How Tire Diameter Affects Scrub Radius, Starting With the Sidewall Numbers

Short answer: A taller tire usually reduces positive scrub radius, and a shorter tire usually increases it. Changing wheel offset has a much larger effect than changing tire diameter.

The reason is steering axis inclination. The steering axis tilts inward at the top, so the line runs outward as it goes down. A larger tire raises the hub and puts the ground farther away. The steering axis then travels farther outward before it meets the ground, which brings its ground point closer to the tire's contact patch center.

The change in scrub radius is roughly the change in loaded tire radius multiplied by the tangent of the steering axis inclination angle. For typical passenger-car angles, that comes to a few millimetres.

What Is Scrub Radius?

Scrub radius is the side-to-side distance at ground level between two points, viewed from the front of the vehicle:

• the steering axis ground intersection, which is where an imaginary line through the upper and lower steering pivots meets the road
• the contact patch center of the tire

It is usually measured in millimetres. The sign depends on which side the steering axis lands:

• Positive scrub radius: the steering axis meets the ground inboard of the contact patch center.
• Negative scrub radius: the steering axis meets the ground outboard of the contact patch center.
• Zero scrub radius: the two points coincide.

Scrub radius affects how strongly braking forces, road bumps and uneven grip on the two front tires push back through the steering. Manufacturers choose its value deliberately alongside the brake and steering design.

Reading the Tire Sidewall: Which Numbers Matter for Geometry

Only the size designation on the sidewall tells you anything about scrub radius. Performance ratings describe what the tire can carry or how fast it can safely run. They do not describe its shape.

Dimensions, Ratios and Units in a Size Like 205/55R16

• 205 is the section width in millimetres. It is measured across the widest point of the sidewalls on a specified measuring rim, not across the tread.
• 55 is the aspect ratio. It is a percentage, not a length: the sidewall height is 55% of the section width.
• R is the construction type, meaning radial. It is not radius.
• 16 is the rim diameter in inches, measured where the tire beads seat.

This mix of units is the most common source of confusion. Width is in millimetres, aspect ratio is a percentage and rim diameter is in inches. To calculate diameter you must convert the rim size to millimetres (1 inch = 25.4 mm).

Some light-truck and off-road tires use a different, inch-based format such as 33x12.50R15. Here the first number is the nominal overall diameter in inches, the second is the section width in inches and the last is the rim diameter in inches.

Markings That Do Not Affect Scrub Radius

• Load index (for example 91) is a code for the maximum load the tire can carry at its specified pressure.
• Speed rating (for example V) is a letter code for the maximum sustained speed under rated conditions.
• Service prefixes such as P or LT identify the tire's intended use and load-rating system.
• Treadwear, traction and temperature grades where shown, plus the manufacturing date code, are not dimensions either.

These values matter for safety and for legal fitment. They do not tell you where the contact patch sits relative to the steering axis.

How Width, Aspect Ratio and Rim Diameter Set Overall Diameter

You can calculate a tire's nominal overall diameter from its size code in three steps:

1. Sidewall height (mm) = section width × aspect ratio ÷ 100.
2. Rim diameter (mm) = rim diameter in inches × 25.4.
3. Overall diameter (mm) = rim diameter in mm + 2 × sidewall height.

From there, the nominal radius is half the overall diameter, and the circumference is the overall diameter × π.

Worked example, 205/55R16:

• Sidewall height: 205 × 0.55 = 112.75 mm.
• Rim diameter: 16 × 25.4 = 406.4 mm.
• Overall diameter: 406.4 + 225.5 = 631.9 mm.
• Radius: about 316 mm.
• Circumference: about 1,985 mm.

Width alone does not set diameter. A wider tire with the same aspect ratio gets a taller sidewall, because the aspect ratio is a percentage of the width. A lower aspect ratio shortens the sidewall. A larger rim adds 25.4 mm of diameter per inch, or 12.7 mm of radius. This is why plus-sizing combines a larger rim with a lower aspect ratio, so the overall diameter stays close to the original.

Why Tire Diameter Changes Scrub Radius

Tire diameter changes scrub radius because it moves the road surface up or down relative to the hub. The steering axis is not vertical, so moving the ground plane changes where the axis meets it.

The Role of Steering Axis Inclination

Steering axis inclination (SAI) is the angle between the steering axis and vertical, seen from the front. The top of the axis leans toward the vehicle's center, so below the hub the axis runs outward toward the wheel.

For every millimetre the ground sits farther below the hub, the steering axis ground intersection moves outward by about tan(SAI) millimetres. The relationship is:

Change in scrub radius ≈ change in loaded tire radius × tan(SAI)

The direction of the change follows from this:

• Larger radius: the axis ground point moves outward, toward or past the contact patch center. Positive scrub radius gets smaller, and negative scrub radius gets more negative.
• Smaller radius: the axis ground point moves inward. Positive scrub radius gets larger.

Why Loaded Tire Radius Matters More Than Nominal Radius

Scrub radius is defined where the tire actually meets the road. That depends on the loaded tire radius, which is the distance from the wheel center to the ground with the vehicle's weight on the tire.

Loaded radius is smaller than the nominal radius calculated from the sidewall because the tire deflects under load. How much it deflects depends on inflation pressure, load, sidewall construction and rim width. A size calculation gives a good estimate of the difference between two tires, but not of either tire's exact loaded radius.

The Effect of Camber

Camber also moves the contact patch slightly as tire radius changes, because a tilted wheel's contact point shifts sideways as the wheel grows.

• Negative camber (top of the wheel tilted inward) moves the contact patch outward as radius increases. This partly offsets the SAI effect.
• Positive camber moves the contact patch inward and adds to the SAI effect.

Typical road camber angles are much smaller than SAI angles, so camber usually modifies the result only slightly.

Example Calculations: How Much Scrub Radius Changes

The examples below assume a steering axis inclination of 13° (tan ≈ 0.231). This is an illustrative value, not a figure for any specific vehicle; check the actual value for the car you are working on. The calculations use nominal size differences and assume the wheel offset does not change.

The pattern is clear. A plus-size package that keeps overall diameter close to the original has almost no effect on scrub radius through diameter. A noticeably taller or shorter tire shifts scrub radius by a few millimetres.

Larger changes produce larger shifts. Going from 265/70R17 (about 803 mm overall diameter) to 285/75R17 (about 859 mm) raises the nominal radius by roughly 28 mm. At an illustrative 10° steering axis inclination (tan ≈ 0.176), that moves the steering axis ground intersection outward by about 5 mm.

Tire Diameter vs Wheel Offset: Which Affects Scrub Radius More?

Wheel offset usually affects scrub radius far more directly than tire diameter does. Offset is the distance in millimetres between the wheel's mounting face and its centerline. When offset changes, the wheel centerline and the contact patch center move sideways by the same amount, which is a 1:1 change.

For example, reducing offset by 10 mm moves the contact patch outward by about 10 mm. That increases positive scrub radius by about 10 mm. A tire diameter change of the same size would shift scrub radius by only around 2 mm at typical SAI angles.

Tire width on its own does not move the contact patch center, as long as the tire is centered on a wheel with unchanged offset. A wider tire spreads the patch farther to each side of the same center point. However, wider tires are often fitted on wheels with different offset, and it is the offset that changes scrub radius.

Limits of Calculated Tire Dimensions

Calculated diameter is a nominal figure taken from the size code. Real tires differ from it in several ways:

• Brand and model: two tires with the same size code can have measurably different actual diameters.
• Tread depth: a new tire is taller than a worn one. For example, losing about 6 mm of tread reduces radius by about 6 mm. At 13° SAI, that shifts scrub radius by roughly 1.4 mm over the tire's life.
• Rim width: mounting on a narrower or wider rim than the measuring rim slightly changes section width and profile.
• Inflation and load: these change loaded radius, which is the radius that actually sets scrub radius.

A favourable scrub radius calculation also does not mean a tire will fit the vehicle. Fitment depends on clearance to suspension parts, brakes, bodywork and steering stops, as well as load and speed ratings and local regulations. These are separate checks from the geometry estimate.

How to Estimate the Scrub Radius Change for a New Tire Size

1. Read the size codes of the current and the proposed tire from the sidewalls.
2. Calculate each tire's nominal overall diameter: rim diameter in mm plus twice the sidewall height.
3. Halve the difference in diameters to get the change in radius in millimetres.
4. Find the vehicle's steering axis inclination from its specifications and take the tangent of that angle.
5. Multiply the radius change by tan(SAI). A positive radius change makes positive scrub radius smaller by that amount. A negative radius change makes it larger.
6. If the wheel offset also changes, add the offset change at full value. Lower offset adds positive scrub radius; higher offset reduces it.
7. Treat the result as an approximation. Actual loaded radius, camber and tire construction will make the real figure somewhat different.

Key Points

• Only the size code on the sidewall (width, aspect ratio, rim diameter) relates to scrub radius. Load index, speed rating and quality grades do not.
• A taller tire moves the steering axis ground intersection outward and reduces positive scrub radius. A shorter tire does the opposite.
• The change is approximately the change in loaded tire radius × tan(steering axis inclination). For typical diameter changes, that is a few millimetres.
• Wheel offset changes scrub radius roughly millimetre for millimetre, so it usually matters more than tire diameter.
• Calculated dimensions are nominal. Actual diameter, loaded radius and physical fitment have to be confirmed separately.