Scrub Radius Calculator

Choose Calculation Mode

Calculate how changing wheels, offset, or spacers shifts your scrub radius from stock.

Offset & Spacer Change Calculator

Full Steering Geometry Calculator

How the Scrub Radius Calculator Works

Scrub radius is one of the most important - and most misunderstood - parameters in front suspension geometry. It defines the lever arm through which every braking force, driving force, and road disturbance creates torque around the steering axis. When you change wheels, offset, or add spacers, you change this lever arm, which directly changes how the car steers and brakes.

In the front view, the steering axis is the line connecting the upper and lower suspension pivot points - the lower ball joint and either the upper ball joint (double wishbone) or strut top mount (MacPherson strut). This axis is tilted inboard at the top by the Kingpin Inclination (KPI) angle, also called Steering Axis Inclination (SAI). When this tilted axis is extended down to the road surface, it intersects the ground at a specific point. The horizontal distance between this intersection point and the center of the tire's contact patch is the scrub radius.

Per SAE J670 (Vehicle Dynamics Terminology), the sign convention is: positive scrub radius means the contact patch center is outboard of the steering axis ground intersection; negative means the contact patch center is inboard. A zero scrub radius means both points coincide - which, contrary to intuition, is not ideal because it creates tire squirm and unpredictable steering torque under braking.

Our calculator uses two modes. The Offset/Spacer Change mode requires only the stock and new wheel offsets plus any spacer thickness. Since the steering axis geometry doesn't change when you swap wheels, the scrub radius change equals the contact patch center shift: ΔSR = (Stock ET − New ET) + Spacer Thickness. The Full Geometry mode computes the absolute scrub radius from the complete front-view steering geometry using the formula below.

The Math Behind It

The full scrub radius formula, derived from front-view projection geometry, is:

SR = S − ET + T − h × tan(KPI)

Where:

• SR = Scrub Radius (mm) - positive outboard, negative inboard
• S = Spindle Length (mm) - lateral distance from the steering axis to the hub/axle center, measured at the lower pivot height
• ET = Wheel Offset (mm) - standard DIN Einpresstiefe; positive means hub mounting face is outboard of the wheel center plane
• T = Spacer Thickness (mm) - pushes the wheel assembly outboard
• h = Lower Pivot Height (mm) - height of the lower ball joint above ground at ride height
• KPI = Kingpin Inclination Angle (degrees) - angle of the steering axis from vertical, viewed from the front

The term h × tan(KPI) represents how far outboard the steering axis projects from the lower ball joint down to the ground. A larger KPI angle or taller lower pivot pushes the axis ground point further outboard, which decreases (or makes more negative) the scrub radius. This is exactly how OEMs design negative scrub radius into FWD vehicles - by using steep SAI angles and tall MacPherson strut pivot points.

Industry Standards & References

• SAE J670 - Vehicle Dynamics Terminology, defines scrub radius as the front-view distance from steering axis ground intersection to contact patch center
• Milliken & Milliken, Race Car Vehicle Dynamics - Chapter 17 covers steering geometry including scrub radius effects on steering torque and stability
• Reimpell, Stoll & Betzler, The Automotive Chassis (SAE International) - Detailed treatment of steering axis geometry and its effects on handling
• Heißing & Ersoy, Chassis Handbook - Discusses the relationship between scrub radius, ABS function, and split-μ braking stability
• ISO 8855:2011 - Road vehicles - Vehicle dynamics and road-holding ability - Vocabulary (international equivalent of SAE J670)

Step-by-Step Example

Scenario: You own a 2020 Honda Civic (FWD, MacPherson strut front suspension). The stock wheels are 18×8 ET50. You want to fit 18×9 ET35 wheels with 5mm spacers. Your alignment sheet shows an SAI (KPI) of 14.3°. How will this affect your scrub radius?

Step 1 - Quick Change Calculation:

ΔSR = (Stock ET − New ET) + Spacer
ΔSR = (50 − 35) + 5
ΔSR = 15 + 5 = +20 mm

Your scrub radius will increase by 20mm in the positive direction. If the stock Civic has approximately −18mm scrub radius (typical for Honda FWD), the new scrub radius would be approximately +2mm - shifting from safely negative to slightly positive.

Step 2 - Full Geometry Verification:

Using typical Civic front suspension dimensions (S ≈ 92mm spindle length, h ≈ 255mm lower BJ height, KPI = 14.3°):

Stock: SR = 92 − 50 + 0 − 255 × tan(14.3°) = 42 − 65.0 = −23.0 mm
Modified: SR = 92 − 35 + 5 − 255 × tan(14.3°) = 62 − 65.0 = −3.0 mm

The change of +20mm matches our quick calculation. The car goes from a healthy negative scrub radius to nearly zero - the squirm zone. This setup would make braking feel inconsistent and reduce the car's inherent stability during split-friction stops. You'd want to either skip the spacers or choose a wheel with more offset (ET40+) to keep the scrub radius safely negative.

Common Mistakes to Avoid

1. Stacking spacers on FWD/AWD cars without checking scrub radius. Every millimeter of spacer thickness directly adds to scrub radius. A 25mm spacer on a car designed with −15mm scrub radius flips it to +10mm positive, potentially causing dangerous brake pull on wet roads and amplifying torque steer. Always calculate the total effect before installing spacers.
2. Confusing wheel offset direction. Lower offset numbers push the wheel outboard (more poke), which increases scrub radius. Many enthusiasts mistakenly think lower offset "tucks" the wheel. If your stock wheel is ET45 and you fit ET20, the wheel moves 25mm outboard - the same effect as a 25mm spacer.
3. Ignoring that zero scrub radius is NOT ideal. Zero sounds perfect, but it causes tire squirm - the contact patch rotates in place when steering, creating unpredictable feedback. Small amounts of positive or negative scrub are actually preferable to zero. Engineers deliberately design for a specific non-zero value.
4. Changing only one side. If you run different offsets or spacers left-to-right, you'll have asymmetric scrub radii. Under braking, the side with more positive scrub radius will generate more steering torque, causing the car to pull. This is a common issue after a single-wheel replacement with a non-matching offset.
5. Forgetting that camber changes affect scrub radius too. While not directly in the basic formula, aggressive negative camber shifts the contact patch inboard, effectively changing the scrub radius. If you add −3° of camber, the contact patch center moves inboard, making the scrub radius more negative - which is why alignment should be considered alongside wheel changes.

When to Use This Calculator

• Before buying aftermarket wheels: Check how the new offset will change your scrub radius. For FWD and AWD cars, try to keep it within ±10mm of stock. For RWD cars, you have more flexibility but should stay within ±20mm of stock.
• When considering wheel spacers: Spacers directly increase scrub radius. Use this calculator to verify the total change is acceptable before you install them, especially on daily-driven vehicles with ABS.
• After a suspension swap or custom knuckle fabrication: Use the full geometry mode to calculate the absolute scrub radius with your new spindle dimensions, pivot height, and KPI angle. This is critical for Formula SAE, track cars, and restomod builds.
• Diagnosing steering problems: If your car pulls under braking, has heavy or vague steering, or exhibits torque steer that worsened after a wheel change, calculate whether the scrub radius has moved out of the safe range.