Scrub Radius: What It Is and Why It Matters

If you’ve ever heard the term scrub radius and wondered if it’s something you need to worry about, you’re not alone. It’s a piece of steering geometry that can change how the wheel feels when you turn the wheel, how the tires wear, and even how stable the car feels under braking. The good news? You don’t have to be a mechanic to understand it, and you can make small adjustments that improve everyday driving.

How Scrub Radius Works

Scrub radius is the distance between two points: the center of the tire’s contact patch on the road and the point where the steering axis meets the ground. When the steering axis is perfectly upright, the scrub radius is close to zero. Most cars have a slight offset, which creates a positive or negative scrub radius.

A positive scrub radius means the steering axis hits the ground inside the tire’s center. This setup gives a stronger steering feel and helps the car pull straight on uneven roads, but it can make the steering feel heavy if the wheels are hit by a pothole.

A negative scrub radius puts the axis outside the tire’s center. This design is common on newer cars with electronic stability control because it reduces the torque felt on the steering wheel during hard braking. However, it can make the steering feel a bit loose, especially on older manual steering systems.

Both setups have pros and cons, and the right balance depends on the car’s weight, suspension design, and what you expect from the steering.

Checking and Adjusting Scrub Radius

Before you try to change anything, you need to know where you stand. The easiest way to check scrub radius is to use a simple measurement tool or a professional alignment machine. If you don’t have access to a machine, you can get a rough idea by measuring the distance from the wheel’s centerline to the point where the steering knuckle pivots, then comparing that to the tire’s offset.

Here’s a quick DIY check:

• Jack up the front wheel and remove the tire.
• Measure the distance from the hub’s center to the outer edge of the wheel mounting surface (this is the offset).
• Measure the distance from the hub’s center to the steering axis (you may need a bolt to mark the line).
• The difference between these two numbers gives you an approximate scrub radius.

If the number is large (positive or negative), you might feel it in the steering feel or see uneven tyre wear on the inner or outer edges. Small adjustments can be made by changing the wheel offset (using spacers or different wheels) or by adjusting the steering rack’s toe settings during a professional alignment.

Most people find the best result by keeping the scrub radius close to zero. That means the steering feels neutral, tyre wear is even, and the car remains stable under braking. If you’re swapping wheels, pick a set with an offset that matches the factory specification, or use hub‑centric spacers that keep the wheel centered on the hub.

Finally, always get a professional alignment after making any changes. The alignment shop can dial in the exact scrub radius, caster, and camber to give you a smooth, predictable steering experience.

Understanding scrub radius doesn’t have to be a headache. With a basic measurement, a few tweaks, and a proper alignment, you can improve steering feel, extend tyre life, and drive with more confidence. So the next time you feel the wheel twitch after hitting a bump, remember it might be scrub radius talking – and now you’ve got the tools to make it behave.