Scrub Radius Calculator

Use the calculator to estimate the scrub radius of a vehicle’s front suspension from steering-axis geometry (SAI/camber), tire diameter, and wheel offset. If you already know the lateral position of the steering-axis intersection with the ground and the lateral position of the tire contact patch center (both at the road surface), you can use the “Direct 2-of-3” tab. Scrub radius is the lateral distance at the road surface between the center of the tire contact patch and the point where the steering axis intersects the ground (or its projection).

Geometry (SAI/Camber)

Change From Stock

Direct 2-of-3

Related Calculators

Scrub Radius Formula

The following formula (definition) is used to calculate the scrub radius:

SR = x_{CP} - x_{SA}

Variables (all measured at the road surface, using the same reference and sign convention):

SR is the scrub radius (mm or in)
x_CP is the lateral position of the center of the tire contact patch (mm or in)
x_SA is the lateral position of the steering-axis (kingpin) intersection with the ground (mm or in)

In other words, scrub radius is the lateral distance from the steering-axis ground intersection to the tire contact patch center. This calculator uses a common sign convention where SR is positive if the contact patch center is outboard of the steering-axis ground intersection, and negative if it is inboard.

What is a Scrub Radius?

Scrub radius is an important aspect of a vehicle’s steering geometry and affects steering feel, stability under braking, and sensitivity to road disturbances. A positive scrub radius occurs (using the convention on this page) when the steering-axis intersection with the ground lies inboard of the center of the tire contact patch. A negative scrub radius occurs when the steering-axis intersection lies outboard of the contact patch center. Zero scrub radius is when the two points coincide.

How to Calculate Scrub Radius?

The following steps outline how to calculate the Scrub Radius.

Determine the lateral position of the steering-axis intersection with the ground (x_SA) using measurements or suspension geometry.
Determine the lateral position of the tire contact patch center (x_CP) at the ground (for example, it can be approximated from wheel offset and camber in a simplified front-view model).
Use the formula from above: SR = x_CP − x_SA.
Finally, calculate the Scrub Radius (SR) in millimeters (or inches).
After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge (inboard = −, outboard = +):

steering-axis ground intersection position (x_SA) = −100 mm

tire contact patch center position (x_CP) = −40 mm

Then SR = (−40) − (−100) = 60 mm (positive).

Follow-Up Calculations

If you move from a +25 mm offset to a +45 mm offset and your scrub radius drops from 38 mm to 18 mm, the wheel centerline is shifting closer to the steering axis — the same geometry you can confirm when you see where the wheel center lands. Getting that relationship right usually reduces kickback and makes the car less sensitive to braking on split-friction surfaces.

A scrub radius of 50 mm turns a 1,500 N side load at the contact patch into about 75 N·m of twisting moment at the hub, which is why many builders estimate hub and bearing stress after changing wheel offset or adding spacers. Even if the steering feel seems fine, that extra leverage can speed up wear on bearings, ball joints, and tie-rod ends.

If you use spacers to bring scrub radius down from 40 mm to 20 mm on the front axle, the next setup check in a race car is often whether the chassis still has the right left-to-right balance, so it helps to check your corner balance. Scrub radius changes how the tire loads during steering, while cross weight tells you whether each corner is carrying its share of the static load before you head to the scales.