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Old 06-29-2010, 12:53 PM
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Offset & Backspace - How Do They Relate?

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.
For Example:

*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.
For Example:

17x9 +12mm offset

*9" rim width + 1" = 10"
*10"/2 = 5" (wheels center line)
*+12mm offset/25.4 = 0.47" (0.50" rounded up)
*0.50" + 5" = 5.50" Backspace


17x9 -12mm offset

*9" rim width + 1" = 10"
*10"/2 = 5" (wheels center line)
*-12mm offset/25.4 = 0.47" (0.50" rounded up)
*0.50" - 5" = 4.50" Backspace

***Please note that the wheels center line is the backspacing for 0 offset wheels.


I hope this helps.

-Matt
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Old 08-24-2010, 09:56 AM
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TTT, lots of questions regarding BS and Offset.
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Old 09-28-2010, 07:45 PM
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I wish people would just stick to backspacing. So easy. lol

But anyways, What is the correct BS for a 2000 f250 SD? I have a set of XD 17in wheels with 315 BFG's and they rub the leaf spring like you wouldn't believe. If I turn the wheel 75% either way it's eating the tires off of it. That's not a very big tire to be causing all that commotion I wouldn't think.

I'm assuming this is a rim made for a newer SD with a different BS? Anyone know the measurements of both and what the heck I can do, if anything?

Thanks!
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Old 09-29-2010, 09:05 AM
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Do you know what backspace or offset and width your XD wheels have? (it should be indicated on the back of the wheel if you don't)

If they are rubbing on your suspension you'll need a lower offset or backspace to push them out.

315mm = 12.4" wide. This is quite a bit wider than the stock tire. (235/85R16 stock tires are only 9.2" wide)
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Old 12-30-2011, 03:24 PM
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This is an older post, I just wish I saw and understood all this before the last set of wheels I bought, they were a piss poor decision because I didn't spend any time looking into it. When I get back from my deployment they will be for sale, and I will start over. They will look much better on a truck that does not have flares!
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Old 12-30-2011, 03:51 PM
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Quote:
Originally Posted by Jasonj_2003 View Post
This is an older post, I just wish I saw and understood all this before the last set of wheels I bought, they were a piss poor decision because I didn't spend any time looking into it. When I get back from my deployment they will be for sale, and I will start over. They will look much better on a truck that does not have flares!
Thank you for your service to our country

Let us know if we can help you with your next set.
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