Backspace and Offset: The Final Frontier

[cruiserguy13]

i've been trying to figure out the whole backspace and offset thing...i've read so many threads and clicked on so many links i'm going a little crazy...

i'm hoping that we can get this whole thing figured out in order to be better educated consumers when shopping for wheels...

so here is what i've figured out so far...i'm not sure this is right so if someone knows more please speak up.

offset = backspace - wheel width/2

based on this equation and wheel width and backspace given by the manufacturer for the 3 following wheels i have figured out the following

wheel 1
wheel width = 8"
centerline = 4"
backspace = 4.5"
offset = 0.5" = 12mm
mfg stated offset = 0" = 0mm

wheel 2
wheel width = 9"
centerline = 4.5"
backspace = 4.75"
offset = .25" = 6mm
mfg stated offset = .25" = 6mm

wheel 3
wheel width = 9"
centerline = 4.5"
backspace = 4.5"
offset = 0" = 0mm
mfg stated offset = .5" = 12mm


Based on the above info my calculations show that only wheel 2 has the correct dimensions as stated by the mfg.

Either that or the equation I'm using is wrong...Or maybe i made some errors in calculating the offset.

Can anyone confirm or refute what is posted above?..Please post ideas and comments.

Let's try to get this figured out!

[BellyDoc]

You're probably better off stating your "equation" as:

offset = backspace - 1/2(rim width)

This is because having the bolting plane INSIDE the centerline is considered NEGATIVE offset. Positive offset is when the bolting plane is outside the centerline.

Therefore, you want the smaller backspace number minus the distance to centerline to come out negative while the bigger backspace number minus centerline should come out positive.

Otherwise, your logic is fine.

I've seen manufacturer's numbers off like that before too. I have to conclude that one (or more) of three possibilities is true:

1) They're screwing up on reporting the real backspace value.
2) They're screwing up on reporting the real offset value.
3) They're measuring the two values in some different way so that they're not equivalent.

An example of (3) would be if the rim had some excessive metal on the inner and outer lip, but it was DESIGNED as an 8" wide rim, and that's where it holds the beads of the tire. The real overall width of the wheel is 8.5", and when they report a 4.25" backspace, they mean that it's 4.25" from the innermost metal material to the bolting plane. They want to specify this because they want you to be ready to have everything out of the way that far in from the hub surface. On the other hand, the wheel may be centered on the bolting plane yielding 0.0" offset...

I'm just theorizing. I prefer to assume that people aren't actually idiots until they give me no other choice.

[cruiserguy13]

ok...i edited the calculations and the equation.