offset and backspace advice all over the place. what is correct?

[smuddy]

Oh boy

Here's the Mopar 4" lift text, wait for it, no mention of offset, because, that's right, it doesn't matter from a CLEARANCE standpoint:

NOTE: It is recommended that this lift be used in junction with 35” to 37” diameter and 12.5” wide tires with a 17” diameter wheel with a minimum of 5” back spacing, check for clearance with the rear stabilizer bar end links first. It is recommended that this kit be used with the Mopar flat top fenders flares (77072342) if 37” tires are being used for clearance.

I am NOT saying offset is irrelevant in the grand scheme, but I am saying that at the end of the day, at least in the JK world, the end measurement you were looking for was backspacing as far as clearance, so whether you took it directly, or calculated it from width and offset, you looked at that number first before looking at how far out the face of the rim and resultant sidewall was pushed.

So, if you have 315/70/17s and the following two wheels:
- 17x8 rim with +12mm of offset which comes out to 4.5" backspace
- 17x9 rim with +12mm of offset which comes out to 5" backspace

Will one of the inner sides of the tread width be closer or farther from the suspension components? Or will that space between them be the same?

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[smuddy]

And it's strange that there isn't consistency in how backspace is measured. Some sites use the rim width and some use rim width plus 1" for the flange to determine the backspacing.

 

[on--belay]

So, if you have 315/70/17s and the following two wheels:
- 17x8 rim with +12mm of offset which comes out to 4.5" backspace
- 17x9 rim with +12mm of offset which comes out to 5" backspace

Will one of the inner sides of the tread width be closer or farther from the suspension components? Or will that space between them be the same?

Taken from TireRack, who I believe offers pretty reliable information:

The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10".

 

[smuddy]

Oh boy

Here's the Mopar 4" lift text, wait for it, no mention of offset, because, that's right, it doesn't matter from a CLEARANCE standpoint:

And, sorry, but that is flat out wrong since backspace is calculated based on the offset 1/2 rim width + offset

 

[smuddy]

Taken from TireRack, who I believe offers pretty reliable information:

The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10".

Does that mean the section width will go up or down as you go wider in rim width? It didn't specify which direction.

Edit: I checked out tirerack and it goes up. I was assuming that but wanted to verify.

 

[on--belay]

And, sorry, but that is flat out wrong since backspace is calculated based on the offset 1/2 rim width + offset

How do you find the actual screaming smiley here. Why do people post misinformation. Backspacing is NOT calculated by the offset. Take a straight edge, lay it across the back of the wheel, measure from there to the hub mating surface of the wheel, that is your backspace, PERIOD, no complicated calculations, no misunderstanding of what you are looking at, it doesn't matter if your backspacing is -12, +12, -40 or +63, that measurement won't change.

From TireRack:

Backspacing is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).

 

[smuddy]

It's not misinformation my friend. You like tirerack, so here is definition of offset from tirerack:
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.
If you don't have the actual wheel in front of you to whip out your trusty ruler, you use a formula, and that is:
1/2 rim width + offset

How in the wide wide world of sports is that misinformation?

 

[on--belay]

Does that mean the section width will go up or down as you go wider in rim width? It didn't specify which direction.

Chart:

Difference from Measuring Rim Rim Width Approximate Tire Section Width
0.5" narrower 5.5" 8.03"
Measuring Rim 6.0" 8.23"
0.5" wider 6.5" 8.43"
1.0" wider 7.0" 8.63"
1.5" wider 7.5" 8.83"

For some reason it compressed, but you should be able to align the 3 columns, and see the section width increasing.

 

[on--belay]

It's not misinformation my friend. You like tirerack, so here is definition of offset from tirerack:
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.
If you don't have the actual wheel in front of you to whip out your trusty ruler, you use a formula, and that is:
1/2 rim width + offset

How in the wide wide world of sports is that misinformation?

Dude, we are talking about measuring backspacing.

Edit* I see now what you were saying, yes, you can calculate the backspacing. When most places just give it to you, I'm not sure why you would, and with all the convoluted calculations and incorrect assumptions (like rim width having no effect on section width) you are throwing around I glossed over what you wrote, so I apologize for that.

 

[tdubs]

It's not misinformation my friend. You like tirerack, so here is definition of offset from tirerack:
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel.
If you don't have the actual wheel in front of you to whip out your trusty ruler, you use a formula, and that is:
1/2 rim width + offset

How in the wide wide world of sports is that misinformation?

Something that I think tire rack and my original post forgot to mention, is that a 17x9" wheel is not always exactly 9" wide. The lip of the wheel could extend another 1/4" or more. Unfortunately I don't think there's an "industry standard" for this so it's again one of those case by case things.

Take the XD 134 Addict 2. It has the following specs:

Size: 17" x 8.5"
Offset: 0

What would my (and tire racks) equation get you for backspace? 4.25"

What is the actual backspacing on the wheel? 4.75" (because the lip adds another .5" on either side of the wheel)

So it appears backspacing does have an advantage over offset in this regard. I'm starting to regret my original post entirely. However, your original question as to why there is so much confusion and misinformation about what works is thoroughly answered by the responses of this thread itself.

There just hasn't been enough time for a consensus yet.

 

[on--belay]

So it appears backspacing does have an advantage over offset in this regard. I'm starting to regret my original post entirely. However, your original question as to why there is so much confusion and misinformation about what works is thoroughly answered by the responses of this thread itself.

AGREED. Thank goodness it is Friday night

But in their product specs for many wheels it looks like they are using the actual backspacing as opposed to the calculated, so perhaps that helps, perhaps it only adds to the confusion. Are some wheels nominal and others not, is every manufacturer's 9" actually 9" outside, or 9" lip to lip, as you said, there is not standard, so we work with what we've got.

 

[tdubs]

AGREED. Thank goodness it is Friday night

Preach

 

[smuddy]

AGREED. Thank goodness it is Friday night

But in their product specs for many wheels it looks like they are using the actual backspacing as opposed to the calculated, so perhaps that helps, perhaps it only adds to the confusion. Are some wheels nominal and others not, is every manufacturer's 9" actually 9" outside, or 9" lip to lip, as you said, there is not standard, so we work with what we've got.

Agree on the Friday night wholeheartedly! I'm leaving work for that now!

Yeah, I've noticed that the backspacing they list doesn't always match the calculation.

Thanks for your input.

Ok...

 

[merilm]

Here's another site that may make it more clear. Guess what, it doesn't take backspacing into account because it doesn't matter. It's the offset and the width of the tire that matter. Check it out. It shows a graphic of where the tire will be as you change the offset.
https://tiresize.com/wheel-offset-calculator/

I like this tire size calculator too - it gives a visual while you are experimenting with options.

 

[smuddy]

I like this tire size calculator too - it gives a visual while you are experimenting with options.

Unfortunately, it would seem that the site I mentioned above with its graphics goes against what most people say about backspacing and it's importance. Honestly, I am still holding to that graphic in how I understand it. Even with so many here insisting that backspacing is what matters, it simply doesn't make sense to me.

The offset is the difference between the centerline and the mounting surface.

Stock rubi rim has a 44mm offset and 6.25" bs. The rim I'm looking at is 17x9 with +12mm offset and 5.5" bs. I'd like to put the 315/70/17 KO2s on. The BFG site says they are 12.7" wide on 9.5" rims so should be slightly narrower on 9". It's a tire width difference of about 1.5" compared to the 33s on the 7.5" wheels. The difference between 44mm and 12mm offset is 1.26", so to me, that means you are pushing the wheel and tire that much further out. 1/2 the difference in tire width is about .75" so that means the larger tire will stick that much further in and out from the centerline compared to the old tire. That means there should be almost 1/2" more clearance between the new tire/wheel compared to the old tire/wheel regardless of backspacing. I simply don't get why this is apparently wrong.

I'm a visual guy and this site just makes sense to me: https://tiresize.com/wheel-offset-calculator/

Let the flaming begin!

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