisible steering pivot axis (the Steering Axis Inclination, or SAI) is called the Scrub Radius.
Positive vs. Negative Scrub Radius
Automotive engineers spend hundreds of hours calibrating the scrub radius to provide optimal steering feel and stability.
- Negative Scrub Radius: The contact patch sits inside the steering axis. This is common in modern front-wheel-drive cars because it naturally stabilizes the steering wheel if one front brake fails or if a tire blows out.
- Positive Scrub Radius: The contact patch sits outside the steering axis. Often found on older rear-wheel-drive cars, this provides heavier steering feel and more feedback, but makes the car "tramline" (aggressively pull left or right following ruts in the road).
The Danger of Wheel Spacers and Low Offsets
When car enthusiasts install wheel spacers or bolt on wide aftermarket wheels with a low offset (pushing the wheels out towards the fenders), they unknowingly increase the positive scrub radius significantly.
Pushing the wheel outward acts like extending a wrench—it gives the road more leverage over your steering rack. If you hit a bump mid-corner with a massive positive scrub radius, the steering wheel can violently snap out of your hands. Furthermore, it accelerates wear on wheel bearings and suspension ball joints exponentially. Always keep offset changes within 5mm-10mm of factory specifications unless accompanied by deep suspension modifications to correct the SAI.