Why the numbers don't add up!

It seems there has been some confusion lately as to how wheel specs are derived. I know I'm confused! The evil specs I am talking about are backspace and offset. I posed the question directly to the Tirerack. Even though they have some help files out there, it seemed that some data was still missing and things still didn't add up. So, here's how they come up with the numbers and hopefully explain where the missing digits are hiding.

Wheel width - is the width between the inner wheel mounting lips, or the inner verticle area where the tire bead seats. It is not the overall width if the wheel.

Centerline - is the line directly in the center of the wheel width.

OFFSET is measured in realtion to the centerline
If the wheel mounting flange is more toward the front of the wheel, compared to centerline, the offset is positive.
If the wheel mounting flange is more toward the rear of the wheel,
compared to centerline, the offset is negative.

BACKSPACE is measured from the wheel mounting flange to the most rearward part of the wheel.

I think this is where the numbers get messed up, at least how it was explained to me. If the rear wheel lip is THICK, this could account for missing digits comparing offset to backspace. Especially in the perfect condition of an 8" wheel w/ 4.5" backspace w/ 0 offset. This simply means that the rear wheel lip is .5" or 12.7mm thick.

Here's a picture to help explain

 

good post

 

You are correct ... The single most common error in calculating wheel dimensions and fitment is failing to take the lip width into account.

 

for reference the image in post 1 has a positive off-set (wheel mounting surface is on the front side of the wheels center line). This creates a larger backspacing, siting your wheel further inside the wheel well.

 

Here's a really cool site...

https://www.rsracing.com/tech-wheel.html