Wheel Fitment question Method Wheels 0MM/4.75 vs -38MM/3.5

[GW Nugget]

Slee comes with 2" bumps.
@baldilocks runs 2.5" bumps & had to do minimal trim.
With a 1" body lift, I have equivalent of 3.25" rear with hammer massaging in the rear & i cut 2" out of my front fenders with equivalent of 1.5" bumps.
I have 9" wheel 3.5" BS
Stock uptravel is 2" then add lift then minus bumps will tell how much uptravel you have.

 

[Bambusiero]

Do yourselves a big favor and just stop thinking / referring / comparing based on "backspace".
Instead, focus on offset (AKA "ET") and wheel width - clarity will come much easier.

 

[cruiserdan]

I disagree. Offset is a variable number based on overall width, is a moving target and can be difficult to translate. Backspace is a one dimensional number.

 

[Bambusiero]

Just for the record, this diagram is fundamentally incorrect (by definition). Please don't carry this idea forward.
I've seen /found pictures just like it sprinkled around the internet. Unfortunate.
It completely forgets to factor in rim thickness, which varies from wheel to wheel, and is almost never specified.

WRONG! :

RIGHT! :

I disagree. Offset is a variable number based on overall width, is a moving target and can be difficult to translate. Backspace is a one dimensional number.

That's not really true Dan, it's sort of turned around.
Both backspace and offset are fixed, dimensional numbers, not variable (for any particular wheel).
And (this is important), neither one is enough by itself.
You need a 2nd number, in both cases, to understand the position of the wheel.
Both are in reference to the mounting (reference) surface, the hub surface.

Offset tells you where the wheel centerline is, relative to the mounting surface, but you need the wheel width too.
Offset tells you exactly the same thing for any wheel width. The centerline is the centerline, no matter how wide.
Knowing where the wheel centerline is gives you an easy, intuitive understanding of how 2 different width wheels will sit.
Fitting and rubbing of big tires gets into issues with both inside and outside tires surfaces, but not the inside rim edge.

Backspace tells you where the inner rim edge is, similarly, relative to the mounting surface, but you need the frontspace too.
With both backspace and frontspace, you could easily calculate where the centerline is, that's true, because : offset = backspace - frontspace, and, again, knowing where the wheel centerline is, relative to the mounting surface is still what you need.
And - you still need to know the wheel width too, right? So you really need 3 numbers in that case.
Who ever tells you the frontspace? Nobody, ever.

But - you cannot translate between offset and back space, either direction, unless you know the rim thickness.
Who tells you the rim thickness? Nobody, ever.
True, you can calculate rim thickness, if you know certain 3 other things...why bother?

And besides, wheel width [inch] and offset (ET)[mm] is the international standard for wheel specs.
Has been for decades.
It's molded / stamped on the wheel.
I believe you'll find 8Jx16, ET0 on your stock Landcruiser wheel.

Talking backspace is just an old-school 1960's American anachronism. Let it go.

Translating to inches is easy:
Offset[inch] = ET[mm] * 25.4

 

[cruiserdan]

Talking backspace is just an old-school 1960's American anachronism. Let it go.

Then I'd have nothing left

Your explanation is very well done and of course accurate. I appreciate the thought put into it.

I, very much like Mark of Mark's Off Road, am a carbureted man in a fuel injected world. I am not totally prepared to simply walk away from a "rule of thumb" that has worked for me for over 40 years. Not quite the 1960s but I was born in the 50s and I got my license in 1974. I can "embrace" modern technology if I really have to.

In my limited experience I don't ever recall seeing a wheel, alloy or steel (excluding split rims) that did not have a flange-to-flange measurement that was one inch (plus or minus) wider than the bead-to-bead measurement. Another thing I have noticed is the outer rim lip diameter is also about an inch greater than the tire mounting diameter.

Applying this observed information, here is a quick result of the 50 odd year old rule. The required second number you mentioned is derived from the plus 1 inch seen historically.

I have a 15x10 wheel with 3.5 inches of back-space and that's all I know about it:

The front-space will be ~7.5 inches and it will have a negative offset of ~2 inches.

I used inches because that is how my carburetor is adjusted.

If all I knew was it was a 15x10 and had a negative 2 inch offset I could figure out the rest. I don't recall offset mentioned that much back in the dark ages.

 

[Bambusiero]

Then I'd have nothing left ...
... am a carbureted man in a fuel injected world.
...got my license in 1974.
... I don't recall offset mentioned that much back in the dark ages.

I, of course, did not mean to imply that you, yourself sir, are an anachronism.
Think I got my license in '73, but was not burdened by any automotive "rules of thumb", 'cause I didn't know diddly squat.
Suspect that the use of offset, especially "ET" in mm, only came to America with the rise of Japanese and Euro imports in the '70s.

... flange-to-flange measurement that was one inch (plus or minus) wider than the bead-to-bead measurement
... plus 1 inch seen historically.

I'll buy that, as a rule of thumb, in fact, I saw the number 1 built into some of the internet offset-backspace calculators you can find out there.
I don't think it so much an exact rule, as more of a guideline (like the Pirate's Code ).
It may be pretty much stuck to by the wheel manufacturers for compatibility with tire mounting machines, so might not vary by a lot.
I might recall seeing some cast aluminum wheel where they actually listed the rim thickness at 1.3", but a rare number to see in print.
But no guarantees, pretty sure it does vary some.
Inside and outside rim thickness on some cast wheels might easily be different, especially the offroad market, with the "similated beadlock" look so popular.

But - using offset / wheel width really is mentally easier to make wheel to wheel comparisons.
Factoring in hub spacers is easy too. They simply add (negative) offset to whatever offset the wheel already has.
I never even consider rim width - doesn't matter at all. Same with back space.
But - here's the thing - I do notice that an awful lot of folks, here on the Mud, go around and around, forever, never getting a handle on what backspace / wheel will work.
And the reason is - It's just harder to grasp the whole picture with only the backspace number.

Anyway - We now return control of this television to it's regular programming...