Offset & Backspace - How Do They Relate? - Chevelle Tech
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post #1 of 15 (permalink) Old Oct 23rd, 10, 12:36 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)
*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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post #2 of 15 (permalink) Old Oct 24th, 10, 12:07 AM
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Re: Offset & Backspace - How Do They Relate?

Great Post Matt! This could be a sticky!

Ron
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post #3 of 15 (permalink) Old Oct 24th, 10, 2:22 PM
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Re: Offset & Backspace - How Do They Relate?

That was a very good description. The one point I would like to add is that backspace is a much easier and better way to determine and discuss wheel fit because backspace is an easily measured dimension (from the mounting surface to the back edge of the rim), where offset is a calculated dimension. There is no physical line or surface on the wheel indicating its center - you can't measure to or from it. Backspace doesn't require trying to figure out if it is positive or negative.

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post #4 of 15 (permalink) Old Oct 24th, 10, 11:09 PM
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Re: Offset & Backspace - How Do They Relate?

I have Cragar S/S wheels on my 67 and with 275/60-15s on them and it is about 1/4 inch from the fender but 2-2 1/2 inches from the frame rail. So does that mean if i were able to find a pair of wheels with more negative offset i could fit wider tires under the rear???

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post #5 of 15 (permalink) Old Jan 18th, 11, 11:07 AM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

^
It depends on the width of the wheel and the negative offset being run.

Lower offset wheels typically push the wheel out toward the fender or away from the suspension. If you only have 1/4" clearance to the fender now, but want a wider tire, it seems you would need to run a higher offset to bring the assembly in more towards the suspension since you have more clearance (2") to the inside.

-Matt
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post #6 of 15 (permalink) Old Mar 27th, 14, 3:37 PM
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Re: Offset & Backspace - How Do They Relate?

Quote:
Originally Posted by sz0k30 View Post
That was a very good description. The one point I would like to add is that backspace is a much easier and better way to determine and discuss wheel fit because backspace is an easily measured dimension (from the mounting surface to the back edge of the rim), where offset is a calculated dimension. There is no physical line or surface on the wheel indicating its center - you can't measure to or from it. Backspace doesn't require trying to figure out if it is positive or negative.
Roman
Just an FYI but many wheels now have the offset number stamped on them. If so,that makes it easy to figure out the backspacing.

Clint Hooper

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post #7 of 15 (permalink) Old Apr 18th, 14, 11:20 PM
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Re: Offset & Backspace - How Do They Relate?

hi, This tire stuff confuses the heck out of me! I have American Racing Torque Thrust II 15x7 rims or 15x8. Cant remember which are on front. Anyway there are just whatever are standard backspacing. I went from drums brakes on the front to disc brakes and it pushed the rims and tires out almost 2 inches. From everything I have seen the standard backspacing is 3.75, so does this mean I need a 5.75 backspacing? is that a positive or negative? Thanks Mike
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post #8 of 15 (permalink) Old Apr 22nd, 14, 12:18 PM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

Quote:
Originally Posted by asstchief12 View Post
hi, This tire stuff confuses the heck out of me! I have American Racing Torque Thrust II 15x7 rims or 15x8. Cant remember which are on front. Anyway there are just whatever are standard backspacing. I went from drums brakes on the front to disc brakes and it pushed the rims and tires out almost 2 inches. From everything I have seen the standard backspacing is 3.75, so does this mean I need a 5.75 backspacing? is that a positive or negative? Thanks Mike
We apologize for the delayed response Mike.

Typically speaking, higher offset wheels (positive for the most part) tuck the wheels in closer towards the suspension. Wheel width however still plays a roll in how far out the wheel sits. For example:

A 15x8 with a -19mm offset (3.75" BS) will sit 1" further out when compared to where a 15x7 ET-6 (3.75" BS) sits. Both have the same backspace/clearance to the inside.
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post #9 of 15 (permalink) Old Nov 17th, 15, 9:45 PM
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Re: Offset & Backspace - How Do They Relate?

I'm trying to find it what the offset should be for a rim if I am running a 28x11.5x15 tire on my 66 Chevelle. Is this tire size possible to run or is it to wide? I'm shopping for some rims but not sure of the correct wheel size and offset. Any help??
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post #10 of 15 (permalink) Old Nov 18th, 15, 12:24 PM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

A 15x9 worth an offset of 0 or -6 would work. PM coming your way.
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post #11 of 15 (permalink) Old Nov 28th, 15, 12:17 PM
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Re: Offset & Backspace - How Do They Relate?

I have a 69 Chevelle that is at the end of restoration. I want to take it to a different level and get away from the "original" look. One thing I want to do is install different wheels and tires. I'm looking at Foose wheels. I'm thinking of 17x7 all around, or maybe just 17x8 in the rear with 17x7 up front. The tire sizes are 225/50/17. Or if I go with a 17x8 in the rear, the tire size is 245/45/17.

I just want to be sure they will not rub the wheel wells, or rub when I turn the front tires. I may end up dropping the car two inches with a set of drop springs. Will this change anything.
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post #12 of 15 (permalink) Old Nov 28th, 15, 12:25 PM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

Quote:
Originally Posted by 69 L79 View Post
I have a 69 Chevelle that is at the end of restoration. I want to take it to a different level and get away from the "original" look. One thing I want to do is install different wheels and tires. I'm looking at Foose wheels. I'm thinking of 17x7 all around, or maybe just 17x8 in the rear with 17x7 up front. The tire sizes are 225/50/17. Or if I go with a 17x8 in the rear, the tire size is 245/45/17.

I just want to be sure they will not rub the wheel wells, or rub when I turn the front tires. I may end up dropping the car two inches with a set of drop springs. Will this change anything.
Thank you for the post. We'd be happy to look into this fitment for you.

Which Foose wheel are you considering?
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post #13 of 15 (permalink) Old Nov 28th, 15, 12:50 PM
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Re: Offset & Backspace - How Do They Relate?

measuring from the face of the drum to the inner wheel well and outer wheel well will give you a good idea. I use a 2' level to get an accurate reading. the side of your level that's against your drum is going to sit where your wheel is. Give yourself and inch to insure zero rubbing but 3/4" will work. you also have to know the section width of the tire. A M/T 275/60/15 has a section width of about 11 inches the tread width is 9 or 9.5. My 67 has an 8" rim with 4.5" of bs I have 3/4" from outer wheel lip and plenty of room inboard. If I put a 8" rim with 5" or even 5.5" bs I could put a wider tire on provided I paid attention to the section width of the tire.
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post #14 of 15 (permalink) Old Nov 28th, 15, 9:17 PM
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Post Re: Offset & Backspace - How Do They Relate?

I want to install Foose PVD Chrome Legends. I think they only come in 17x8. However, the non PVD Chrome come in 17x7 and 17x8. I want to install BF Goodrich G-Force Comp 2A's
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post #15 of 15 (permalink) Old Nov 30th, 15, 11:27 AM Thread Starter
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Re: Offset & Backspace - How Do They Relate?

Quote:
Originally Posted by 69 L79 View Post
I want to install Foose PVD Chrome Legends. I think they only come in 17x8. However, the non PVD Chrome come in 17x7 and 17x8. I want to install BF Goodrich G-Force Comp 2A's
Thank you for the additional information.

According to the manufacturers technical specs, the Foose Legend (PVD) is available in 17x7 +1 (4" BS) and 17x8 +1 (4.5" BS) in the 5-120.65 bolt pattern. From what we've measured, you shouldn't have any issues running the tires sizes you proposed earlier on your application, even lowered
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