Offset Vs. Backspacing?

[camino70]

I am not sure if I am thinking about offset correctly and whenever I talk to a wheel shop I get different answers and become more confused
I have been using an online calculator to calculate offset given the width and backspacing: http://www.geocities.com/braveheart_808/'Rim Calculator'!A1

What I come up with for the front is 8" w/ 4.25"bs gives -13mm
8.5" w/4" bs yields -13mm as well.
For the rear I get -13mm as well, 9.5" w/4.75"bs.

Am I doing this correctly? Anytime I try to talk to a local wheel shop, they never have any clue about backspacing and talk only in reference to offset and I want to be clear with it before I order anything.

Thanks for any help.

Ron

 

[stickybones]

Let's see if we can make this simple.

Backspacing is the distance from the mounting surface of the wheel to the OUTER edge of the inside of the rim.

Offset is the distance from an imaginary plane running thru the center of the wheel to the mounting surface.

Check out the image at the bottom of this page on gnttype.org

Wheel

In this figure, rear spacing is the same thing as backspacing.

 

[camino70]

Thanks,
That is the way I understand it, but my calculations don't match that of what people are trying to sell

I am thinking they are less then honest, but I am not 100% on my calculations.

Ron

 

[whitess]

Ron,
It confusing/wierd to say the least.
Some places use "offset" and some use "backspacing" for the specs. Here's some conversions for reference:
Wheel width---------Backspace-------------Offset
....7"...........................3.75".....................-6mm
....8"...........................4.00"....................-12mm
....8.5"........................4.00"....................-19mm
....8.5"........................4.25.....................-12mm
....9"...........................4.50"....................-12mm
For example, if you want an 8" wide rim with a 4" backspacing the offset will be -12mm, or a 9" rim with a 4.5" backspacing will be a -12mm offset. It's kind of screwy but that's how they do it. If you come up with any other sizes you need converting let me know. I have a chart off the net but the link doesn't work anymore. I printed it out so I still have it.
HTH.
Larry.

 

[Leviathan]

Everything you wanted to know about wheel fitup, includes diagrams and descriptions.

Wheel fitment technical definitions

 

[camino70]

Thanks guys,
Lev, the link is great!
Larry, that is pretty much what I was getting, but no one I talked to was able to confirm or deny them, wasn't sure if I was crazy or them...

Thanks again.
Ron

 

[49-blues]

Definently weird!

 

[Pronto]

Someone could make a lot of $ if they came up with a cheap plastic tool to measure what rims/tire combos would fit on your car. I thought I saw some kind of caliper type of tool on a car show once that had scales on it to measure and test each diamention.

 

[GN ERGY]

Leviathan, Thanks, thats some good info.
Tarey D.