Wheel offset/backspacing question

[super-gt]

Ok, I'm talking to a salesman from Iforged for my custom wheels I'm ordering and I need to know the offset of the wheels.

Fronts are 18X7 with a 4 inch backspacing

Rears are 18X12 with a 4 inch backspacing

-Now at first I thought that I just took rim width(12 inches)
-Divided it by 2 to get my centerline( 6 inches)
-Than subtracted by my backspacing(4 inches)
-This yields a negative 2 inch offset, or -51mm.

The only problem I have with this is that the salesman is telling me this offset is impossible.

Now I found a wheel offset vs. backspacing chart here:

This chart shows the offset for a 12 inch wide wheel with 4 inch backspacing to be -63mm.

So basically, I am missing about 12mm from my offset spec.

I realised that 12mm is about equal to a half an inch.

Then after looking around abit it seems that wheels are usually about an inch wider than they actually say they are if you measure the overall wheel width instead of just where the tire mounts.

With this tid bit of information, I can account for my missing half inch or 12mm of offset.


So now I ask you guys, am I doing this correctly?

Would my offset infact be -63mm on an 18x12 wheel with 4 inch backspacing?

And going off the same principle, my 18X7's with a 4 inch backspace would have a 0mm offset?

-Taylor

 

[jjorgensen52]

Re: Wheel offset/backspacing question

Unless I am mistaken, backspacing is usually measured from the inside face of the rim and offset from centerline. I might be wrong though. I run 8" width rims on my GT with 13mm of offset and they are flush to fenderline.... if that helps at all

 

[Don W.]

Re: Wheel offset/backspacing question

Try this:

Our fronts are 41mm, rears are 13mm

http://www.nice-wheels.co.uk/pcdandoffset.html

I also think backspacing is measured from a straight edge across the (inside) edge of the wheel flange to the mounting surface.

 

[super-gt]

Re: Wheel offset/backspacing question

Hmm...ok so when I measure backspacing on a wheel, I use the actual wheel width and not just the manufacturer stated wheel width? This is where I am having a problem is that it seems when manufacturers talk about wheel width, they talk about where the tire mounts on the rim and not the overall rim width from the outside edge to outside edge.

If this is the case and I do have to measure backspacing from overall width, then that would put my actual wheel width for a 12 inch wheel closer to 13 inches. In which case, I would use a backspacing of about 4.5 inches instead of 4. with the 4 inch backspacing, I would end up with -51mm offset again.

 

[jjorgensen52]

Re: Wheel offset/backspacing question

Hmm...ok so when I measure backspacing on a wheel, I use the actual wheel width and not just the manufacturer stated wheel width? This is where I am having a problem is that it seems when manufacturers talk about wheel width, they talk about where the tire mounts on the rim and not the overall rim width from the outside edge to outside edge.

If this is the case and I do have to measure backspacing from overall width, then that would put my actual wheel width for a 12 inch wheel closer to 13 inches. In which case, I would use a backspacing of about 4.5 inches instead of 4. with the 4 inch backspacing, I would end up with -51mm offset again.

Yes, that's correct for the measurement. But if your width is 13, your centerline is now 6.5 and with your 4 inch backspace you've got 2.5 inches of offset, or -63mm as would agree with the chart

 

[super-gt]

Re: Wheel offset/backspacing question

Yes, that's correct for the measurement. But if your width is 13, your centerline is now 6.5 and with your 4 inch backspace you've got 2.5 inches of offset, or -63mm as would agree with the chart

Yea, but I originally figured on a 4 inch backspacing when I was using the measurment of 12 inches. If I figure on 13 inches than that means the wheel will stick out another half inch on the inside and the outside making my total rear end width another inch wider. I'm trying to keep my total rear width around 63 inches. Assuming the wheel is 13 inches wide, the rear end is 45.5 inches wide from axle face to axle face, and I use a 4.5 inch backspace, that should bring me to approx 62.5 inches of width. So the wheels will sit in the fender well about 1/4 of an inch more than they used to. Since the truck will sit lower and have taller tires, this will be a good thing as it will provide more tire/inner fender clearance.

lol....sorry, done rambling..

Thanks for the input guys, I appreciate it. I've posted on a few different forums and it suprises me how little feed back I've gotten. I think offset and backspacing scare people away or something...