What is scrub radius?

Scrub radius is the lateral distance, measured at the road surface, between the point where the steering axis meets the ground and the centre of the tire's contact patch. It is measured in millimetres. A positive scrub radius means the steering axis meets the ground inboard of the contact patch; negative means it meets outboard.

What is a good scrub radius for a road car?

Most modern front-wheel-drive cars are factory-engineered with a negative scrub radius between −5mm and −20mm for braking stability and torque steer control. Rear-wheel-drive performance cars typically run 0mm to +15mm for stronger steering feedback. Values beyond ±30mm for street use are generally considered outside the safe range.

Is negative or positive scrub radius better?

Neither is universally better — it depends on the vehicle's drivetrain and purpose. Negative scrub radius improves braking stability on split-grip surfaces and reduces torque steer on FWD cars — which is why most modern passenger vehicles use it. Positive scrub radius gives more direct steering feel and road texture feedback, which is why RWD sports cars often use mild positive values. Very large values in either direction are problematic.

What scrub radius do FWD cars use?

The vast majority of front-wheel-drive passenger cars are engineered with a mildly negative scrub radius, typically between −5mm and −20mm. This geometry is specifically chosen to improve braking stability on uneven surfaces and to suppress torque steer — a front-wheel-drive-specific problem where driving force through the front wheels pulls the steering to one side under acceleration.

How does wheel offset (ET) change scrub radius?

Every 1mm reduction in wheel offset (going to a lower ET number) moves the contact patch 1mm further outboard relative to a fixed steering axis — making scrub radius approximately 1mm more positive. Going from ET49 to ET35 is a 14mm offset reduction, which shifts scrub radius by approximately +14mm. If your car was designed with −12mm scrub radius at ET49, those ET35 wheels would put it at approximately +2mm — crossing zero into positive territory.

Do wheel spacers change scrub radius?

Yes. Wheel spacers are functionally identical to running a lower-offset wheel. A 20mm spacer pushes the entire wheel-and-tire assembly 20mm further outboard, moving the contact patch away from the steering axis and making scrub radius approximately 20mm more positive. On FWD cars engineered with mild negative scrub radius, even a 10–15mm spacer can push the geometry toward zero or beyond — changing steering feel noticeably.

Does tire size affect scrub radius?

Yes, but the effect is smaller than an equivalent offset change. A taller tire has a larger radius, which increases the horizontal distance the steering axis travels from hub height to the road surface (the SA ground projection). A larger SA projection makes scrub radius more negative. In practice, a 20mm increase in overall tire diameter shifts scrub radius by roughly −2mm to −4mm depending on your SAI angle — noticeable but secondary to the ET change.

Can I measure scrub radius at home?

Not accurately. Measuring actual scrub radius requires knowing the precise projected position of the steering axis at the road surface — which requires either manufacturer engineering data or specialised alignment equipment that measures SAI. What you can do at home is estimate the scrub radius shift from a wheel or tire change using the offset-delta mode of this calculator.

Scrub Radius Calculator

Find out how a wheel or tire swap shifts your scrub radius — or calculate the exact value from full suspension geometry data. No engineering degree required.

Quick Answer

Most modern FWD cars run −5mm to −20mm negative scrub radius — this self-correcting geometry keeps the car tracking straight under hard braking on split-grip surfaces. RWD sports cars typically use 0mm to +15mm for better steering feel. Every 1mm reduction in wheel offset ET shifts scrub radius approximately +1mm more positive — fitting ET35 wheels where ET49 was stock moves scrub radius by approximately +14mm.

Scenario	Result
Typical FWD passenger car	−5mm to −15mm — improves braking stability
Typical RWD sports car	+5mm to +20mm — improves steering feel
Acceptable positive scrub radius	0mm to +30mm — beyond +50mm causes heavy steering
Zero scrub radius	Steering axis intersects ground at tire contact centre — neutral
ET49 → ET35 wheel swap (+14mm offset reduction)	Scrub radius shifts ≈ +14mm more positive

Negative scrub radius (FWD norm): braking force on each front wheel creates a self-correcting moment — safer under split-μ braking. Beyond ±50mm in either direction is outside safe street geometry.

Drivetrain

Units

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.

Load OEM Preset — optional

Fills factory ET and sets the drivetrain selector above. Change either field to override.

Factory Wheel ET (mm)

Stamped on your OEM wheel. Typically ET35–ET55 on passenger cars.

New Wheel ET (mm)

Stamped on the aftermarket wheel (e.g. ET35, ET25).

Spacer Thickness (mm) — optional

A spacer pushes the wheel outward, reducing effective ET. A 10 mm spacer on an ET35 wheel behaves like ET25 — enter it here and the effective offset is calculated automatically.

Optional — Tire Diameter Change

Factory Tire Overall Diameter (mm)

New Tire Overall Diameter (mm)

Leave blank if only changing wheels. If both diameters are filled, an adjustment is calculated using a typical 12° SAI assumption.

AI Insight

Enter your values and calculate first to enable AI insights.

Scrub Radius Reference — Geometry vs. Driving Effect

Setup	Scrub Radius	Steering Effect	Braking Behavior
Modern FWD (factory)	−5mm to −20mm	Stable, slight understeer feel	Stable on split-grip — self-correcting
Modern RWD/AWD sport (factory)	0mm to +15mm	Communicative, more road feel	Slight pull toward higher-traction side
Zero scrub radius	0mm	Light, neutral	Neutral
Large positive (enthusiast fitment)	+20mm to +50mm	Heavy, strong kickback over bumps	Notable pull under hard braking
Excessive (either direction)	> ±50mm	Very heavy, tiring	Unstable — safety concern
After 20mm spacer (typical result)	+20mm from factory	Noticeably heavier	Some pull on hard braking

Scrub Radius Formulas

Mode 1 — Full Geometry Calculation Tire Radius (mm) = Overall Tire Diameter ÷ 2 Axle Dist (mm) = Spindle Hub Pivot Offset − Wheel Offset (ET) SA Ground Proj (mm) = Tire Radius × tan(SAI in degrees) Scrub Radius (mm) = Axle Dist − SA Ground Projection Sign: positive = contact patch outboard of steering axis negative = contact patch inboard of steering axis Mode 2 — Offset/Tire Delta (no SAI required) ET Delta (mm) = Factory ET − New ET Tire Radius Δ (mm) = (New Tire Diameter − Factory Tire Diameter) ÷ 2 Tire Adjustment (mm) = Tire Radius Δ × tan(12°) [typical SAI assumed] SR Delta (mm) = ET Delta − Tire Adjustment Key rule: every 1mm reduction in ET → scrub radius shifts ≈ +1mm

SAI / KPI — Steering Axis Inclination / Kingpin Inclination. The inward tilt of the steering axis from vertical when viewed from the front. Typically 8°–15° on passenger cars.

Spindle Hub Pivot Offset — Lateral distance from wheel/hub centerline to steering axis pivot at hub height. Requires manufacturer data or alignment equipment to measure.

ET (Einpresstiefe) — Wheel offset in mm. Lower ET = wheel sits further out = contact patch moves outboard = SR shifts more positive.

Worked Example — Fitting Aftermarket Wheels on a 2020 VW Golf

Stock setup: 225/45R17 on ET49 wheels · estimated factory scrub radius: −12mm

Stock Setup

Wheel offset: ET49
Tire: 225/45R17 (~650mm overall diameter)
Estimated scrub radius: −12mm
Steering: stable, brakes track straight on uneven grip

After Swap — ET35 Wheels

New wheel offset: ET35
ET delta: +14mm (49 − 35)
New estimated scrub radius: +2mm (−12 + 14)
Effect: crossed zero — now mildly positive. More steering feel, small brake pull becomes possible on uneven grip.

The −12mm factory value is an illustrative estimate. Actual scrub radius depends on precise knuckle and strut geometry. Use Mode 1 with manufacturer SAI data for an exact figure.

How to Use This Calculator

1Choose your mode: Use Mode 2 (Wheel/Tire Swap) if you're fitting aftermarket wheels or spacers — just enter factory ET and new ET. Use Mode 1 (Full Geometry) if you have your vehicle's SAI angle and spindle offset from manufacturer data.
2Enter factory and new ET (Mode 2): Enter your OEM wheel's ET (stamped on the wheel barrel or found in spec sheets) and the new wheel's ET. Leave the optional tire diameter fields blank if only the wheel is changing.
3Add tire diameter if changing (Mode 2): If you're also upsizing or downsizing tires, enter both the factory and new overall tire diameters in mm. This adds a small adjustment to the result based on a typical 12° SAI assumption.
4Read the scrub radius delta: The result shows how many millimetres scrub radius shifts and in which direction — along with a plain-English explanation of what that shift means for steering feel and braking behavior.
5Compare to factory spec: Use the reference ranges (FWD: −5 to −20mm; RWD: 0 to +15mm) to judge whether your proposed fitment stays within safe factory geometry. If in doubt, consult a suspension specialist.

Where to Find Your Input Values

Wheel Offset (ET)

Stamped on the inside of the wheel barrel. Also shown on the manufacturer's spec sheet (e.g. ET35, ET45, ET49).

SAI / KPI Angle

Listed in your vehicle manufacturer's alignment specifications. Displayed by modern 4-wheel alignment machines as a diagnostic reading. Typically 8°–15°.

Spindle Hub Pivot Offset

Manufacturer engineering data or suspension geometry software. Not commonly published in owner manuals — typically requires factory service data.

Overall Tire Diameter (mm)

Calculate from tire size: (rim dia × 25.4) + 2 × (section width mm × aspect ratio ÷ 100). Or use a tire size calculator.

Frequently Asked Questions

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