Offset & Backspace - How Do They Relate? - Vintage Mustang Forums

 
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post #1 of 9 (permalink) Old 12-30-2010, 03:48 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

Last edited by Discount Tire; 02-12-2011 at 01:41 PM.
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post #2 of 9 (permalink) Old 02-11-2012, 05:03 PM
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Not all true

at least as far as wheels from vintage wheel works go. everything is essentially right. but here's the reality i personally experienced. the 1" of total bead on the wheel is not included in the advertised wheel width. This is .5" of bead on the backside and .5" of bead on the front side. So a 10" rim is really 11". Now I calculated everything based upon the advertised 10" width. By calculated, I mean I measure my rear wheel wells, taking into account the mounting surface, leaf spring clearance, tire on wheel width(see bfg site for this). I ordered 4.5" backspace. And they came with 4.5" back space. But my backspace ended up being off by 0.5" inches with respect.

So, use the advertise wheel width to get mounted tire width. But use wheel width + 1" to calculate how the tire will center in your wheel well.
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post #3 of 9 (permalink) Old 08-30-2013, 10:51 PM
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Question Wheel Spacers

So what is the general consensus on using wheel spacers to make up for offset?
Found the wheels i want but in order to make them fit i will need a 35mm spacer....

Its fun, when it runs......
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post #4 of 9 (permalink) Old 02-02-2014, 02:02 PM
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Quote:
Originally Posted by Pynikal View Post
So what is the general consensus on using wheel spacers to make up for offset?
Found the wheels i want but in order to make them fit i will need a 35mm spacer....


Are spacers a usable item with front Disc brakes?
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post #5 of 9 (permalink) Old 02-02-2014, 11:56 PM
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I believe so. All the spacer does is put more space between the hub and the wheel. It does not move the rotor or caliper in any way.

Its fun, when it runs......
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post #6 of 9 (permalink) Old 03-26-2014, 09:36 PM
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Okay Matt

Help me understand this. I measured my 67 styled steel wheels originals and chipped. The approximate wheel width measuring with a tire mounted and measured from the filleted outside rim to inside filleted rim. The bottom of that fillet not the top or outer most fold of the wheel edge.

I got 6.5 inches total.

Measure to hub

I got 3.75

cut by half the 6.5 to find center at 3.25

then subtract 3.75 that makes my offset -.05 ( negative ) is this correct ?

so by your graphic. My hub shoves outward to the outside of the car by half an inch?



Quote:
Originally Posted by Discount Tire View Post
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 #7 of 9 (permalink) Old 03-28-2014, 12:22 PM Thread Starter
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Quote:
Originally Posted by malstrom View Post
Okay Matt

Help me understand this. I measured my 67 styled steel wheels originals and chipped. The approximate wheel width measuring with a tire mounted and measured from the filleted outside rim to inside filleted rim. The bottom of that fillet not the top or outer most fold of the wheel edge.

I got 6.5 inches total.

Measure to hub

I got 3.75

cut by half the 6.5 to find center at 3.25

then subtract 3.75 that makes my offset -.05 ( negative ) is this correct ?

so by your graphic. My hub shoves outward to the outside of the car by half an inch?
X = Backspace: The distance from edge of rear (inner) wheel lip to mounting pad.
Y = Wheel Width: The distance between front and rear bead flanges.

Offset = [x - ((Y+1)/2]*25.4

Given how you measured your wheel width, your wheels are 5.5" wide (bead flange to bead flange, roughly). So by following the math the center would be at 3.25". With a backspace of 3.75", this means the offset would be +12.7mm (1/2")

[3.75-((5.5+1)/2]*25.4 = 12.7mm
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post #8 of 9 (permalink) Old 03-11-2015, 09:51 AM
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If you want to cheat:

Offset to backspacing conversion

Justin Novick
Cool, Shiny Things
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post #9 of 9 (permalink) Old 07-31-2016, 09:33 AM
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