Measurement from the pad of the wheel (which sits against the hub surface of the rotor) to the very backside of the wheel. This determines suspension and inner fender clearance of the wheel. Note: This measurement does not designate caliper clearance.

Measurement from the pad of the wheel (which sits against the hub surface of the rotor) to the very front side of the wheel. This determines outer fender clearance of the wheel. Note: This measurement usually equates to lip size however since the center of the wheel is included is not always the case as pad thickness (controls caliper clearance) can change.

Both backspacing and frontspacing combined equate to the wheels overall width which will be 1" wider that the wheels actual width. Confused? It's ok! A wheels actual width is measured from beadseat to beadseat and not to the very outside of the wheel. The extra material on the outsides of the beadseat area to keep the tire on the wheel are a 1/2" wide each adding 1" to the actual width of the wheel.

When anyone lists a size of a wheel they are always referring to the actual width which is the real width of the wheel. The only case where I've seen this not true is if someone measure the width themselves and mixed up the overall and actual widths.

Relationship from the pad of the wheel (which sits against the hub surface of the rotor) to the very center of the wheel (normally displayed in millimeters). This distance can be even with the centerline (zero offset), positive of the centerline, or negative of the centerline of the wheel. It's the most commonly used method of listing a wheels fitment.

The higher the offset the further the pad sits forward to the centerline of the wheel pulling the wheel under the car more. The lower the offset the further the pad sits backwards to the centerline of the wheel pushing the wheel out from under the car.

Only real downside with offset is it's not a measurement you can easily obtain (if no sticker or stamp exists) and is harder to understand and recalculate when widths of wheels change.

Modular 3pc wheels consist of a inner shell, outer shell (lip), and center. Almost every manufacture makes shells that are in 1/2" increments which once the two shells are combined creates the width of the wheel. The center of the wheel which makes up the spoke design and pad designates both offset range and caliper clearance.

The shape/contour of the center along with the pad length has a minimum and maximum range that it can be cut (offset range). If the backspacing/offset number your trying to hit doesn't fall within the range of the center with the shells you have chosen you would have to shift the stack.

Shifting the stack will either increase or decrease the offset range of the center. For Example; If you need a lower offset range you can reduce the width of the inner shell and increase the width of the outer shell thus moving the center inwards. The same thing in reverse would need to happen to allow for a higher offset range moving the center forwards.

Depending on a wheels center design and pad thickness you can achieve multiple setups using different shell stacks and pad thickness while still maintaining the same width and backspacing/offset. The only way this works since the shells are in 1/2" increments is using centers with 1/2" different lengths. This will result in wheels with bigger lips and less caliper clearance and smaller lips and more caliper clearance as these two go hand in hand. In most cases you can build the same size wheel front and rear with more lip on the rear by reducing the pad length since the caliper is smaller and has less overhang to clear. This setup would not be able to be rotated as the shorter pad length on the rear will not get around the larger brakes on the front.

Diameter is a visual or functionally choice. In almost every case wheel diameter is chosen based on looks (aspect ratio to the body). In racing applications the smallest diameter possible would be the best as weight is less reducing rotational mass and unsprung weight. Factors such as rotor diameter, calipers, and tire selection will be the only things that would keep the diameter being as small as possible.

When wheel diameters change your tire selection will also change. Going for a larger diameter the sidewall of the tire will be shorter and vise versa for going smaller. You'll want to keep the overall diameter of the tire to stay constant from what was stock thus the reason for the sidewall thickness change.