Offset alone is useless.
The combination of wheel diameter, wheel width, offset and disk/face is needed to know fitment.
Offset is the distance between the centerline of the wheel and the hub mounting face.
This distance dictates the wheel’s placement within the wheel well. Offsets are described in three different ways, Positive, Zero and Negative.
Positive Offset: Hub mounting face is on the outside (towards the wheel face) of the wheel centerline.
Zero Offset: Hub mounting face is the wheel centerline.
Negative Offset: Hub mounting face is on the inside of the wheel centerline.
There are other important measurements that include offset. Forward Spacing is the distance from the front of the wheel to the hub mounting face. Backspacing is the distance from the hub mounting face to the inside of the wheel. Both of these measurements are key to calculating fitment.
The manufacture of a given vehicle designed their wheels (offset included) around the chassis, suspension, braking and steering of said vehicle. Changing the offset can lead to problems if it is changed.
The change in offset can be calculated by finding the difference between the offset of the new wheel minus the offset of the factory wheel. When this number is large side effects start to become apparent.
Simply, when the difference in offset is positive the wheel will sit further in the wheel wells. When the difference in offset is negative the wheel will be pushed further outward.
This is the effects of offset very simply, ignoring wheel width can lead to serious problems. DO NOT LOOK AT ONLY ONE MEASUREMENT, all measurements of a wheel are key and must be considered when purchasing a new wheel.
Knowing the potential side effects of changing the offset drastically is key.
Potential Side Effects of drastically changing offset.
- On suspension components
- Control arms
- Trailing arms
- On body panels
- Fender liners
- On brakes
Knowing your clearances to all of these components is key before purchasing or deciding on a given offset.
I will be going into detail on specific models.
- Acceptable Offsets
- Acceptable Diameters
- Acceptable Widths
Offset is generally measured in millimeters (mm) although converting an offset from inches (in) to millimeters (mm) is very simple. mm = in * 25.4
Change in Offset = New wheel offset* – Factory wheel offset*
Wheel Width in mm’s = Wheel Width in inches * 25.4
Centerline in mm’s = Wheel Width / 2
Backspacing in mm’s = Centerline + Offset* + 12.7
Forward spacing in mm’s = Centerline – Offset* + 12.7
*this formula applies for both Positive and Negative offsets, use the appropriate sign for a given offset to calculate both back and forward spacing.