Why if B/S is half rim width, offset isn't zero?

MtnLuvr · **Posted:** Sat Apr 18, 2009 10:18 pm

Ok, I apologize if this has already been clearly explained before. After 20 minutes of using the board's "search" function, I haven't found a clear explanation on this. Please educate the ignorant....

I thought I understood the definitions that:

Backspace is the distance from the inboard (e.g. facing the engine compartment) wheel flange to the hub mounting surface on the wheel backside.

Offset is the distance from the centerline of the rim widthwise to the hub mounting surface (where a positive offset would mean the hub mounting surface on the outer rim face side of centerline and a negative would mean it is on the side closer to the engine compartment or inboard side).

I've been research aftermarket wheel options and apparently don't correctly understand the above because I would think an 8" wide wheel with 4" of backspace would have an offset of 0 mm since the hub mounting surface is at the centerline (widthwise) of the wheel rim.

Instead, I'm finding 8" wide wheels with 4.5" of backspace and 0 mm offset. This is confusing to me. I would think if an 8" wheel had 4.5" of backspace then it would have +1/2" (a.k.a. +13mm) offset or conversely if an 8" wheel had 0 mm offset, then it would have 4" backspacing.
:?:

I'd appreciate any help / insight to get me straightened out.

JeepinJarhead03 · **Posted:** Sun Apr 19, 2009 2:47 am

im pretty sloshed right now but your methodology is correct

an 8 inch rim would have center of 4 and if it had 4 inch back space it would have 0 offset

if it had 4.5 bs then it would have 4-4.5 for +.5 offset

its RW / 2 = X

X - BS = Z

when BS > X , hub surface is closer to outside of rim

Boiler · **Posted:** Sun Apr 19, 2009 3:12 am

Is backspacing the measure from the edge of the rim to the center of the mounting flange, or to the mounting surface of the mounting flange? If it was to the surface, it would be less than half the width, due to flange thickness.

No clue though, personally.

MtnLuvr · **Posted:** Sun Apr 19, 2009 9:30 am

Boiler wrote:

Is backspacing the measure from the edge of the rim to the center of the mounting flange, or to the mounting surface of the mounting flange? If it was to the surface, it would be less than half the width, due to flange thickness.

No clue though, personally.

Here's some reference info sources:

1. Wheel backspacing and offset
2. Definitions
3. Wheel technical info
4. Offset info here and here.

MtnLuvr · **Posted:** Sun Apr 19, 2009 9:58 am

JeepinJarhead03 wrote:

im pretty sloshed right now but your methodology is correct

an 8 inch rim would have center of 4 and if it had 4 inch back space it would have 0 offset

if it had 4.5 bs then it would have 4-4.5 for +.5 offset

its RW / 2 = X

X - BS = Z

when BS > X , hub surface is closer to outside of rim

I'm still unclear. If my methodology and understanding is correct, then why is there all types of wheel info on-line that shows examples where this is not true?

Here's just a few quick examples:
1. Look at the "Backspace to Offset Conversion Chart" (after scrolling down a little) for an 8" wheel width at 4" of backspace and the offset is NOT shown at 0, but at -12mm.
2. Look at the very first row in this table for this ProComp 8" wide wheel yet it shows 4.5" backspacing and 0mm offset.
3. Look at AEV's Bridger wheel that says in this table its 16x8 wheel has an offset of 6mm (for the 5x4.5" bolt pattern we need), yet here it says 4.5" backspacing.
4. This American Racing ATX Predator wheel is listed as 16"x8" with 4.5" of backspacing yet 0 mm offset.
5. and I could keep digging up more examples....

That is why I'm confused. Please help.

tommudd · **Posted:** Sun Apr 19, 2009 10:09 am

are you taking into account steel -vs- aluminum etc ?

MtnLuvr · **Posted:** Sun Apr 19, 2009 10:18 am

tommudd wrote:

are you taking into account steel -vs- aluminum etc ?

Right now I'm trying to figure out what's wrong with my understanding regardless of materials of construction. These were just examples I could quickly dig up on-line.

Yes, I do plan to give consideration to materials of construction versus application when I go to make my selection but it is bugging me that the "math" doesn't work out given the definitions.

Can you explain what I'm missing?

tomsjeep · **Posted:** Sun Apr 19, 2009 12:50 pm

I am by no means "smart" in this stuff... however, some smart folks exist, and I'll try to put their info up here in a coherent fashion:

Quote:

Rear spacing or back spacing is the distance from the backside of the wheel mounting pad to the outside of the rim flange.
Offset The distance from the centerline of the wheel to the mounting surface of the wheel.
Negative offset When the back of the bolt pad is closer to the inside of the wheel; when mounting surface is inboard of the rim centerline.
Positive offset When the back of the bolt pad is closer to the street side of the wheel; when the mounting surface is outboard of the rim centerline.

This is from 4 Wheel Drive Hardware's site...
http://jeep-forums.4wd.com/showthread.php?t=13876
If you follow the link you'll see a diagram that lays it out... the reason for the descrepancy is that the wheel has thickness... at least that appears to be the answer based on what I'm seeing/reading. There may be a better, more exacting explanation, but if so, I'd love to be educated.

JeepinJarhead03 · **Posted:** Sun Apr 19, 2009 2:30 pm

the thickness shouldn't come into play, the measurements are to the same location, inside of rim to hub mount surface and outisde of the rim to hub mount surface

i think the problem is, is that they're two different ways of measuring something and you typically get info for one or the other and have to do the math yourself for the other measurement

there might be a chance the strange-math you've got from some of the sites is just wrong

i would email the manu directly for whatever rim you have in question and get them to explain it

if the descrepancy is because of the thickness of the wheel, then from what i've understood.. would mean they're measuring to the wrong location :lol:

if both measurements are to the surface that touches the hub, then it shouldn't matter on the rim thickness ?

MtnLuvr · **Posted:** Sun Apr 19, 2009 3:28 pm

JeepinJarhead03 wrote:

if both measurements are to the surface that touches the hub, then it shouldn't matter on the rim thickness

That's what I'm thinkin...

JeepinJarhead03 · **Posted:** Sun Apr 19, 2009 3:43 pm

i'd just contact the manu, get their explanation

could be a case of an advertising website doing secondary math and making a mistake

Rich · **Posted:** Thu Jan 07, 2010 1:08 pm

Sorry to dig up an old post, but I think I can shed some light on the subject.

Back Space =mounting flange to wheel flange
Offset = mounting flange to bead lip

Remember we are talking about a SAE standard and a rest-of-the-world metric standard. This accounts for any discrepancy, as wheel flanges can vary in thickness. Let's say you have a wheel that is designated a 15x8JJ (which means its actually 15x7.5ish bead to bead) Your backspace of 4" will put the mounting flange smack dab in the middle according to SAE. But because the metric system is based on the bead, that same 4" back space will put the flange just about .5" off center favoring the inside (- offset), which puts the offset at right around -12mm.

Keep in mind that the offset system is not exactly representative of true outer wheel dimensions, while the BS is closer. Neither system is exact, as a PC wheel will be wider than one with a simple spray paint finish.

mattflo · **Joined:** Wed Jun 22, 2022 7:41 pm **Posts:** 1

This drove me crazy when I sat down to make some calculations.

I finally found this at tiresize.com. It explicitly states that a wheel width is from one tire bead to the other. I'm a bit unclear if flanges always and consistently add 1/2 inch to each side of a wheel. However, that appears to be the common practice when performing these calculations.

https://tiresize.com/wheel-offset-calculator/

Obviously, offset isn't affected by the fact that flanges aren't included in a wheel's official width. The center of the wheel is the center of the wheel, whether flanges are included or not. Of course, this assumes flanges are the same size on both sides of the wheel....

However backspacing is most certainly affected by the flange. Since backspacing is used to ensure the wheel clears suspension, brakes, etc., including the flange in the backspace measurement is important.

Why if B/S is half rim width, offset isn't zero?

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