Scrub Radius Change Calculator

How to use it: enter your vehicle's actual stock scrub radius (from a spec sheet or alignment printout - not something this tool can look up or derive), your stock wheel offset, your new wheel's offset, and any spacer you're adding. The tool tells you how much the scrub radius changes and its new value.

Why offset alone can't give you an absolute scrub radius. Scrub radius is the distance, at the ground, between where the steering axis (kingpin line) crosses the ground and the tyre's centerline. The steering axis position is fixed by your suspension's physical geometry - it doesn't depend on which wheel is bolted on. Wheel offset only tells you where the tyre centerline sits relative to the hub face, not where the hub face sits relative to the steering axis, so offset alone can only tell you the change in scrub radius, never the absolute value.

Formula. A lower offset (or an added spacer) always pushes the wheel centerline outboard by that same amount, and since the steering axis doesn't move, the gap between them changes by exactly that amount:

Outward shift = (stock offset − new offset) + spacer thickness
New scrub radius = stock scrub radius + outward shift

Worked example. With the defaults (10 mm known stock scrub radius, 35 mm stock offset, 25 mm new offset, no spacer): outward shift = (35 − 25) + 0 = 10 mm. New scrub radius = 10 + 10 = 20 mm - the new wheel roughly doubles the scrub radius in this example.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Known stock scrub radius

10 mm

Stock wheel offset

35 mm

New wheel offset

25 mm

Added spacer thickness

0 mm

Primary result: New scrub radius: 20.0 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This tool uses the common textbook convention where scrub radius is positive when the steering axis crosses the ground inboard of the tyre centerline. Some sources use the opposite sign convention - if your reference material disagrees with the sign here, flip the sign of the result; the magnitude of the change is correct either way.

Tyre or wheel diameter does not affect scrub radius (it's a ground-plane, side-to-side measurement, not related to rolling radius), so it isn't an input here. This also assumes the tyre is centered on the wheel and doesn't model steering-axis inclination changes from suspension modifications (lowering, camber/caster plates, different control arms), which can change scrub radius independently of wheel offset.

A larger scrub radius increases steering effort and can amplify the effect of a front brake failure or a pothole strike pulling the steering wheel - this is a real handling consideration for significant offset or spacer changes, not just an arithmetic curiosity. If you don't know your vehicle's stock scrub radius, an alignment shop or the vehicle manufacturer's technical data can usually provide it.