Scrub Radius Explained: What Wheel Offset Does to Your Steering
Scrub radius is the sideways distance, measured on the road surface, between the point where the steering axis meets the ground and the center of the tire's contact patch. It is the one piece of steering geometry that a wheel change alters directly: every millimeter of offset or spacer moves it by a millimeter. If you are choosing wheels, this is the geometry guide that applies to you.
Positive, zero, negative
| Scrub radius | Where the axis meets the road | Typical behavior |
|---|---|---|
| Positive | Inboard of the contact patch center | More steering feel; braking and bumps tug at the wheel |
| Zero | At the contact patch center | Light, but the tire scrubs on the spot when parking |
| Negative | Outboard of the contact patch center | Self-stabilizing if one side has more braking force |
The contact patch pushes on the car with braking, drive and bump forces. Scrub radius is the lever those forces use to turn the wheel about its steering axis. With equal forces left and right the two sides cancel through the steering rack. When they are not equal — one wheel on a wet patch under braking, a pothole on one side, a front-drive car putting down power unevenly — the difference arrives at the steering wheel.
Many front-wheel-drive cars and cars with diagonally split brake circuits are designed with a small negative scrub radius, so that uneven braking steers the car slightly back toward straight. Older rear-drive designs often have a positive value.
The geometry
The first term is how far the wheel centerline sits from the steering axis at the height of the hub. Kingpin inclination then carries the axis outward on its way down to the road. A taller tire gives the axis further to travel, so it reduces scrub radius slightly; a shorter one increases it.
What a wheel change does
Offset sets where the wheel centerline sits relative to the hub face. Lower offset or a spacer moves the centerline outboard, and the scrub radius moves in the positive direction by the same amount. Wheel width does not matter here — only where the centerline ends up.
| Change | Centerline moves | Scrub radius |
|---|---|---|
| ET45 → ET35 | 10mm outboard | +10mm |
| ET45 → ET35 with a 5mm spacer | 15mm outboard | +15mm |
| ET45 → ET50 | 5mm inboard | −5mm |
| 8" ET45 → 9" ET45 | None | No change |
On a car designed with −5mm of scrub, going from ET45 to ET30 lands at +10mm: the sign has flipped, and the stabilizing behavior the designers built in is gone. Whether that is noticeable depends on the car, but it explains a familiar set of complaints after fitting low-offset wheels or thick spacers:
- The steering follows ruts and cambers in the road (tramlining).
- More kickback through the wheel over one-sided bumps.
- The car pulls under hard braking on uneven surfaces.
- Heavier steering, and more torque steer on a front-drive car.
- Higher load on wheel bearings and ball joints, because the tire's force now acts further from them.
Keeping it in check
The straightforward approach is to stay near the original centerline: if you want a wider wheel, choose an offset that splits the extra width between inside and outside rather than pushing all of it outward. The calculator shows how far a wheel's position moves against the recorded OEM baseline; the outward movement of the centerline is the change in scrub radius. Corrected knuckles and relocated ball joints exist for cars built around very low offsets, but they change other geometry at the same time.