After 22+ years in the custom wheel and performance tire business, I never get tired of turning someone's dream into a reality. Many times I look at a vehicle that has been built and think to myself "Those wheels really set that car off and fit the overall theme perfectly". Then I look at the gap between the tire and the fender and walk away in disgust because the width or offset was incorrect.
Over the years, I have heard people talking about offset and backside setting in ways that are everything but correct. Wheels are by far the most distinctive part of a vehicle. They can make or break the total image of the vehicle. That being said, I want to demonstrate how offset and backside settings are measured and their mathematical correlation to each other. First, offset is the distance from the mounting pad to the centerline of the wheel measured in millimeters. Referring to Diagram 1, you can see that the mounting pad of the wheel (the surface of the wheel that touches the hub of the vehicle) is toward the rear of the wheel in relation to the centerline of the wheel. This is known as negative offset. If the mounting pad lines up with the plane of the centerline, this is known as 0 offset. The more negative the offset, the deeper the face of the wheel tends to be. Most older rear-wheel drive cars and trucks needed negative offset wheels to fit properly. Most newer cars and trucks have a positive offset, which makes the face of the wheel almost flat with the outer flange. If we install a 0 or negative offset wheel on a late model truck, the tire is pulled out toward the street and the turning radius increases. We would typically have to install a lift kit so that the tires do not rub.
Next, backside setting, also known as rear spacing, is measured by placing a wheel face down and laying a straight edge over the rear flange. We then drop a ruler perpendicular to the straight edge down to the mounting pad. The distance is measured in inches. Refer to Diagram 2.
We point out that there is a mathematical relationship between offset and backside setting. It is expressed in the following formula:
Measured Width of the wheel (Bead to Bead) + 1"= Overall width Overall width/2 = Centerline Backside Setting - Centerline = Offset in inches. Offset in inches x 25.4 = Offset in millimeters. Click Here to Continue to Wheel Tech 201