We determine offset by first visualizing the wheel with a centerline drawn through the middle. Next, the distance from the centerline to the mounting pad (where the wheel touches the vehicle) in millimeters is the offset. When the mounting pad is toward the front of the wheel, this is (+) positive offset. Most newer vehicles have positive offset wheels, which have a flat face and little if any outer lip. When the mounting pad is toward the rear of the wheel, this is (-) negative offset. Older model vehicles typically used this offset because the frame was narrow and the suspension was tucked further under the vehicle than newer vehicles.
Backside setting is determined by lying the wheel face down (on a cloth or rug to protect the finish) and placing a straight edge over the rear flange until it overhangs the wheel. This is also known as Rear Spacing or Back Spacing. They are synonymous .
From here, measure perpendicular to the straight edge to the mounting pad. This distance from the mounting pad to the bottom of the straight edge is backside setting. It indicates how far that the wheel will tuck under the vehicle once it is bolted up to the vehicles hub. Too much backside setting and the wheel or tire will hit the inner fenderwell or other suspension component. Not enough backside setting and the lip of the fender will make contact with the tread of the tire.
The X-Factor is the amount of caliper clearance needed to keep the face of the wheel from contacting the face of the brake caliper. This is a critical measurement, especially when installing a big brake package.
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