After seeing countless threads where people are selling wheels of unknown offset I've decided to write a step by step guide to measuring offset. It is a quick and easy process that involves taking two measurements and some simple maths and it can even be done when the wheel still has a tyre on it.
What you will need:
- A smooth flat surface such as concrete
- A tape measure
- A long straight edge such as a level or straight piece of wood
Procedure:
- Place the wheel face down on the concrete. Inside of tyre is now pointing up, outside pointing down.
- Place the straight edge across the top of the wheel passing over the centre hole.
- Measure (in mm) from the wheel mounting surface to the inside edge of the wheel (bottom of the straight edge). Call this measurement A.
- Measure (in mm) the overall width of the wheel (concrete to the bottom of the straight edge). Divide the measurement by 2 and call it B.
Calculate:
The offset of the wheel can now be calculated by subtracting B from A.
Offset = A – B
Example:
Using the wheel and measurements shown in the pictures above.
Wheel mounting surface to back edge of wheel (A) measured at 109.
Overall wheel width is measured as 242, divide by 2 to get B = 121.
This gives an offset (A - B) of -12.
In this case I also know that the wheel has an offset of -12 because it is stamped on the rim. The exact stamping is "7JJX15 12 N", where "7" is the rim width (inside the bead), "JJ" is the type of bead on the rim, "15" is the rim diameter and the "12 N" tells us the offset is negative 12. On the other side of the valve it is stamped "HYUNDAI SAE 43 87" which simply gives manufacturer information and presumably a date code, manufactured in week 43 of 1987.
Useful notes:
- The more positive offset the narrower the track width will be. The more negative offset, the wider the track width will be. Zero offset rims will have equal distances from the wheel mounting surface to each side of the rim.
- Just because a rim is 7 inches wide doesn't mean you can skip the overall width measurement and use 7 inches instead. The specified rim width is measured between the inside of the beads. For this purpose we need the overall width of the rim.
- Offset is sometimes referred to as ET. In this case an ET10 rim would have a +10 offset, ET-13 would be -13 etc.
- There is no such thing as F100 offset. Measure your rims.
- Tim.
