Wheels can be a huge headache. Particularly, laying out the specifications to clear big brakes, custom suspension components, fit within fenders and allow full travel of the suspension.
One thing that got me frazzled in the past was offset versus backspacing.
I grew up learning from old men and working on classic cars. You never heard the term offset and anything metric discussed when looking for Cragars at the swap meet.
The term you would always hear would be backspacing.
It's a very simple measurement. Take the width of the wheel, divided by two and that would give you a wheel with a mounting surface dead center of the hoop.
Add an inch, and the wheel moved inboard an inch, subtract an inch and it moved outboard an inch. Simple enough right?
Fast forward to today, wheel shopping and looking at some contemporary options for my Fox Body Mustang. I find things like +25mm and -10mm describing offset.
First, I had to determine the difference between positive and negative offset.
Positive offset moves the mounting face outwards from the center plane of the wheel.
Negative offset moves the mounting face inwards from the center plane of the wheel.
In otherwords, +25mm offset would move the wheel hoop inwards towards the suspension roughly an inch. It would give a 10 inch wheel roughly 6 inches of backspacing. Make sense?
An offset of -25mm would move the wheel hoop outwards away from the suspension roughly an inch. It would give that same 10 inch wheel roughly 4 inches of backspacing.
Here's an illustration that might make it a bit more clear.
Keep in mind that all of these measurements are based off of the wheel's mounting surface in reference to the wheel's centerline.
Here's an example of a simple way to measure both terms.
Until I grasped the simple arithmetic of offset in the same way that I calculated backspace in my head, it never clicked.
Here's my little trick. 25.4mm or 2.54cm is equal to exactly one inch. I still think about it in rough inches unless it's something that needs laser precision. That helps me figure it out in my head and move on.