How Wheel Offset Changes Affect Suspension and Steering Geometry

Short answer: Lowering the offset of a wheel (for example, from ET45 to ET35) moves the wheel centerline and the tire contact patch center outward by the same number of millimetres. In this example the shift is 10 mm. This makes the scrub radius more positive (or less negative) by that amount and widens the track by twice that amount per axle. It also lengthens the lever arm that loads the wheel bearing. Raising the offset does the opposite and moves the wheel inboard, closer to the suspension. Changing tire width does not move the wheel centerline. Changing overall tire diameter shifts the scrub radius only slightly, because it changes where the steering axis meets the ground.

To see these effects with real numbers, you first need to read two sets of markings: the tire sidewall and the wheel itself.

What do the numbers on the tire sidewall mean for suspension geometry?

A tire size such as 225/45R17 94W combines dimensions, a ratio, a construction code and two performance ratings. Only the dimensions and the ratio describe the tire's size. The ratings describe load and speed capability, not geometry.

Note the mixed units. Tire width is in millimetres, rim diameter is in inches, and the aspect ratio is a percentage. Any calculation has to convert inches to millimetres (1 inch = 25.4 mm).

How to calculate sidewall height, overall diameter and circumference

The nominal dimensions of a tire can be calculated from its size code in three steps:

1. Sidewall height = section width × aspect ratio ÷ 100. For 225/45R17: 225 × 0.45 = 101.25 mm.
2. Overall diameter = rim diameter in mm + 2 × sidewall height. For 225/45R17: (17 × 25.4) + (2 × 101.25) = 431.8 + 202.5 = 634.3 mm.
3. Circumference = π × overall diameter. For 225/45R17: 3.1416 × 634.3 ≈ 1,993 mm.

Half the overall diameter, about 317 mm here, is the unloaded radius. It is the vertical distance from the wheel center to the ground before the tire carries weight. This value matters for scrub radius, as explained below.

Comparing two tire sizes

Width, aspect ratio and rim diameter interact. A wider tire with a lower aspect ratio on a larger rim can end up with a similar or larger overall diameter.

In this comparison, the extra 20 mm of width is split evenly about the wheel centerline. That holds only if the wheel offset stays the same. The inner tire edge moves 10 mm closer to the suspension and the outer edge moves 10 mm closer to the fender. The 9.5 mm increase in radius raises the hub by roughly that amount and slightly changes where the steering axis meets the ground.

Calculated dimensions versus actual tire dimensions

The figures above are calculated nominal values, not measurements. Actual tires differ in several ways:

• Brand and model variation: Real section width and diameter vary between brands and models of the same size.
• Rim width: Mounting a tire on a rim wider or narrower than the measuring rim changes its actual section width.
• Loaded tire radius: The radius under the vehicle's weight is smaller than the unloaded radius. It depends on load, inflation pressure and tire construction, so measure it with the vehicle on level ground.

A favourable calculation also does not confirm fitment compatibility. Fitment still depends on the vehicle manufacturer's approved sizes, load and speed requirements, brake and suspension clearance, and local regulations.

How do you read wheel markings, including offset?

A wheel marked 8Jx18 ET45 has a nominal rim width of 8 inches between the bead seats. J is the flange profile, 18 is the rim diameter in inches, and ET45 means an offset of +45 mm. Bolt pattern (for example 5x112) and center bore are also stamped on many wheels. They affect whether the wheel mounts at all, not the geometry discussed here.

What is wheel offset?

Wheel offset is the distance, in millimetres, from the hub mounting face of the wheel to the wheel centerline. The wheel centerline is the plane halfway across the rim width.

• Positive offset: The mounting face sits outboard of the centerline, so most of the wheel sits inboard toward the suspension. This is common on modern passenger cars.
• Zero offset: The mounting face is exactly on the centerline.
• Negative offset: The mounting face sits inboard of the centerline, so the wheel sticks out further from the hub.

Every 1 mm reduction in offset moves the wheel centerline 1 mm outward, provided the hub stays where it is. Every 1 mm increase moves it 1 mm inward. A wheel spacer has the same effect as reducing the offset by the spacer's thickness.

Offset is related to backspacing, the distance from the mounting face to the inner edge of the wheel. Backspacing is roughly half the overall wheel width plus the offset. The overall width is slightly more than the nominal J width because of the rim flanges.

What is wheel scrub radius?

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

• the point where the steering axis meets the road
• the tire contact patch center

The steering axis is the imaginary line the wheel pivots around when steered. On a MacPherson strut it typically runs from the upper strut mount to the lower ball joint. On a double-wishbone suspension it runs from the upper to the lower ball joint. The axis is tilted inward at the top, and this tilt is the steering axis inclination (SAI).

The sign depends on which side of the steering axis the contact patch sits:

• Positive scrub radius: The contact patch center is outboard of the steering axis ground point.
• Negative scrub radius: The contact patch center is inboard of the steering axis ground point.
• Zero scrub radius: The steering axis meets the ground exactly at the contact patch center.

Kingpin offset vs scrub radius: what is the difference?

Kingpin offset and scrub radius both measure the lateral distance between the steering axis and the wheel. They are measured at different heights:

• Scrub radius is measured at the ground, between the steering axis ground point and the contact patch center.
• Kingpin offset, in the commonly used vehicle-dynamics definition, is measured at wheel center height. It is the horizontal distance between the steering axis and the wheel centerline. It is sometimes called spindle offset.

Because the steering axis is inclined, the two values differ. Below the wheel center, the axis moves outboard by the height × tan(SAI). As a result, kingpin offset at hub height is always larger than the scrub radius at the ground on a conventional design with inward-leaning SAI.

Terminology is not consistent across sources. Some use kingpin offset to mean scrub radius, so check which height a stated figure refers to.

The distinction matters because different forces act at different points:

• Braking with brakes mounted at the wheel: The braking force acts at the contact patch, so its lever arm about the steering axis is the scrub radius.
• Driving force on driven front wheels: The force is transmitted through the wheel center, so its lever arm is the kingpin offset at hub height.

Does wheel offset affect scrub radius?

Yes. Changing wheel offset changes scrub radius by the same amount, millimetre for millimetre.

The steering axis is fixed to the knuckle and hub, so it does not move when you change wheels. The wheel centerline and the contact patch center do move:

• Lower offset pushes the contact patch outward. This makes the scrub radius more positive, or less negative.
• Higher offset pulls the contact patch inward. This makes the scrub radius less positive, or more negative.

This one-to-one relationship ignores the small sideways shift of the contact patch caused by camber.

Scrub radius calculation step by step

A simplified scrub radius calculation in the front view works like this:

Scrub radius ≈ kingpin offset at wheel center height − loaded tire radius × tan(SAI)

All lengths are in millimetres. A positive result means positive scrub radius. To estimate how a wheel offset change affects scrub radius:

1. Find the kingpin offset. This is the horizontal distance from the steering axis to the wheel centerline at hub height, for the current wheel.
2. Find the SAI angle. Use the vehicle's specification or an alignment report.
3. Measure the loaded tire radius. This is the distance from the hub center to the ground with the vehicle on level ground.
4. Calculate the current scrub radius using the formula above.
5. Calculate the offset change: old offset − new offset. The result is positive when the wheel moves outward.
6. Add the offset change to the kingpin offset and repeat step 4. Equivalently, add the offset change directly to the scrub radius.

Illustrative example. The input values below are chosen only to show the arithmetic and do not describe a specific car.

• Inputs: kingpin offset 75 mm, SAI 13° (tan 13° ≈ 0.231), loaded radius 300 mm.
• Original wheel: 300 × 0.231 ≈ 69.3 mm, so scrub radius ≈ 75 − 69.3 = +5.7 mm.
• After ET45 → ET35: kingpin offset becomes 85 mm, so scrub radius ≈ 85 − 69.3 = +15.7 mm.

This is calculated geometry. Measuring the steering axis position precisely on a real vehicle requires suitable alignment equipment or accurate suspension data.

How do tire diameter and width change scrub radius?

Tire width: A tire mounted on a given rim is centered on the wheel centerline. A wider tire therefore does not move the contact patch center or change the scrub radius by itself. It does widen the contact patch, which spreads the area that scrubs around the steering axis, and it reduces clearance on both sides.

Tire diameter: A larger loaded radius places the ground further below the hub. Because the inclined steering axis moves outboard as it descends, the axis meets the ground further outboard. This reduces the scrub radius by about Δradius × tan(SAI).

In the 225/45R17 to 245/40R18 example, the radius grows by about 9.5 mm. At an SAI of 13°, that is 9.5 × 0.231 ≈ 2.2 mm less scrub radius. The effect is small compared with an offset change of 10 mm or more. Fitting a larger wheel with lower offset and a wider tire usually makes the scrub radius more positive overall.

How does scrub radius affect handling?

Scrub radius sets the lever arm that converts longitudinal forces at the contact patch into torque around the steering axis. Its size and sign influence braking stability, steering feedback and steering effort.

• Braking on uneven grip: With positive scrub radius, braking force at each front contact patch tries to turn that wheel outward around the steering axis. If one side has more grip, for example one wheel on ice, the imbalance pulls the steering toward the higher-grip side. A negative scrub radius produces a torque that tends to counteract this pull. Many front-wheel-drive cars, particularly those with diagonally split brake circuits, are designed with small or negative scrub radius for this reason.
• Steering feedback and kickback: A larger scrub radius, positive or negative, transmits more of the forces from bumps, potholes and grip changes into the steering wheel. Some drivers read this as stronger steering feedback, but it also means more kickback and tramlining on rutted roads.
• Steering effort at low speed: With some scrub radius, the tire rolls around a small arc when steered at a standstill. With near-zero scrub radius, the contact patch mostly twists in place. Very large scrub radius increases the leverage that road forces have on the steering.
• Brake balance sensitivity: As scrub radius grows, small differences between left and right braking force, from wear or contamination, produce larger steering torques.
• Driven front wheels: Torque steer depends mainly on kingpin offset at hub height and differences between the left and right driveshafts. A lower wheel offset increases kingpin offset by the same amount as scrub radius, so it can make torque steer more noticeable.

The manufacturer chose the original scrub radius together with spring rates, steering ratio, power assistance and brake layout. A changed value is not automatically worse. It does move the vehicle away from the conditions it was tuned for, and large changes tend to be more noticeable than small ones.

How does wheel offset affect wheel bearing load?

Wheel offset changes the lateral distance between the tire contact patch center and the center of the wheel bearing. That distance acts as a lever arm. The vertical and cornering loads at the contact patch create a bending moment on the bearing, hub and spindle.

Moving the wheel outward with lower offset or spacers lengthens this lever arm. It increases the moment for the same wheel load. Moving the wheel inward with higher offset shortens the lever arm.

Illustrative arithmetic:

• A corner load of 450 kg is about 4,410 N.
• An extra 10 mm (0.010 m) of lever arm adds about 4,410 × 0.010 ≈ 44 N·m of bending moment from vertical load alone.
• Cornering forces add further moment.

How much this shortens bearing life depends on the bearing design and its intended load center. Simple arithmetic cannot give a figure in kilometres. It does show why bearing manufacturers and vehicle makers specify an offset range, and why lug nut torque and wheel stud condition matter more as lever arms grow.

How does wheel offset change track width and suspension clearance?

Track width is the distance between the left and right tire contact patch centers on the same axle. A change of offset on both wheels changes track width by twice the offset change. Going from ET45 to ET35 on both sides widens the track by 20 mm.

Offset also decides where the wheel and tire sit relative to the surrounding parts:

• Higher offset moves the wheel inward. Check clearance to the strut body, spring, spring seat, control arms and sway bar links. This is especially important when the tire is also wider.
• Lower offset moves the wheel outward. Check clearance to the fender lip and wheel arch liner through full suspension compression and full steering lock.
• Wider tires reduce clearance on both sides by half the width increase each, if the offset stays the same.

The inner and outer edge positions can be compared between setups with simple arithmetic. Track the change in offset and half the change in tire or wheel width. For example:

• Change: 225 mm tire on ET45 to 245 mm tire on ET35.
• Outer edge: moves 10 mm (offset) + 10 mm (half the width increase) = 20 mm further out.
• Inner edge: stays roughly where it was, because the offset moves it 10 mm outward and the width moves it 10 mm inward.

Whether that new position actually clears depends on the real tire dimensions and the vehicle's suspension travel. Only a physical check can confirm it.

What these calculations can and cannot tell you

Sidewall and offset arithmetic reliably shows the direction and approximate size of geometric changes. For a given change of wheel and tire, it tells you how far the wheel centerline moves, roughly how much the scrub radius and track width change, and how the lever arm on the wheel bearing grows or shrinks.

It cannot tell you:

• the exact loaded tire radius or actual tire width of a specific tire on a specific rim
• whether a combination clears brakes and bodywork through the full range of suspension and steering travel
• whether it meets the load requirements and legal rules for the vehicle

Use the numbers to compare options and understand the consequences. Treat fitment as something to verify against the vehicle's specifications and by physical inspection.