Thanks for the kind words and info. I can figure the metric conversion part but what I'm in the dark about is the offset verses b.s. example, a 15x7 with +38 mm offset. How do you determine the back spacing. Do you deduct the 38mm(1.5 inch) from the 7 inch width?
OK, bare with me.I think I may have this figured out. The offset is measured from the center line of the wheel to the mounting surface. In above example of 38mm(1.5 inches) the mounting surface will be 1.5 inches to front side of wheel plus 3.5 inch(1/2 of 7" width) from center line to back side of wheel equals 5 inch backspacing.
Am I close to being on the right track here?
BS is the distance from the mounting plate and the inner edge of the rim surface.
Offset is the distance from the wheels center line and the outer edge of the rim.
You would have to take into consideration the distance between the mounting plate in the wheel and the distance to the wheels centerline when working any equation with the wheels offset.
I know it all sounds pretty daunting, but I think most of it is because of the investment involved. The last thing you want to do is drop that dime on those shinny new wheels and hydes only to find out they do not fit or rub the wheel wells.
I was not so concerned with the offset when I did my calculations. I concentrated my efforts inbound and outwards. So taking the posistion of the mounting posistion on my diff. loaded on the perches. I determined the max depth I could obtain without clearance issues then measure outward to determine the max wheel size I could get in there without issues on the other end. Offset was not even a consideration, it just eneded up being what it was. I knew I was not going to get a deep dish in there and I did want a bit of a relief.
I ended up with an 8" rim would prefer a 7" in the front so I can loose the spacers. Also take into consideration I was a non traditionalist when I set up this Falcon I run 17" wheels. Many of you Falconites frown on that.
Don't forget to take into consideration how much your tires will protrude past the edges of the rims.
I saw this on line and hope it shines some light on this topic:
Using a 10-inch-wide Niche Over Ride wheel as an example, we can easily find the offset using the backspace measurement.
Wheel centerline (wheel width divided by two) and backspacing
Backspacing minus wheel centerline equals offset
If the answer is negative, then you have a negative offset; if it's positive, you have a positive offset. A 10-inch-wide Niche Over Ride has a wheel centerline of 5 inches and the backspace is 6-1/2 inches
6-1/2 (backspace) minus 5 (wheel centerline) equals 1-1/2 inches (positive offset)"