Scrub Radius Explained
What positive, negative and zero scrub radius mean, how wheel offset and spacers change it, and what manufacturers say about changing wheels.
Quick answer
Scrub radius is the distance at the road between the center of the tire contact patch and the point where the steering axis meets the ground. It is positive when the axis meets the ground inboard of the contact patch center, negative when outboard, and zero when they coincide. With the steering axis unchanged, lowering wheel offset by 10 mm adds 10 mm of scrub radius.
What scrub radius is
When the front wheels steer they pivot about the steering axis. Scrub radius (also called steering offset) is the horizontal distance at road level between that axis and the center of the contact patch.
Positive Scrub Radius
The steering axis (the line through the upper and lower steering pivot points, extended to the road) meets the ground inboard of the center of the tire contact patch.
Negative Scrub Radius
The steering axis meets the ground outboard of the center of the tire contact patch.
Zero Scrub Radius
The steering axis meets the ground at the center of the tire contact patch.
How scrub radius affects steering feel, braking and torque steer is covered in suspension engineering texts. We have no manufacturer or official source we can cite for those effects, so this guide does not state them.
How wheel changes affect scrub radius (geometry)
These are geometric relationships, assuming the steering axis does not change. Wheel offset (ET) is the distance from the wheel mounting face to the wheel centerline; a lower ET places the wheel further out. If a wheel is listed by backspacing, the wheel offset and backspacing calculator converts it to ET.
| Change | Effect at the contact patch center | Why |
|---|---|---|
| Lower wheel offset (for example ET45 to ET35) | +10 mm per 10 mm of offset reduction | The wheel centerline, and with it the contact patch center, moves outward relative to the steering axis, so scrub radius becomes more positive (or less negative). |
| Higher wheel offset (for example ET35 to ET45) | −10 mm per 10 mm of offset increase | The wheel centerline moves inward, so scrub radius becomes less positive (or more negative). |
| Wheel spacer (for example 15 mm) | +15 mm for a 15 mm spacer | A spacer moves the wheel outward by its thickness, which has the same geometric effect as reducing the offset by that amount. |
| Wider tire on the same wheel (for example 225 mm to 245 mm) | 0 mm at the contact patch center; +10 mm at the outer edge | A tire is centered on the wheel, so a wider tire on the same wheel does not move the center of the contact patch. Its outer edge moves outward by half the width increase (20 mm ÷ 2), which matters for fender and suspension clearance. |
All figures are calculated from the geometry. Enter your own wheel and tire in the scrub radius calculator for your own numbers.
The scrub radius formula
1. Find where the steering axis meets the ground (x_SA)
The steering axis is the line through the upper and lower pivot points of the steering knuckle (the upper strut mount on a MacPherson strut, or the ball joints on a wishbone or multi-link front suspension). Extend it down to the road. The lateral position where it meets the road is x_SA.
2. Find the center of the contact patch (x_CP)
The center of the contact patch is the middle of the tire where it touches the ground, which is the wheel centerline (with camber it shifts slightly).
3. Apply the formula
Scrub radius = x_CP − x_SA, with positions measured outward from the vehicle centerline. A positive result means the contact patch center is outboard of the steering axis (positive scrub radius); a negative result means it is inboard (negative scrub radius).
4. Account for a wheel offset change
If nothing else changes, the change in scrub radius equals the reduction in wheel offset: ET35 replacing ET45 moves the contact patch 10 mm outward and adds 10 mm.
5. Do not add a tire width term
A wider tire fitted on the same wheel is centered on the wheel, so it does not move the contact patch center. Only the wheel offset, spacers and the steering axis position change the scrub radius.
What manufacturers and NHTSA say about changing wheels
Ford (2024 F-150 and Mustang owner's manuals)
Wheels and tires must be the same size, load index and speed rating as those originally fitted. Use of any other tire or wheel can affect the safety and performance of your vehicle.
General Motors (2025 Chevrolet Silverado 1500 owner's manual)
The wrong wheel can cause problems with bearing life, brake cooling, speedometer or odometer calibration, headlamp aim, bumper height, vehicle ground clearance, and tire or tire chain clearance to the body and chassis. When replacing wheels, use a new GM original equipment wheel.
Michelin
Replace tires with the same size designation, or approved options, recommended by the vehicle manufacturer, with load and speed ratings that meet or exceed the specified values.
NHTSA TireWise
A wheel alignment maximizes the life of your tires and prevents the car from veering. Stop using tires that show signs of irregular wear. Rotation, balance and alignment can help tires last longer.
The vehicle manufacturer or a dealer can tell you the approved wheel sizes and offsets for your vehicle. Alignment specifications and suspension geometry data are in the manufacturer's service information. For the alignment angles themselves, the wheel alignment guide explains camber, caster and toe, and the step-by-step wheel and tire fitment guide covers the rest of a wheel change.