Scrub Radius Calculator finds signed scrub radius from wheel offset, spacer thickness, tire diameter, and steering axis inclination on a car, truck, or SUV front suspension setups.
Find Positive or Negative Scrub with the Scrub Radius Calculator
This tool works out signed scrub radius – the lateral gap at ground level between the steering axis and the tire’s contact patch center – from wheel offset, spacer thickness, tire diameter, and steering axis inclination.
It’s built for anyone changing wheel offset, adding spacers, or fitting a different tire size and wanting to know how that shifts scrub radius before it’s on the car.
Reading the Scrub Radius Calculator’s Inputs and Outputs
Enter steering axis inclination (SAI) in degrees, the hub-face-to-steering-axis distance, current and new wheel offset (ET) in millimeters, current and new tire diameter, and any spacer thickness. The hero result is signed scrub radius for the new setup; the cards below break out how much of the change came from offset versus tire size, what offset would zero out scrub entirely, and how one more size step in tire or spacer would move it further.
The Geometry Behind Scrub Radius
Scrub radius is the hub-face distance to the steering axis, minus wheel offset and any spacer, minus half the tire diameter multiplied by the tangent of the steering axis inclination.
$$SR = (d – ET + s) – \frac{D_{tire}}{2}\tan(\theta_{SAI})$$
That last term exists because an inclined steering axis doesn’t hit the ground straight below the hub – it walks sideways by tire radius times tan(SAI) as it travels down to the road, which is straightforward geometry rather than an automotive-specific convention. A common input mistake is entering wheel diameter instead of full tire diameter, which understates the SAI correction term and throws off the whole result.
Steering axis inclination on production cars typically runs somewhere in the single digits up to around 15°; entering 0° removes the tire-size term from the math entirely, and entering a very large angle (above roughly 30°) is outside what any real production suspension uses, which is why the calculator flags it rather than returning a number.
Common Scrub Radius Calculator Input Errors
Entering ET as a positive number when the actual stamped offset is negative, which flips the sign of the entire result.
Mixing millimeter offset figures with an inch-based tire diameter without converting, since the tool expects offset in millimeters and length fields in whichever unit system is selected.
Forgetting to zero out the spacer field after removing a spacer, which leaves a stale value quietly skewing every scrub radius calculation that follows.
Scrub Radius Calculator: Frequently Asked Questions
What’s the actual difference between positive and negative scrub radius?
Positive scrub radius means the tire’s contact patch center sits outboard of the steering axis; negative means it sits inboard. Most modern front- and all-wheel-drive cars are built with a small negative or near-zero scrub radius specifically to reduce torque steer.
Does a lower offset wheel really change scrub radius that much?
Yes – the relationship is direct: each 1 mm reduction in wheel offset moves scrub radius roughly 1 mm more positive, and a spacer of the same thickness has the identical effect. A 16 mm offset drop is a 16 mm shift in scrub radius before the tire-size term is even considered.
Do taller tires make scrub radius better or worse?
It depends on the sign you’re starting from. Since the tire-size term subtracts from scrub radius, a taller tire pulls scrub in the negative direction – which helps counteract a lower-offset wheel that pushed scrub positive, but pushes an already-negative setup further negative.
What actually happens with a high positive scrub radius?
A high positive scrub radius means the tire’s contact patch effectively arcs around the steering axis as it turns, rather than pivoting cleanly on its own center, which is felt as heavier steering effort and more kickback over bumps. On a driven front axle it also amplifies torque steer under hard acceleration.
Is zero scrub radius actually the ideal target?
Not in practice – many independent build writeups describe true zero scrub as producing vague, disconnected steering feel with poor self-centering, which is why manufacturers deliberately target a small negative value on FWD/AWD platforms rather than chasing exactly zero.