Can this be right?: Offset & Backspacing

[Otis Campbell]

I recently saw a wheel I was interested in from Raceline. Its a 16x8 with advertised 0 offset and 4.5" backspacing. My understanding of offset is that its the distance from the mounting surface of the wheel to the centerline of the wheel. In this case 0, so the mounting surface is on the wheel's centerline. 4.5" backspacing, as I understand it, is the distance from the mounting surface of the wheel to the back of the wheel. But, an 8" wide wheel with 0 offset leaves only 4" not the 4.5" that is advertised.... I wrote Raceline asking them but haven't received a response. Do I have a misunderstanding of the terms?

 

[DirtDonk]

Yes, that can be correct.
A full half an inch is quite a bit of difference, where the difference is typically more like a quarter inch.
But the main thing is that the measurements are taken from different points on the wheel. The only common point is the wheel mounting surface.
I think one measures from outside the bead (backspace?), and the other from inside the bead.
With certain aluminum wheels that have thick castings, this can equate to a bit of a difference. That’s why, for our Bronco purposes, I have always preferred backspacing over offset.
This gives you a hard, repeatable number to the back/inside edge of the wheel. Which is better for gauging clearance to a radius arm or body part.

At least that’s the way I believe I understand it. Be sure to let us know what the manufacturer says when they get back to you.

 

[jamesroney]

Yes, that can be correct.
A full half an inch is quite a bit of difference, where the difference is typically more like a quarter inch.
But the main thing is that the measurements are taken from different points on the wheel. The only common point is the wheel mounting surface.
I think one measures from outside the bead (backspace?), and the other from inside the bead.
With certain aluminum wheels that have thick castings, this can equate to a bit of a difference. That’s why, for our Bronco purposes, I have always preferred backspacing over offset.
This gives you a hard, repeatable number to the back/inside edge of the wheel. Which is better for gauging clearance to a radius arm or body part.

At least that’s the way I believe I understand it. Be sure to let us know what the manufacturer says when they get back to you.

@DirtDonk, you nailed this.

The backspace number includes the thickness of the rim at the sealing lip, which also includes any wheel weight flange feature as well. 1/4 inch difference is typical for a steel wheel, and 1/2 inch is typical for a modern aluminum wheel.

A 16x8 wheel with the mounting face directly in the center of the wheel (offset = 0) will measure 9 inches wide from outside-to-outside, and thus 4.5 inches from the back of the wheel to the mounting face. So yes, 16x8, ET-0, 4.5 BS is common.

One might wonder why anyone would buy a 16 inch wheel for a Bronco in 2024, since tire size is limited, and tie rod clearance is not much improved...but that's a different discussion.

 

[Apogee]

I'm with Paul preferring BS, mostly because it's something you can directly measure with a tape and a straight edge, whereas offset measures to a centerline that isn't marked on the wheel, nor exists with any physical feature to reference. X2 on the whole Ø16" tire selection issue...some decent load range E stuff, not much else.

 

[Broncobowsher]

8" wide wheel is 8" at the tire bead surface. But outside of rim to outside of rim is 9". The rim has ½" of lip. Standard part of wheel design. It's radiused to not cut into the tire.

Backspacing is done from the outside of the lip to the mounting surface. Not the tire mating surface to the wheel mounting surface. Thus the half inch error.

Until you start working with wheels and have to cross the backspacing and offset numbers it doesn't make sense and looks wrong. But after dealing with it for awhile, you learn the difference and it is normal

I hate it when they only list one or the other. There are times when I really need to know the backspacing and other times the offset means more,

 

[Yeller]

This is what happens when engineers and machinists get involved along with other cultures and countries in wheel descriptions and manufacturing, no disrespect intended to any of those professions, sometimes just doesn't make sense to the rest of the free world that doesn't calculate everything from a step file into CNC equipment from an XL file of days gone by. James is right it is from the outside lip of the wheel, not the tire edge. which can be confusing, but once it is understood, it does eliminate some confusion and additional math, especially when it comes to aluminum wheels. For an 8" wheel, 0 offset is 4-1/2" of back spacing, to get 4" it would be a -12 offset (12 is 12mm, approximately 1/2"). Virtually all international and many domestic machine shops use metric measurements because they are easier to program, hence the millimeters in the dimension. I know we just want to know the backs spacing, we don't care about how we get there. The easy solution is a calculator that does it all for you, Here is a link to the calculator that I use so I don't have to be smart LOL

 

[Otis Campbell]

Thanks guys! I get it now.

 

[Broncobowsher]

They tried metric tire sizes in the 80s. Anyone have a mustang with the TRX package? A few years ago I found a set of NOS TRX tires for a Tempo(?)
Somehow the world runs inches for rim sizes. Just think of everyone in the rest of the world that is all metric and is totally confused by needing 18 inch tires. About the only time in there life they will ever use inches.