How do you convert mm offset to inch backspacing?

[4birdman]

Is there some magic conversion or something? I have a set of Sport Max 962s in 18x9.5 all around on my 98 Trans Am. They are great looking wheels, but the clear is peeling off in big flakes and they are starting to look rough. Sucks since they have maybe 15k miles on them and the only time it has seen rain is when I get caught heading home from a show. It sits covered most of the time. I have the very first set made, so I guess I got the manufacturing defects or something.

Anyway, I am thinking about picking up a second set of wheels so I can have some variety, and I can refinish my wheels and keep them in case I want to swap back and forth.

The wheels currently have 35mm offset, and I'd really like to keep the same stance. Can anyone help me out?

[Project69]

Go there, you can enter your wheel specs and it will convert your offset to inches.



http://www.rims-n-tires.com/info_spe...20/%20Comparer

[Apogee]

Is there some magic conversion or something?

Yep...it's magic. It's called math.

If you don't want to use an online calculator, just convert your offset dimension to be consistent units, inches in this case, 25.4mm = 1 in, then add half of the wheel width if it is a positive offset and you've got the backspacing dimension.

For example, 35mm = (35mm)/(25.4mm/in) = 1.378 in
1/2 Wheel Width = (.5)*(9.5 in) = 4.75 in
BS = (4.75 in) + (1.378 in) = 6.128 in

[4birdman]

Psh, math. I'm a designer, I leave the math to the engineers.

Thanks for the help!

[Bigblue73]

It's the other way around.....you need to prove the engineers wrong!

[4birdman]

I left the engineering field for design for a reason! I hate math!

[TonyL]

Use this handy calculator

[SickSpeedMonte]

Yep...it's magic. It's called math.

If you don't want to use an online calculator, just convert your offset dimension to be consistent units, inches in this case, 25.4mm = 1 in, then add half of the wheel width if it is a positive offset and you've got the backspacing dimension.

For example, 35mm = (35mm)/(25.4mm/in) = 1.378 in
1/2 Wheel Width = (.5)*(9.5 in) = 4.75 in
BS = (4.75 in) + (1.378 in) = 6.128 in

Dont you also have to add the width of the wheel "lip" (that the tire seats on)? Usually about 1/4"

The rim width is measured from inside of that lip, where as backspacing is measured to the outside.

http://home.earthlink.net/~bzdel/TireWheel.htm

[Apogee]

Dont you also have to add the width of the wheel "lip" (that the tire seats on)? Usually about 1/4"

The rim width is measured from inside of that lip, where as backspacing is measured to the outside.

http://home.earthlink.net/~bzdel/TireWheel.htm

Yes, you are correct. I should've used the actual wheel width, outside to outside, not the nominal wheel width noted by 4birdman. My 19x10 C6 wheels are ~11" wide outside to outside, so the inner lip definitely makes a difference when specifying backspacing dimensions...that's why I use offset as much as possible.

Tobin
KORE3

[Boesch]

As mentioned by others, wheel width is measured from bead seat to bead seat. Outside to outside will generally be about 1" wider than the actual rim width. Backspace is measured from the outside.

[Turbo67camaro]

Or, use the Google.com search feature, which acts as a calculator also.

Examples (key and click Google search):

245mm in inches
result -> 245 millimeters = 9.64566929 inches

11 inches in mm
result -> 11 inches = 279.4 millimeters

It will handle most conversions you'll ever need.

Unfortunately, it will not fetch you a beer though :(

[DriverzInc]

it's rather simple if you haven't figured it out already. For offset, or backspace you include the wheel beads on each side, which is .50". So an 8" wheel will measure 9" overall. Now, say that 8" wheel has a 4.5" backspace. That also means that it has a 4.5" front space. (4.5" + 4.5" = 9") So the mounting surface of the wheel sits centered between the two rim edges. This is a 0mm offset. Now say, you have an 8" wheel with a 12mm positive offset. This means that the wheels mounting surface is pushed outboard towards the front of the wheel by 12mm. 12mm is roughly .50 of an inch, so a +12mm offset is the equivalent to a 5" backspace on an 8" wheel.

Hope that helps.