Calculating Offset from Backspacing

[CJay]

I'm trying to order a set of Adaptec adapters, and they've been very helpful. Brad's trying to help me find how thick I need my adapters to be. The problem is I have no idea what offset my wheels are.

I measured the width and the fronts appear to be 7.5s, while the rears appear to be 8.5s. The 7.5s seem to have a little less than 5" backspacing, while the 8.5s are right on the dot at 5" backspacing.

I checked this thread and found the Offset/Backspacing chart very helpful. According to the chart, my 8.5" wheels with 5" backspacing would result in it being ET+6. Did I figure this out correctly, or did I do something wrong somewhere?

However, the fronts are a mystery to me. Here are some pictures, and hopefully someone can help me out.

The 7.5" wheel as measured from bead to bead.


The markings inside the 7.5" wheel. Not very helpful..


Backspacing on the 7.5" wheel.




Here's the 8.5" wheel as measured from bead to bead.


Backspacing on the 8.5" wheel.


I also found this calculator and I put in some mock numbers. I think the rears are accurate, but the fronts seem to be off.



Hope someone can help so I can make it easier on the guys at Adaptec Speedware.

[DrieStone]

Your calculations seem close. Keep in mind, though if you're measuring from the mounting pad to the edge of the bead, then you need to make sure you calculate using the width of the wheel including the bead. The offset is measured as mm from the centerline of the rim (it doesn't mater if you include the bead or not, the centerline doesn't change, well unless your bead is a different size on the inside and outside).

So using the numbers in the screenshot above, the centerline of your front wheel is at 102.5mm (measuring from the edge of the bead), your mounting pad is 129mm, so your front ET is actually ET+26.5 (but what's half a mm between friends?)

Your rear center line is at 112.5mm, your mounting pad is at 127mm, so your rear ET is actually ET+14.5.

[indy510]

.
Mounting pad has nothing to do with it ...... .. you need to measure the actual width of the wheel

a 16x8.5 wheel measures 9.5" wide........ and 4.75" backspacing would be a zero offset

so you have 5" backspacing ..... and rougly a positive quarter inch offset ..... or ~+6mm


A 16x7.5 wheel measures 8.5" wide ..... and 4.25" backspacing would be a zero offset

so you have about 5" backspace?? ..... . so a positive 3/4" offset .... or ~+18mm

[DrieStone]

.
Mounting pad has nothing to do with it ...... .. you need to measure the actual width of the wheel

a 16x8.5 wheel measures 9.5" wide........ and 4.75" backspacing would be a zero offset

so you have 5" backspacing ..... and rougly a positive quarter inch offset ..... or ~+6mm


A 16x7.5 wheel measures 8.5" wide ..... and 4.25" backspacing would be a zero offset

so you have about 5" backspace?? ..... . so a positive 3/4" offset .... or ~+18mm

The mounting pad is vital to it. Since offset is the difference between the mounting pad to the center line of the wheel. We're saying the same thing though, and I agree that the actual width of the wheel is vital to the calculation (or at least makes it easier). I don't know what wheels he's using, but my 8" wheels are not 9" wide, they're 8.75" wide. You can't assume that his beads are both 1/2" wide.

Assuming his numbers in the screenshot are correct. His wheel width for the front is 8.1" not 8.5" which his centerline us 4.05", his backspacing (edge of rim including bead to mounting pad) is 5.1", so he has a 1.05" offset (or 26.67mm).

His rear wheel width is 8.9" so centerline is at 4.45", his backspacing is 5". So his offset is 0.55" (or 13.97mm).

I know my numbers came out slightly off due to rounding, etc.

I would say that CJay needs to verify the total width of the rims he's using (bead to bead).

( [Total Width Bead to Bead] / 2) - [Backspace from Mounting Pad to Bead] * 25.4 = [Wheel Offset]

[CJay]

Thank you for the replies. My front wheels have a total width (including the beads) of 8 and 3/8ths" (8.375") and a backspacing from the mounting pad to the bead of 4 and 7/8ths" (4.875"). My rear wheels have a total width (including the beads) of exactly 9.5" and a backspacing of exactly 5".


( [Total Width Bead to Bead] / 2) - [Backspace from Mounting Pad to Bead] * 25.4 = [Wheel Offset]


Using this formula I got numbers like ET-17.4625 for my front wheels and ET-6.35 for my rear wheels. I feel like I did the math completely wrong? I never was good at math, lol.

[DrieStone]

Sorry... I flipped the sides that you subtract when I wrote it out.

[Backspace from Mounting Pad to Bead] - ( [Total Width Bead to Bead] / 2) * 25.4 = [Wheel Offset]

4.875 - (8.375 / 2) = 0.6875 (that's inches of offset, then multiply by 25.4 to covert to mm) = 17.46mm
5 - (9.5 / 2) = 0.25 (inches offset) * 25.4 = 6.35

So ET+17 and ET+6.

[CJay]

Ahh, that's fantastic, DrieStone. I really appreciate all the help. Thanks, indy510 as well.

I have already ordered the adapters from Adaptec. But, it is good to know what offset my wheels are at now. I ordered 15mm adapters all around. So final specs of the wheels will come out to:

16x7.5 ET+2
16x8.5 ET-9

Sounds good to me, can't wait to mount 'em.