Offset and backspace are essentially two different ways of looking at the same thing. They determine the location of the wheel and tire assembly when bolted to the vehicles hub.
Offset is the measured distance between the hub mounting surface and the wheels center line.
With X being the determined offset, the hub mounting surface on positive offset wheels is X amount forward from the wheel center line. The hub mounting surface on negative offset wheels is X amount backwards from the wheel center line. The hub mounting surface on 0 offset wheels is the wheel center line.
Backspacing is the measurement from the hub mounting surface to the back edge of the wheel.
***Typically speaking, the higher the offset/backspace, the more the wheel will tuck inwards towards the suspension or away from the fender. The lower the offset/backspace, the more the wheel will push out away from the suspension or towards the fender.
*If the offset on a 9" wide wheel is +12mm, the hub mounting surface will be 12mm forward from the wheels center line. Measuring from the hub mounting surface to the back edge of the wheel, the backspacing is 5.5"
*If the offset on a 9" wide wheel is -12mm, the hub mounting surface would be 12mm towards the back of the wheel from the wheels center line. Measuring from the hub mounting surface to the back edge of the wheel, the backspacing is 4.5"
Here's how to calculate the backspacing using the rim width and offset:
First - add 1" to the rim width and then divide by 2 to find the wheels center line (you have to account for the wheel flange which is why you add 1" to the rim width) . Second - convert the offset which is in millimeters into inches. There is 25.4mm in 1 inch so divide the offset by 25.4. Lastly - add the converted offset to the wheels center line if the offset is positive for the correct backspacing. Subtract the converted offset to the wheels center line if the offset is negative for the correct backspacing.
But the offset won't change if the mounting surface is thicker (or thinner), but the back spacing may.. correct?
I mean if you have two rims with 0mm offset, but one is an alloy with a thicker mounting surface, and one is a steel wheel with a thinner, then the back spacing between the two would be slightly different (may 1/4" or something).. or am I thinking that wrong in my head?
Edit> After re thinking and re -reading I noticed the 0mm offset uses the back side of the mounting surface as the center mark, so it wouldn't affect it. I was thinking at a time I shouldn't be, early in the morning lol
So I have been trying to figure this out, I am looking at getting http://www.uswheel.com/products/stealthstar in a 16x8 with 6x5.5 for my 2003 xterra, They have a 0mm offset, now in my calculations that will make the wheels stick out away from the center of the Xterra a half inch over stock, is this correct or am I off? What offset am I looking for to get them to stick out past the fenders about a inch or so in a 16 inch rim?
The OEM rims are 5.5" BS and the wheels you are looking at are 4.5" BS. Go look through the tacked wheels and tires compatibility thread to give you an idea of what it will look like and ho much trimming you may have to do.
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