Alignment & Scrub Radius
Scrub radius decides how the steering talks to you — and how brake torque pulls the wheel. Track it through offset changes, convert camber and toe between units, and set up with the conventions real alignment racks use.
Scrub radius
Estimated scrub (geometry)
9 mm
e − R·tan(σ)
Offset change effect
+15 mm
ET 45 → 30, 1:1 rule
New scrub radius
24 mm
Positive scrub: good feedback band for sporty road/track cars.
The geometric estimate is only as good as your e figure — measure it on the car or use a known baseline scrub radius instead. The offset rule is the part you can bank on: pushing the wheel 10 mm outboard adds ~10 mm of positive scrub. Camber changes, SAI changes and taller tyres all move it too.
Camber & toe converters
“18-inch” wheel ≈ 460 mm + 2× sidewall
total 0.176°
Toe conventions trip everyone up: string-alignment reads total, laser racks usually quote per side, and “1/16 inch” (1.6 mm) total toe ≈ 0.14° per side on a 650 mm tyre. A touch of toe-out front (0.05–0.15°/side) sharpens turn-in; toe-in rear keeps a RWD car calm. Camber figures the same way: 2° on a 650 mm tyre is ~23 mm of visible lean top-to-bottom.
Good to know
- Scrub radius: distance at the road between where the steering axis hits and the tyre centreline. Axis inside the patch = positive; outside = negative.
- The geometric estimate (e − R·tan σ) needs a measured hub offset — but the offset rule is exact enough to bank on: pushing a wheel 10 mm outboard adds ~10 mm of positive scrub.
- Sign matters: modern cars run slightly NEGATIVE scrub for stability and split-μ braking safety; big positive scrub adds steering feel but also torque steer and brake pull.
- Camber and toe are geometry, not opinion: run the numbers so “1/16 inch of toe” and “0.15°” mean the same thing across your string alignment and the shop’s laser rack.