Scrub radius is the lateral distance on the ground between the tire's center contact patch and the imaginary line drawn through the upper and lower ball joints (steering axis).
Look at the front tire from straight ahead: draw a line through the upper and lower ball joints down to the pavement or dirt, and compare where it hits against the middle of the tire tread. When the tire centerline sits outside the steering axis line, scrub radius is positive, which is standard on oval cars fitted with wide wheel offsets or wheel spacers. Adding a half-inch wheel spacer on the right-front widens track width and increases positive scrub radius, giving stronger steering feedback and jacking effect, while excessive scrub radius increases steering kickback over ruts and drags speed on braking.
Swapping wheel backspacing or adding hub spacers changes scrub radius alongside track width, altering steering effort, brake pull and front tire heat.
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Trackside setup apps vs chassis simulation (Performance Trends & SusProg3D)