Offset & Backspace - How Do They Relate? - Toyota FJ Cruiser Forum
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post #1 of 20 (permalink) Old 06-29-2010, 01:27 PM Thread Starter
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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)
*5" - 0.50" = 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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Last edited by Discount Tire; 09-19-2013 at 11:53 AM.
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post #2 of 20 (permalink) Old 07-03-2010, 08:46 AM
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Re: Offset & Backspace - How Do They Relate?

Thanks! This is very helpful.

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post #3 of 20 (permalink) Old 10-08-2010, 09:37 AM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

Bump. Lots of questions about this.

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post #4 of 20 (permalink) Old 10-08-2010, 09:55 AM
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Re: Offset & Backspace - How Do They Relate?

Makes sense now!
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post #5 of 20 (permalink) Old 10-08-2010, 09:56 AM
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Re: Offset & Backspace - How Do They Relate?

This is great for someone like myself who doesn't know a great deal about cars. Wheels and tires (the sizing, will it fit with X lift, etc.) are particularly mysterious, so pictures always help.

Can you comment on what sort of offset is generally regarded as better for offroading FJs and why? Intuitively it seems to me that having a negative offset would put more strain on the other components, but that's just a hunch.

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Last edited by Dest; 10-08-2010 at 10:00 AM.
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post #6 of 20 (permalink) Old 10-08-2010, 12:57 PM
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Re: Offset & Backspace - How Do They Relate?

...one thing I've always wondered about is space needed due to the brake caliper on our FJ's. BellyDoc explains it as 'X factor'. Is that a normal measurement that you can just go look up when looking at different wheels?

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post #7 of 20 (permalink) Old 10-08-2010, 03:26 PM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

Quote:
Dest previously said: View Post
This is great for someone like myself who doesn't know a great deal about cars. Wheels and tires (the sizing, will it fit with X lift, etc.) are particularly mysterious, so pictures always help.

Can you comment on what sort of offset is generally regarded as better for offroading FJs and why? Intuitively it seems to me that having a negative offset would put more strain on the other components, but that's just a hunch.
This really depends on your suspension set-up and how far you want to push your fitment limits (how much trimming or modifications you are willing to do). Most of the guys here want to run the "biggest" tire they can with their stock suspension, leveling kit or lift.

If you have a lift (more than a mini lift or level) a wheel with a 4.5" backspace is generally the backspace that people go with for off-roading. (8" wide wheel with 0mm offset, 8.5" wide with -6mm or 9" wide with -12mm) With a decent lift most people want to run wider and taller tires than what came on the vehicle stock. To properly run an extra wide and extra tall tire you need a wider wheel and you need the wheel to sit out away from the vehicle further than stock to avoid rubbing the suspension components and/or frame.

If you only have a leveling kit, mini lift or no lift at all you are more limited on how tall and wide of a tire you can run. In this situation most people choose to run a wheel with closer to stock specs so that they can keep the wheel and tire tucked under the vehicle in the deepest part of their fenders. (picture your fender cavity as bowl shaped) This allows them to run a taller tire but limits how wide they can go.

Will lower than stock offsets or low backspace increase strain on suspension and or other components? Its possible, but as far as I have seen, no one has been able to figure out to what extent.

Quote:
sinerasis previously said: View Post
...one thing I've always wondered about is space needed due to the brake caliper on our FJ's. BellyDoc explains it as 'X factor'. Is that a normal measurement that you can just go look up when looking at different wheels?
I wish there was. Unfortunately "x Factor" is really only used by wheel manufacturers. It would be nice if there was a simple 1,2,3,4 etc scale of something for brake clearance.
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post #8 of 20 (permalink) Old 10-10-2010, 05:23 PM
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Re: Offset & Backspace - How Do They Relate?

This should be a sticky. Many people could use this.
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post #9 of 20 (permalink) Old 04-02-2012, 11:39 AM
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Re: Offset & Backspace - How Do They Relate?

good info!

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post #10 of 20 (permalink) Old 05-31-2013, 10:26 AM
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Re: Offset & Backspace - How Do They Relate?

I was at Discount Tire Direct yesterday and met all the nice folks that help us out on the forum .
We were talking about backspace/offset and I didn't realize they posted this reference guide last year.
Since this seems to be a topic that has a lot of gray area, I decided to sticky it for easy reference.
Thanks to discount for a really well put together explanation of this.

There are several useful links included in the first post, that may also be helpful

"The Rock"
https://www.fjcruiserforums.com/forum...8490-rock.html

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