Offset and Backspacing

[TexasBlaZeR2]

Everyone seems really confused about offset and backspacing. I was too, so I sat down and tried to figure it out mathematically. Let me try and explain it here the way I understand it:

First of all, the reason that offset and backspacing are so important is because the wheels on our trucks are more like wheels on front wheel drive cars than rear wheel drive. A large amount of backspacing is needed to properly accomodate the four wheel drive components. If you don't have enough backspacing wheels will rub.

Unfortunately, a lot of cool aftermarket wheels will not fit our trucks due to inadequate backspace and offset. The set of wheels on eBay should fit with no problem because they are close to stock.

Now comes my attempt at explaining some of these terms:

Backspacing and offset are dependent on the width of the wheel. Stock wheels and some aftermarket wheels are 15x7. A lot of aftermarket wheels are also 15x8.

Here goes:

15X7 CALCULATION

Stock wheels are 15 x7". Therefore, the centerline of the wheels is in the middle at 3.5".

|-------| 7" width
|---| 3.5" centerline

Now we need to convert this to mm. There are 25.4mm in an inch.

Converting from inches to mm for the whole wheel = 7 * 25.4mm/in = 177.8mm

Converting the centerline distance from inches to mm = 3.5 * 25.4mm/in = 88.9mm

Therefore, the centerline of a 7 inch wheel is at 3.5" or 88.9 mm.

Offset is the distance from this centerline to the surface of the wheel that mounts to the vehicle.

There are currently a set of wheels on eBay for auction that have the +35mm offset.

Custom S-10 wheels on eBay

Backspacing is the distance from the mounting surface to the back of the wheel. To calculate this, we need to add the centerline distance plus the offset:

88.9mm + 35mm = 123.9mm backspacing

now we can convert from mm back to inches

123.9mm * 1in/25.4mm = 4.88in

However, you also need to add an extra 1/2" for the bead of the wheel. Therefore, for a 15x7 wheel, you actually have a 4" centerline:

Actual backspacing is 101.6+35=136.6mm or 5 3/8" BS

15X8 CALCULATION:

Now let's do the same calculation for a 15x8:

For an 8 inch wheel, the numbers are different:

Most aftermarket wheels are 15 x8". Therefore, the centerline of the wheels is in the middle at 4.0".

|-------| 8" width
|---| 4.0" centerline

Now we need to convert. There are 25.4mm in an inch.

Converting from inches to mm for the whole wheel = 8 * 25.4mm/in = 203.2mm

Converting the centerline distance from inches to mm = 4.0 * 25.4mm/in = 101.6mm

Therefore, the centerline of a 8 inch wheel is at 4" or 101.6 mm.

Offset is the distance from this centerline to the surface of the wheel that mounts to the vehicle.

Again, using a +35mm offset

Backspacing is the distance from the mounting surface to the back of the wheel. To calculate this, we need to add the centerline distance plus the offset:

101.6mm + 35mm = 136.6mm backspacing

now we can convert from mm back to inches

136.6mm * 1in/25.4mm = 5.38in

However, remember that you also need to add an extra 1/2" for the bead of the wheel. Therefore, for a 15x8 wheel, you actually have a 4.5" centerline:

Actual backspacing is 114.3+35=149.3mm or 5 7/8" bs

So, you can see, that the same wheel in 8 inches with the same offset requires 5 7/8 in backspacing

Most of the aftermarket wheels which are available for our rigs are 15x8. However, most of them also stick out too far because they only have something like 3.75 to 4 inch backspacing. Very few 15x7s are available for our trucks. However, with a 15x7 you can usually get away with less backspacing...remember that the centerline distance of a 15x7 is a full half inch smaller than for a 15x8.

Here is a link that shows a picture too:

Picture of offset and backspacing

If I am incorrect in any of my math, please let me know.

Thanks,

[jeff024]

this is the size of a stock s-10 CC wheel
Stock wheel size: 15" x 7"
Stock backspacing: 6"
Stock offset: +2.5" (63.5mm)
Pattern Circle Diameter: 5x4.75" (120.65mm)
Thread size: 12mm x 1.5
Pilot diameter (center bore): 2.75"

[TexasBlaZeR2]

Is that the same for N90 and ZR5 wheels?

What is your opinion of a wheel with +35mm offset?

[jeff024]

yes its the same for both wheels as far as options I just called the place I got mine from and had them give me a list of what would work and went from there

[TexasBlaZeR2]

Okay. According to the calculations above for a wheel with +35mm offset:

15x7 with a 4"centerline (including 1/2" for the bead) would be 101.6 + 35 = 136.6mm or 5 3/8" BS

15x8 with a 4.5" centerline (including 1/2" for the bead) would be 114.3 + 35 = 149.3mm or 5 7/8" BS

So, that does seem reasonable. A wheel with +35mm offset should work.