Scrub Radius Calculator
Compute your exact scrub radius from SAI and spindle offset geometry, or instantly calculate how much a wheel change, spacer, or wider tire shifts your scrub radius — with drivetrain safety rating.
Quick Example
A Golf Mk8 GTI (ET51, FWD, 12° SAI) has a factory scrub radius of −12 mm. Fitting ET35 aftermarket wheels shifts the scrub radius to approximately +4 mm — a +16 mm swing that noticeably increases torque steer and steering kickback over bumps. Mode 1 (Quick Delta) confirms the change instantly without needing SAI data.
Scrub Radius Change — Wheel or Tire Swap
Enter your factory and new wheel offset to see exactly how scrub radius shifts. No SAI data needed — just your ET values.
Pre-fills factory ET and drivetrain. You can still edit values below.
Stamped on your OEM wheel. Typically ET35–ET55 on passenger cars.
Stamped on the aftermarket wheel (e.g. ET35, ET25).
A spacer reduces effective ET by the same amount. 15 mm spacer on ET45 = effective ET30. Stacks with the new wheel ET above.
Optional — Tire Diameter Change
Leave blank if only changing wheels. If both diameters are filled, an adjustment is calculated using a typical 12° SAI assumption.
Affects safety interpretation of the result.
How to Use This Calculator
- 1Choose Mode 1 (Quick Delta) if you only have your OEM and new wheel ET values — no SAI data required.
- 2Choose Mode 2 (Full Geometry) if you have your vehicle's SAI/KPI angle and spindle offset from manufacturer data or an alignment printout.
- 3In Mode 1: enter OEM ET, new wheel ET, and optionally old/new tire diameters if upsizing.
- 4In Mode 2: enter SAI (degrees), spindle offset (mm), wheel ET (mm), and overall tire diameter (mm).
- 5Read the result: the calculator shows your scrub radius value, its sign (positive/negative/zero), and a drivetrain safety rating.
- 6Cross-reference the OEM table below to see how far your result deviates from factory geometry.
Formula Reference
Mode 1 — Quick Delta (ET-based)
ΔSR = −ΔET + (ΔTire Diameter / 2) × tan(SAI) Quick shortcut (no SAI data): ΔSR ≈ −ΔET (1 mm less offset = +1 mm scrub radius) Tire width correction (half of width increase): ΔSR(tire) ≈ (new_width − old_width) / 2
Mode 2 — Full Geometry (SAI-based)
SR = (Spindle Offset − ET) − (Tire Radius × tan(SAI)) Where: Spindle Offset = lateral distance from hub centerline to steering axis pivot ET = wheel offset in mm (stamped on barrel) Tire Radius = (overall diameter × 25.4) / 2 [mm] SAI = Steering Axis Inclination in degrees Sign convention (SAE/ISO standard): Positive SR → contact patch outboard of steering axis Negative SR → contact patch inboard of steering axis
ΔSR = −ΔET
Every 1 mm reduction in wheel offset = +1 mm scrub radius. Every 1 mm spacer = +1 mm scrub radius.
How to Interpret Your Result
Very light, vague steering. Poor self-centering. Likely caused by very high ET aftermarket wheels. Can produce bump steer.
Target range for most front-wheel-drive and AWD crossovers. Minimises torque steer and provides stable braking.
Acceptable for all drivetrains. Light steering with slightly reduced self-centering. Common on performance FWD.
Ideal for RWD. Provides good road feel and feedback. Acceptable for AWD with symmetric torque split.
Fine for RWD, marginal for AWD, problematic for FWD. Torque steer becomes noticeable. Verify with alignment.
Problematic for all drivetrains except purpose-built trucks. Strong brake pull, steering kickback, accelerated bearing wear.
Scrub Radius Safety Limits by Drivetrain
The same scrub radius value carries very different risk depending on whether the front wheels only steer (RWD) or also deliver power (FWD/AWD).
Front wheels steer AND drive. Positive scrub amplifies torque steer. Stay negative or near-zero.
Max positive: +15 mm
Every 10 mm spacer adds +10 mm scrub — already risky past 15 mm on performance FWD.
Front wheels only steer. Small positive scrub gives feel and feedback without the torque-steer penalty.
Max positive: +60 mm
Large spacers push beyond 60 mm — brake pull and heavy steering become noticeable.
Similar to FWD when front-axle dominant. Most crossovers (CX-5, Tucson, Q5) use slight negative for braking stability.
Max positive: +10 mm
Worst case: spacers push positive, combining with AWD torque distribution for corner fight.
Balanced torque split reduces front-axle torque steer. Can tolerate slightly more positive than biased AWD.
Max positive: +20 mm
Subaru symmetric AWD is especially sensitive — stay within ±10 mm of OEM.
OEM Scrub Radius Reference
Estimated factory scrub radius and safe aftermarket offset ranges for popular platforms. Use the calculator above to verify your specific setup. "Safe range" = stays within ±15–20 mm of OEM scrub radius on standard suspension.
| Platform | OEM ET | Est. Scrub | Safe ET Range | Drivetrain | Spacer Risk |
|---|---|---|---|---|---|
| VW Golf Mk8 (FWD) | ET51 | ≈ −12 mm | ET41–ET61 | FWD | Low |
| Honda Civic FE (FWD) | ET50 | ≈ −8 mm | ET40–ET60 | FWD | Low |
| Toyota Camry XV70 (FWD) | ET45 | ≈ −10 mm | ET35–ET55 | FWD | Medium |
| Mazda CX-5 KF (FWD/AWD) | ET50 | ≈ −9 mm | ET40–ET60 | AWD | Medium |
| Hyundai Tucson NX4 (AWD) | ET51 | ≈ −11 mm | ET41–ET61 | AWD | Medium |
| Subaru WRX VB (AWD) | ET55 | ≈ −15 mm | ET45–ET65 | AWD | High |
| BMW 3 Series G20 (RWD) | ET30 | ≈ +18 mm | ET20–ET40 | RWD | Low |
| Audi Q5 FY (AWD) | ET38 | ≈ −8 mm | ET28–ET48 | AWD | Medium |
| Ford F-150 14th Gen (4WD) | ET12 | ≈ +40 mm | ET2–ET22 | 4WD | Low |
Estimates based on published OEM suspension geometry and SAI data. Real-world values vary by trim, model year, and suspension modifications. Verify with an alignment rack.
What is Scrub Radius?
Scrub radius is the lateral distance at road level between the center of the tire contact patch and the point where the steering axis (kingpin/SAI line) intersects the ground. When you turn the steering wheel, the tire pivots around the steering axis ground intersection — scrub radius is how far offset that pivot point is from the tire's own contact center.
The sign matters: positive scrub radius means the contact patch is outboard of the steering axis (pivot is inside the contact area) — common on older RWD vehicles and aftermarket fitments with reduced ET. Negative scrub radius means the pivot is outside — the designed state for most modern FWD and AWD platforms to minimise torque steer. Zero sounds ideal but produces vague, disconnected steering in practice.
Three variables directly control scrub radius: SAI/KPI angle (fixed by suspension hardpoints), spindle offset (fixed by hub design), and wheel offset ET (changeable by the owner). This calculator exposes that third variable and shows the exact consequence.
EPS Masking — Modern Cars Hide the Problem
Electric power steering (EPS) systems can partially filter the kickback and self-steer caused by incorrect scrub radius. Symptoms may not appear in normal driving — but will surface under hard braking, aggressive FWD acceleration, or sharp road impacts where the mechanical force exceeds EPS compensation. Never rely on "it feels fine" to validate geometry on a modern EPS vehicle. Always check the numbers.
Frequently Asked Questions
Scrub radius values and safe offset ranges are estimates based on published suspension geometry data, engineering literature, and community-verified measurements. Real-world values depend on specific model year, trim, suspension modifications (lowering, coilovers), and component wear. The safe ranges are guidelines, not guarantees. Always consult a qualified alignment technician and verify scrub radius on an alignment rack before driving on any significant wheel or suspension modification. TireCalculatorHub is not liable for any damages arising from use of this calculator.