maybe become a sticky???
But here's how..
a rim is 8" wide but 9" overall because of the bead lips(edge to edge and each bead lip is 1/2"), now the backspace is 4". The backspace is measured with that 1/2" lip from that 1 side of the rim.
The wheel is 228.6mm or 9"(25.4millimeters to 1 inch), the bs is 4" or 101.6mm(thats inches x 25.4= width in mm). Now the rims centerline is 114.3mm(half the overall width) and the backspace is 101.6mm, you'll know it's negative offset because the backspace in millimeters is less than the centerline of 114.3mm. Now subtract the 101.6mm backspace from the centerline measurement of 114.3mm and you get -12.7mm offset.
Its the distance measured between the bs and the wheels centerline. If that distance is towards the "show" side of the wheels centerline it's positive offset, if it's to the inside of the centerline it's negative offset.
Here's another..
8" rim with 6"bs.
9"/228.6mm total width
centerline is 114.3mm
backspace is 6"/152.4mm
since the backspace in millimeters is greater than the centerline you know it's positive offset, and it's 114.3mm subtracted from 152.4mm which is +38.1mm offset.
wheel width is measured from inside bead lips, or just take overall width and subtract 1" for the lips to get width.
hope I described it so not to confuse you :?
