Backspacing and Offset - the never ending headache of wheel shopping
Put together some info from my own personal research. Hoping it helps someone else out when they look for wheels.
The relationship between
backspacing and
offset is based on the wheel's width and how the mounting surface (hub) is positioned in relation to the centerline of the wheel. Here's a breakdown to help you understand and convert between the two:
Backspacing
- What It Is: The distance from the mounting surface to the inner edge of the wheel (lip).
- What It Affects: Determines how far the wheel and tire stick into the wheel well toward the suspension and other components.
- Measured In: Inches.
Offset
- What It Is: The distance from the mounting surface to the centerline of the wheel.
- What It Affects: Determines how far the wheel sits inward or outward from the hub.
- Positive Offset: Mounting surface is toward the front of the wheel (closer to the outside).
- Negative Offset: Mounting surface is toward the back of the wheel (closer to the inside).
- Measured In: Millimeters.
The Relationship
The key difference between backspacing and offset lies in the
reference point:
- Backspacing uses the inner edge as the reference point.
- Offset uses the centerline of the wheel as the reference point.
Conversion Formula
To convert between backspacing and offset, you’ll need:
- Wheel width (in inches).
- Backspacing (in inches).
- Conversion factor: 1 inch = 25.4 mm.
Formula:
- Find the centerline of the wheel: Centerline = Wheel Width/2
- Calculate the offset: Offset (mm) = (Backspacing−Centerline) × 25.4
Example
Suppose you're looking at a
10-inch wide wheel with a
5.5-inch backspacing (Adjust for your wheel combo):
- Find the Centerline:
Centerline = 10/2 = 5 inches
- Subtract the Centerline from Backspacing:
5.5 − 5 = 0.5 inches (positive offset)
- Convert to Millimeters:
0.5 × 25.4 = 12.7 mm offset (positive)
So, a 10-inch wide wheel with a 5.5-inch backspacing has a
+12.7 mm offset.
Quick Reference Table (Approximate)
For a 10-inch wide wheel:
- 5.5-inch backspacing = +12 mm offset.
- 5.0-inch backspacing = 0 mm offset.
- 4.5-inch backspacing = -12 mm offset.