I thought I'd do a bit of a guide to the basics of measuring wheels. Hope it's useful.
Width
Width is measured between the bead seats, not the overall width of the wheel.
The wheel below is 6" wide (quite handily confirmed by the manufacturers stamp on the barrel).
PCD
PCD (Pitch Circle Diameter) is the diameter of the circle that can be traced through the center of the mounting holes.
This is most easily measured on four stud cars by measuring from the outside of one stud hole, to the inside of the opposite one. Using the hole edges give a good accurate datum to work to, rather than trying to find the centre of the hole.
Centre bore is that hole in the middle of the wheel.
Then it gets a bit tricky...
Offset
Offset is how far the mounting face of the wheel (the inside bit that presses against the hub) is offset (moved in or out from) the centre line of the wheel.
To calculate offset you will need to know two things, the width of the wheel, and the back space (distance from the mounting surface to the inner edge of the wheel. You can take these measurements two ways, the overall width, and to the overall inner edge of the wheel from the mounting face, or to the bead seat. Whichever you choose make sure that you do the same for both.
On this wheel the overall width was 177mm
The back space was 105mm
Now you can use a simple equation to calculate the offset:
Backspace - (Width/2)
If you get a positve answer, you have a positive offset, if you get a negative answer you have negative offset.
I calculated the offset of the trial wheel using the bead seat and the inner edge,
Considering I had to guestimate the place to measure the backspace to the bead seat I'm pretty pleased that I was only 0.5mm out between the two.
Width
Width is measured between the bead seats, not the overall width of the wheel.
The wheel below is 6" wide (quite handily confirmed by the manufacturers stamp on the barrel).
PCD
PCD (Pitch Circle Diameter) is the diameter of the circle that can be traced through the center of the mounting holes.
This is most easily measured on four stud cars by measuring from the outside of one stud hole, to the inside of the opposite one. Using the hole edges give a good accurate datum to work to, rather than trying to find the centre of the hole.
Centre bore is that hole in the middle of the wheel.
Then it gets a bit tricky...
Offset
Offset is how far the mounting face of the wheel (the inside bit that presses against the hub) is offset (moved in or out from) the centre line of the wheel.
To calculate offset you will need to know two things, the width of the wheel, and the back space (distance from the mounting surface to the inner edge of the wheel. You can take these measurements two ways, the overall width, and to the overall inner edge of the wheel from the mounting face, or to the bead seat. Whichever you choose make sure that you do the same for both.
On this wheel the overall width was 177mm
The back space was 105mm
Now you can use a simple equation to calculate the offset:
Backspace - (Width/2)
If you get a positve answer, you have a positive offset, if you get a negative answer you have negative offset.
I calculated the offset of the trial wheel using the bead seat and the inner edge,
Considering I had to guestimate the place to measure the backspace to the bead seat I'm pretty pleased that I was only 0.5mm out between the two.
