Can someone explain wheel offset?

I am looking for new wheels for my new addition and have been perusing the classifieds here and other places and I have no idea how to determine if certain wheels will work on my car (2002 Targa). Stock sizes were 17" and 18" with tirerack and other places reporting optional 19" but that is apparently dependent on offset. (And of course bolt pattern, but that one I understand.) Anyone have a primer on this stuff, or even a good link to one?

Not sure what you want to know... but follow the link to an offset calculator: Offset Calculator Hope that helps

I guess, all else equal, what affect does changing the offset have? The linked calculator indicates that increasing offset moves the wheel in toward the car and decreasing offset moves the wheel away from the car. (Keeping the wheel width the same.) Is that right? How do I determine whether a certain width and offset will fit on a certain car? If I go from a 17*9 wheel to a 18*10 wheel, what offset ranges do I need to be within? etc. I can't be the only one who has no idea about this stuff.

Quotes source post: 10701300

You will need to know your current wheel Width and Offsets. Enter those first. Then enter the potential new wheels width and offsets. Press calculate and it will show how much inner clearance and outer wheel difference you will have... then check your current setup clearances on your car. if you have the adequate room you will be good. if not adjust the offset until you are within spec, then find wheels with those specs.

Offset is where the hub mounting surface is in relation to the center of the wheel. Positive offsets indicate the hub mount is closer to the outside edge of the wheel, while negative offsets indicate the hub mount is closer to the inside edge of the wheel. How a wheel fits with X offset is a factor of the width of the wheel. An 18x9 et30 wheel will not fit the same as an 18x10 et30 wheel (offset is identified by the letters "et", which comes from the German word "einpresstiefe" which translates to "insertion depth"). While the offset is the same, the 18x10 et30 wheel is ~26mm (1 inch) wider than the 18x9 et30 wheel, so each side of the wheel will stick out 12.5mm more than the 18x9 wheel. So, if you wanted the 18x9 wheel to fit the car like the 18x10 wheel, you would need to lower the offset 12.5mm, so the offset would need to be et17.5. Does that help at all? Here is something I made awhile back that might help visualize it as well:

Offset will vary depend on the thickness of the wheel as you can see in the picture. Offset is the area between the mounting pad of the wheel and the center line of the wheel. The centerline would change based on thickness of the wheel. What this means is a 8.5" wheel with a offset of 50 will sit much differently than a 9.5" wheel with the same offset, for two reasons. One being the wheel is wider, second being the offset as it determines how that extra width will sit, either towards the fender or towards the strut. Here is a article written by the guys who made the above picture explaining Offset. They are currently making me a spacer setup. http://42draftdesigns.com/tech/offset.html

All of that is really helpful. Thanks for the summaries. Next step: How do I translate knowing the wheel diameter, width and offset to knowing whether the wheel will work with a specific model of car?

Quotes source post: 10701479

Tire size is an equally important variable. For example, on an 18"x10" rear wheel of arbitrary offset, you might mount multiple different tires which have differences in width and diameter including but not limited to: 265/35-18* 265/40-18 285/35-18* 295/30-18 *OEM sizes And there are also considerations beyond 'will it work'... Understeer/oversteer can be affected by changing front/rear tire width radio and large changes in wheel position through offset can effect handling. Your best bet is to post up what you are thinking about for wheels/tires, on what car, and for what kind of driving and get some feedback from folks who might have a similar setup. This diagram shows how tire selection is equally important to offset for proper clearanceL Also note that 'offset' and 'backspace' are both used to define the position of the wheel once mounted. Offset is from the centerline of the wheel, regardless of wheel width. Backspace is the inside clearance, taking the wheel width into account.

Off topic, but I've always found the odd mix of measurements used in tires & wheels a bit strange... Wheel diameters in inches Wheel width in inches Wheel offset in metric Bolt patterns in metric or inches Tire inner diameter in inches Tire width in metric, occasionally inches Tire diameter as a calculated value of percent of the metric width (x2) plus the inches diameter, unless width is in inches, then overall diameter in inches as well. Clear as mud?

Great set of explanations. I certainly understand it better now. Any yeah, why aren't we all on metric by now ?

Found this doing some random googling in my quest for new wheels. http://www.wheeldynamics.net/appguide.html