WTF: Anyone know about wheel offsets?

[Tyrant Turbo]

OK, so the question has to do with positive and negative offset. Rota just hooked up some new wheels for the RX7 and they are asking for size, bolt pattern and offset. The size I am going with is 18x9.5 with a offset that has yet to be determined. Now when I ask people about offset no one seems to have a clue. My question is what makes for a larger lip? Currently I have a 40mm offset on my other wheels and that put me near a 2 inch lip. The wheels I am getting come in only positive offset. The offsets they come in are +12/+20/+25/+30/+35/+48 which of these has the smallest lip and which one would have the largest lip?

The wheels I am getting are here:

If I am missing something here or am way off on something I said, it's because I have no clue about wheels offsets. So help me! Thanks guys

[Polish Guy]

sorry, i dont know either. 

[new2me]

http://www.1010tires.com/WheelOffsetCalculator.asp

Does this help?

[new2me]

The offset of a vehicle's wheel is the distance between the centerline of the wheel and the plane of the part of the wheel bolted to the hub. It can thus be both positive and negative, and is typically measured in millimeters. Offset has a significant effect on many elements of a vehicle's suspension, including suspension geometry, clearance between the tire and suspension elements, the scrub radius of the steering system, and visually, the width of the wheel faces relative to the car's bodywork.
Zero Offset - The hub mounting surface is even with the centerline of the wheel.
Positive Offset - The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative Offset - The hub mounting surface is toward the back or brake side of the wheels centerline.
"Deep dish" wheels typically have negative offset or a very low positive offset.
To maintain handling characteristics and avoid undue loads on bushings and ball joints, the car maker's original offset should be maintained when choosing new wheels unless there are overriding clearance issues.
Wheels are usually stamped with their offset using the German prefix "ET", meaning "Einpresstiefe" or, literally, "insertion depth". An example would be "ET45" for a 45mm offset.
Calculating the offset of a wheel is a fairly easy mathematical equation. First, measure the overall width of the wheel (remember, just because a wheel is 18x7.5, does not mean that the OVERALL width is 7.5”. It means that the measurement from outboard flange to the inboard flange is 7.5”). Next, divide that width of the wheel by two; this will give you the centerline of the wheel.
Overall width/2 = Centerline
After determining the centerline, measure from the mounting pad to the edge of the inboard flange (if the wheel were laying flat on the ground – face up – your measurement would be from the ground to the mounting pad). This is your back spacing.
Back Spacing - Centerline = Offset in Inches
Inches x 25.4 = Offset in mm
Source: American Racing Wheels

http://en.wikipedia.org/wiki/Offset_%28wheel%29

[Rizzo]

new2me great job.

[Tyrant Turbo]

indeed. Thanks new2me very helpful.

[Bill-B]

You posted first dangit

Pretty much positive brings the design to the front of the wheel, 0 in the middle, and negative in for some CRAZY lip action   I chose 15x8 et0  for a 3.5" lip

[JVC]

great post new2me, i was wondering about offset a bit too but that pretty much cleared it up!