Scrub radius is the distance on the road surface between the center of the tire contact patch and the point where the steering axis intersects the road. It connects alignment geometry with wheel offset and tire fitment.
Scrub radius is measured at the road surface.
SAI, wheel offset, tire radius and ride height all influence it.
Changing wheels can change scrub radius without changing normal alignment readings.
Large changes can affect steering effort, kickback and split-friction braking.
Positive, negative and zero scrub radius
If the steering-axis intersection and tire-center point meet at the same location, scrub radius is zero. When they are separated, the direction of separation determines whether the design is described as positive or negative. Sign conventions may differ, so the geometry diagram is more useful than the label alone.
As the wheel steers, the tire contact patch moves around the steering-axis intersection. The offset between these points creates a lever arm through which braking and road forces act on the steering.
What determines scrub radius
| Influence | Effect on geometry |
|---|---|
| Steering Axis Inclination | Moves the axis intersection inward or outward at the road. |
| Wheel offset | Moves the tire centerline relative to the hub. |
| Tire diameter | Changes where the axis reaches the road surface. |
| Ride height | Changes suspension position and sometimes SAI/camber relationship. |
| Spacers or adapters | Move the complete wheel outward. |
Effect on vehicle behavior
Scrub radius influences steering effort at low speed, road-shock feedback, torque steer and the steering response to unequal braking forces. A design may use a small or negative scrub radius to reduce steering disturbance if one front wheel has less grip during braking. A large positive lever arm can increase kickback and sensitivity to wheel imbalance or road irregularities.
These effects depend on the complete suspension and steering system. Scrub radius is not an isolated “good or bad” number.
Aftermarket wheels and spacers
A wheel with lower offset usually moves the tire outward and changes scrub radius. The normal alignment machine may still show toe, camber and caster inside specification because those angles do not directly measure the contact-patch offset. The vehicle can nevertheless feel different, load wheel bearings differently and create tire or body clearance issues.
Diagnostic use in the workshop
Suspect a scrub-radius or fitment change when steering effort, brake sensitivity or kickback appears after wheels, spacers, hubs or suspension height are changed while the normal alignment report remains acceptable. Compare wheel part numbers, offset markings, tire size and track width with the approved configuration.
Relationship to alignment service
Camber and SAI readings help describe the steering-axis and wheel relationship, but scrub radius also needs wheel and tire dimensions. An alignment can optimize adjustable angles, yet it cannot restore original scrub radius if the wheel offset or tire centerline has been altered.
Frequently asked questions
Can an alignment machine measure scrub radius?
Some systems can calculate or estimate it when the necessary geometry and tire dimensions are available. Standard toe, camber and caster measurements alone are not enough.
Does a wider tire always increase scrub radius?
Not by itself. The contact-patch center depends on wheel offset, rim width, tire shape and mounting position.
Can incorrect scrub radius cause tire wear?
It can change steering and load behavior, but normal tire-wear diagnosis should still include toe, camber, pressure, rotation and suspension condition.
Technical content reviewed for TreadPlus Learn v1.0 · Updated July 16, 2026
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