Re: Wheel math (backspacing and width)
[Re: ahy]
#828500
10/12/10 11:07 PM
10/12/10 11:07 PM
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Joined: May 2005
Posts: 598 NC, USA
davenc
mopar
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mopar
Joined: May 2005
Posts: 598
NC, USA
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Could someone demonstrate the math on this? The way I look at it, the 15 X 7 rims should result in the tire sticking out 1/2 inch more than his current 15 X 8 rims. Here is my reasoning: a 15 X 8 rim which is perfectly centered on the hub has a 4.5" BS. This is because the backspace is measured from the rim edge, which is 9" wide on a 8" rim (8" is the inside distance where the tires seats). Therefore, his 5.5" BS rim has the wheel centerline moved in towards the springs by 1".
Now, the new 15 X 7 rims. A perfectly centered rim should have a 4" BS, because again of the 1" difference between the outside width of the rim and the inside width of the rim. If the 15 X 7 rim has an actual BS of 4.5", that means the rim centerline is only moved 0.5" in towards the spring.
If the tire section width is the same, and the tire is centered on the rim (always is), then the outside edge of the tire section width will stick out 0.5" more.
Granted the more narrow rim will actually decrease the section width some, but my guess is this will not totally account for the offset change.
Is there a flaw in this reasoning?
Dave
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Re: Wheel math (backspacing and width)
[Re: davenc]
#828501
10/12/10 11:46 PM
10/12/10 11:46 PM
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Joined: Jan 2003
Posts: 8,398 Highland, MI.
Sunroofcuda
master
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master
Joined: Jan 2003
Posts: 8,398
Highland, MI.
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Factory 15x7" Mopar rims SHOULD have a 4.25" backspacing - any OE ones I've measured anyway.
No Man With A Good Car Needs To Be Justified
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Re: Wheel math (backspacing and width)
[Re: chache876]
#828503
10/13/10 12:14 AM
10/13/10 12:14 AM
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Joined: Feb 2009
Posts: 182 NW of Indy
Thackdaddy
member
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member
Joined: Feb 2009
Posts: 182
NW of Indy
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There are two things to consider, wheel backspace and offset backspace, To find the offset, start by determining the wheels' centerline. To obtain that, measure the wheel's overall width from one outside edge to the other. This is not always the same as the width given in the manufacturer specs. For example, a width of 15x8 inches refers to the distance between the inner flange and outer flange. Next, divide the width in inches by 2 to find the centerline. If the actual width of our example wheel is 9 inches, its centerline would be at 4.50 inches. Then, subtract the centerline to get offset 9 - 4.5 = 4.5. 1: Measure the room in your wheel wells relative to the wheel hub. 2: Backspace = The distance from back of the wheel to the wheel's mounting space, in inches. 3: Centerline = The wheel width divided by 2, in inches. 4: Offset = centerline - backspace, which will be positive or negative, in inches. 5: Offset in Millimeters = Offset x 25.4 (they use mm in their literature) So...measure your wheel, then measure the new one and you will know exactly how it will fit. In your post you say you are going from a 15x8 with 5.5 backspace to a 15x7 with 4.5 backspace, if those numbers are correct you will only gain half inch.
Last edited by Thackdaddy; 10/13/10 12:47 AM.
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Re: Wheel math (backspacing and width)
[Re: Thackdaddy]
#828504
10/13/10 08:14 AM
10/13/10 08:14 AM
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Joined: May 2005
Posts: 4,730 Florida
BDW
master
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master
Joined: May 2005
Posts: 4,730
Florida
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