Calculating wheel offset involves taking specific measurements of the wheel to determine the distance between the wheel's mounting surface and its centerline.
Wheel offset, often measured in millimeters (mm), is a crucial dimension that affects how your wheels sit within the fender wells, impacting suspension clearance, braking components, and vehicle handling. It is the distance from the hub mounting surface to the centerline of the wheel.
Here's how to calculate it:
Understanding the Key Measurements
To calculate wheel offset, you need two primary measurements:
- Backspacing: The distance from the wheel's hub mounting surface to the inner edge of the wheel rim.
- Wheel Width: The width of the wheel from the inside of the outer flange to the inside of the inner flange (where the tire beads seat). Note: The advertised wheel width is typically this bead seat measurement, not the overall width.
Step-by-Step Calculation
Follow these steps to calculate your wheel offset:
1. Measure the Wheel Width
- Measure the width of the wheel from the inside lip of one flange to the inside lip of the opposite flange. This is typically done where the tire bead sits.
- Practical Tip: Mount the wheel on a flat surface face down. Lay a straight edge across the back of the wheel. Measure from the surface to the straight edge to get the overall width, then subtract the thickness of both flanges to estimate the bead width, or use calipers to measure directly across the bead seats if possible. The advertised wheel width is often the easiest number to use here.
2. Calculate the Wheel Centerline
- Divide the measured wheel width by two. This gives you the distance from either flange to the exact center of the wheel width.
- Formula:
Centerline Position = Wheel Width / 2
3. Measure the Backspacing
- This is the distance from the wheel's hub mounting surface (the part that bolts to the car's hub) to the inner edge of the wheel rim.
- As shown in the provided video reference at 0:57, you measure "from The Hub face the mounting Point again to the same point here," which is the inner edge of the wheel.
- Practical Tip: Lay the wheel face down on a flat surface. Place a straight edge across the back of the wheel (across the inner rim flange). Measure vertically from the straight edge down to the hub mounting surface.
4. Calculate the Offset
- Subtract the calculated wheel centerline position (from Step 2) from the backspacing measurement (from Step 3).
- Formula:
Offset = Backspacing - (Wheel Width / 2)
The result will be in the same unit as your measurements, typically millimeters (mm).
Understanding Offset Types
Based on the calculation, offset can be:
- Positive Offset: The hub mounting surface is located outboard of the wheel's centerline. This is common on front-wheel drive cars and newer vehicles. The wheel sits further in towards the vehicle body.
- Negative Offset: The hub mounting surface is located inboard of the wheel's centerline. This is common on many rear-wheel drive cars and trucks, or aftermarket wheels for a wider stance. The wheel sits further out from the vehicle body.
- Zero Offset: The hub mounting surface is exactly aligned with the wheel's centerline.
Example Calculation
Let's say you measure:
- Wheel Width (bead seat to bead seat) = 8 inches (convert to mm: 8 * 25.4 = 203.2 mm)
- Backspacing (from hub face to inner edge) = 5.5 inches (convert to mm: 5.5 * 25.4 = 139.7 mm)
- Wheel Centerline Position = 203.2 mm / 2 = 101.6 mm
- Offset = Backspacing - Wheel Centerline Position
- Offset = 139.7 mm - 101.6 mm
- Offset = 38.1 mm
In this example, the wheel has a positive offset of approximately +38 mm.
Summary Table
| Measurement | Description | How to Measure (Practical) | Used For Calculating... |
|---|---|---|---|
| Wheel Width | Distance between tire bead seats (inside of flanges). | Measure inside lip to inside lip, or use advertised width. | Wheel Centerline |
| Backspacing | Distance from hub mounting face to inner edge of rim. | As shown in video: measure from hub face to inner rim edge. | Offset |
| Wheel Centerline | Half of the Wheel Width. | Calculated: Wheel Width / 2 |
Offset |
| Offset | Distance from hub mounting face to wheel centerline. | Calculated: Backspacing - (Wheel Width / 2) |
N/A |
Understanding and correctly measuring wheel offset is crucial when selecting new wheels to ensure proper fitment, clearance, and vehicle dynamics.
