Ok Ive searched for the answer to this one and Im stuck.
Looking at a set of American Outlaw Buckshot wheels with the following specs.

17X9 5-127 15MTB

So can someone help me understand the backspacing of 15? Is this mm?

Thanks in advance and hopefully I didnt miss this answer somewhere else in the forum.

 

The 15 is in mm. Check out this site. It really helps with understanding wheel sizing:

h ttp://www.rimsntires.com/specs.jsp

 

The 15m is offset. It is the measurement of how far from center instead of how far from the back of the rim the hub is. For positive off set you take rim width and add 1" for the lip on both sides and divide by 2 for the distance from back to center then add the offset. Since it is MM you divide by 25.4 so 15/25.4=.61 add offset to center for backspacing and you have 5.61" of backspacing.

The formula looks like this. (Width + 1)/2+(offset /25.4) so (9+1)/2+(15/25.4)=5.6"

 

Thanks guys. While I had a basic grasp of the concept of backspacing, I was lost with the "15" measurement.
So with the 5.6" of backspacing Im thinking I need to look at some spacers as well.

 

Depending on what tire width but if you are up in the 12.5" or 70mm range then yeah you will be about an inch shy on those. Don't know how set you are on that rim but you might want to look at a 17x8 like the Black Rock d-window I've got. Kind of the same look without the gun theme but 4.5" back.

 

I totally agree.. this whole backspacing thing had me going bonkers when I first starting looking into it.

 

It's the kind of thing I don't mind explaining repeatedly cause I forget it too if I go too long without working the numbers. Don't know why they don't just list backspacing so we don't have to do the math part. Mine were easy though. 8" wheels 0 offset, add one inch for each side lip and divide by 2 for center is 4.5".

 

Thanks for all the input Keith. Since Im still working out the tire choice, (between 35x12.5x17, or a 315/70/17) Ill still have to be sure of my calculations when I get the tires\wheels and decide on a spacer at that point.
And I agree, make our lives easier and just list the backspacing. I hate math. :-)

 

Ok now I want to make sure Im using the tool shown here correctly. I apoligize in advance if Im being dense.

With another wheel as an example.

KMC XC Rockstar 17x9 - Offset =35, centerbore 72.6

So the math here goes: (Width + 1) / 2 + (Offset/25.4) = backspacing
(9 + 1) / 2 + (35/25.4) = 2.96

The only Question is does the centerbore number come into play as you mentioned by taking it (72.6) and adding it to the backspacing total that came from the formula shown above?

So would it be : (9 + 1) / 2 + (35/25.4) = 2.96 + (72.6/24.5) ?

 

The centerbore is the diameter of the inside of the wheel where it meets the hub. It just needs to be bigger or the same size as the hub or it won't fit but does not affect backspacing.

The numbers you have 9" with 35 offset would be
(9+1)/2 * (35/25.4)= 6.38" backspace which is a lot for a 12.5" tire on a JK

Looking at their website I don't see an XC but an XD at 17x8 with a 35 offset or 5.8" backspace but the 17x8 with a 10 offset, 4.8" backspace which is better.

They also have the 17x9 with -12 offset. That would be a 4.53" backspace which is also good.