Tech: How To Measure Backspace and Offset
Do you know the difference between backspace and offset? Or how to measure backspace and calculate offset? These are questions that we hear often. So here is a quick explanation, courtesy of Kevin Oeste at V8TV.
Backspace is simply the distance from the mounting surface to the inside edge of the wheel, and offset is simply the distance from the mounting surface to the center of the wheel. So if you can measure your backspace and overall width, then everything else is simple math.
In this example in the video below, Kevin is working with an 11-inch wheel (a stated width of 11 inches or 11 inches of tire mounting surface inside the wheel flanges). When he measures the overall width including the wheel flanges, it measures 12 inches. (Note: As a rule, your overall width will almost always be one full inch larger than the stated width.)
So now that you know your overall width, you can easily calculate the center of the wheel by simply dividing the overall width in half. So 12 inches ÷ 2 = 6 inches.
To measure backspace, Kevin places a flat edge against the inside wheel flange and measures to the back of the hub mounting surface to get a distance of 5 3/16 inches. So our backspace is simply 5 3/16 inches.
Now we have everything that we need to calculate offset. Offset is simply the distance from the hub mounting surface to the center of the wheel. In other words, Offset = Measured Backspace – Calculated Wheel Center Distance.
In this example, we already know that our Measured Backspace = 5 3/16 inches.
And we know that our Calculated Wheel Center Distance = 6 inches.
So 5 3/16 – 6.0 = –13/16 inch.
-13 ÷ 16 = –.8125 inch.
(Note: Pay attention to the minus sign here. In this case, the hub mounting surface is to the INSIDE of the wheel center, so this wheel has a negative offset. If the backspace measurement was larger than the wheel center distance, then the hub mounting surface would be to the outside of the wheel center, and the wheel would have a positive offset.)
Now since offset is conventionally expressed in millimeters, we just need to convert –.8125 inch to millimeters. There are 25.4 millimeters per inch. So –.8125 inch x 25.4mm/in = –20.6375mm.
So this 11 inch wheel has a 12 inch overall width, 5 3/16 inch backspace, and a –20mm offset.
