Wreck Me Otter

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Mr Michael B

Hey Kim,

Wheel offset is a term used to describe the difference between the mounting surface of the wheel, the centerline, and its inner and outer rims. Porsche wheel offset is measured in terms of "backspacing." This is the depth from the wheels mounting flange to the inner rim edge. Find your wheel's back-spacing by placing the wheel face down; laying a straightedge across the wheel rim (not the tire); and then measuring down to the mounting flange.

Hope that is helpful.

shaaark89

actually,
offset and backspacing are two different things. backspacing refers to the distance between the mounting point of the wheel and the inner rim lip.
offset refers to the distance between the mounting point and the centerline of the wheel. you can have wheels with the same offset and different backspacing and vice versa.
all porsche factory wheels have the offsets stamped on them somewhere preceded by the letters et.

Bill Verburg

Kinesis does not put the o/s on the wheel.
<img src="http://www.pelicanparts.com/MotorCity/wmv/WheelsFAQ-diagram.gif" alt=" - " />

It is the normal distance from the wheel centerline to the mounting face. It is difficult to measure directly so backspace is often used as an easily measured proxy. The formula for o/s is;

o/s = Backspace - (wheel width)/2

Mr Michael B

"o/s = Backspace - (wheel width)/2"

Bill,

That is fabulous. Thank you. A lot easier than the stuff I wrote!

shaaark89

if you measure from lip to lip you will typically get a measurement that is an inch wider than the true wheel width. the lip is not included in the true wheel width. look at rim width vs overall rim width on bills picture.

Bill Verburg

Thats correct, as indicated in the diagram you measure the normal width from edge plane to edge plane and the normal backspace from edge plane to inner mounting face for the calculations. You do not use the advertised wheel width for any calculation

Danno

"I was wondering what width is common to use when stating the width of a wheel."

Bill's right. Never, ever use the 'official' width of the rim in your calculations. That's the 18x8" and 18x10" sizes that's stamped on your wheel.

To compute offset & backspace, you should use the 'measured' overall-width which is typically 0.75-1.0" wider than the bead-width. There are a lot of errors in the various wheels-FAQs out there because they get those two mixed up (bead vs. overall width).