Let’s start by measuring your Rim Width!
Measure from the inside bead flange – the opposite bead flange. This is where your tire is secured, This measurement is noted on most tire specifications as “Measured Rim Width”
The Rim Lip measures between the bead flange and the back edge of wheel. (Industry standard is .500″)
Backspace is best measured first by placing a straight edge across the back edge of the wheel, then measuring from the hub mounting surface to the straight edge.
The most important information we would normally ask for is “A” and “E” from this brake diagram.
“A” is the distance from the hub pad to the face of the caliper. (In other words: How much spacer would you need to get a straight edge to clear the caliper)
Combining “A” and “E” we now know how much room, as well as what kind of step pattern we need to follow to fit over any brake system.
Offset Conversion
(XXmm) Offset X .03937 ((This first step converts Metric to Standard))
Add Half of Wheel/Rim Width
Add .500 for the Back Bead Flange ((Remember industry standard adds this Half Inch))
Negative Offset Conversion
When ordering, Please call out a required negative offset. See how the formula changes below!
(XXmm) Offset X .03937 ((This first step converts Metric to Standard))
Subtract Half of Wheel/Rim Width
Subtract .500 for the Back Bead Flange ((Remember industry standard adds this Half Inch to the formula))
Rich’s Example:
We are looking at a 35mm Offset 15×8.
35mm X 0.03937 = 1.37795
1.37795 + 4 (Half of Rim Width) = 5.37795
5.37795 + .500 (Back Flange) = 5.877 Inch Backspace (Or 5&7/8 Backspace if that’s how you roll)