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That's a good way to go. Then when you go bigger, you won't have to get wheels with less backspacing. Heck, you could even go less. I'm running 4" backspacing and I had plenty of room for 35x12.5's on my stock axles.
 

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FYI You should also mention the width of the wheel in conjunction to the backspacing since it influences tire interference.
 

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Only if you're worried about interference with the outside edge of the tire, too. If your only worry is the inside part of the tire, then backspacing is enough to take into consideration.
Yeah however the majority of interference issues on our JK is with the tire edge thus wheel width + Backspacing are important values to mention ( or wheel offset alone will do as well )
 

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any more backspacing than 4.5" will cause you problems in running 35"x..."x12.5" tires (or 315x...x75s). I am running the Pro Comp 7069s which gives me 4" of backspacing and I am good to go! :)


Also, if you get replacement wheels, try to stick with 16s or bigger....there are some success stories of getting 15" wheels and not having problems with the brake calipers, but there are also stories of guys having to grind down the edges of their calipers to prevent the wheels from hitting. :beer:
 

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any more backspacing than 4.5" will cause you problems in running 35"x..."x12.5" tires (or 315x...x75s). ....
As for inside interference considering "minimal" mods on a JK tires running more true to the 35X12.5" size will rub at stock steering lock with 4.5" backspacing on 8" wide wheels( ~0 Offset), likely clear at steering lock with 4.5" backspacing on 9" Wheels (~-12 Offset) and would clear for sure at steering lock with 4.5" backspacing on 10" wide wheels (~-25 Offset).
 

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LOL

Backspacing is independent of the rim width.

OFFSET is changed by the rim width though.

BS is simply the depth of the brake side of the rim's bowl...

So, If the bowl is 4.5" deep on a 7" rim, its ALSO 4.5" deep on a 10" rim, etc.


As the inboard edge of the bowl is DEFINED by the BS (BS is measured from the outer flange edge, down to the hub face) - The BS essentially defines where the inboard side of the tire is.



OFFSET is measured from the hub face to the CENTER of the rim's width.


In practical terms, that means that if you get what is labeled a 8" rim, its 9" wide overall. (Adding the typical flange thicknesses).


That also means that for that rim, if it has 4.5" of BS, it also has ZERO OFFSET.

If I kept the 4.5" of BS, but made it on a 10" rim, which would be 11" wide...I changed the offset, but the BS is the same, and the inboard edge of the rim is in the same place.

If the inboard edge is in the same place, my tire clearance is in the same place too.


So, LESS BS means the inboard edge is moved OUTBOARD...increasing clearance for inboard side components.


If a stock rim has 6.25" of BS, a 4.5" BS rim is moving the tire's inboard edge (outboard) by 1.75"...increasing tire clearance proportionally.

If a stock tire is 255 mm (~ 10"), and a 12.5" tire is therefore ~ 2.5" wider, it will require about 1.25" of additional clearance inboard (Half of the total 2.5" difference).

As 1.75" is more than 1.25", that's enough room (Barely).

You want a little fudge factor of course, as what's labeled a 12.5" tire might be really 12.8" or 12.2" in ACTUAL width, etc....

...and a TALLER (Larger diameter) tire tends to swing in a wider arc, hitting more stuff...requiring some more room for that too.


Just wanted to clarify all that for you guys.

:D
 

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Discussion Starter · #11 ·
The wheel width Im looking for is between 8 and 9 inches. Im relatively new to most things about wheels and tires so if this creates a problem with the inner wheel rubbing against anything I would like to know.
 

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I have 3" of lift, 2dr, 4.5" backspacing on 17x9 wheels, 33x12.50's. no rubbing, ever. Also, makes a mean stance. hope this helps.

it rubbed slightly at full turn before the lift went on with the same setup otherwise. i have also fit 35x12.50s with no rubbing, after lift.

in short "no." it isnt too much. it is different from using spacers. as descibed by one person above - it changes the location of the hub in the wheel - pushing the wheel away form the center of the vehicle.



see: "rear spacing"
 

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LOL
Backspacing is independent of the rim width.

OFFSET is changed by the rim width though.
......
Well offset is also independent of wheel width if you look at it this way because -25mm on a 8" Wheel or on a 9" Wheel is -25mm ;)
It changes only if you compare same Backspacing with the offset number and same would happen if you compare Backspacing with same offset numbers.

Backspacing value without wheel width is fine if you look at the rim only but the scenario changes once you throw a tire into the mix and discuss possible tire interference because the part of a tire that hangs past the outer wheel flanges changes with wheel width and there offset is a "better" value if you understand it

e.g.: 12.5" wide tire on a 8" wide 4.5"BS wheel would go about 1.75" past the inner wheel flange.
(12.5" tire width - 8" Wheel bead width = 4.5" / 2 since we are only looking at one side = 2.25" - .5" for wheel flange width* = 1.75" tire wall past the inner wheel flange)
Final tally 1.75" tire over + 4.5" BS = 6.25" WMS to inner tire wall

So now the same 12.5" wide tire on a 9" wide 4.5"BS wheel would per above calculation just with the 8" replaced with 9" wheel width:
(12.5"- 9")/2 - .5" + 4.5" = 5.75" WMS to inner tire wall
Dang .5" less but it's the same Backspacing ???!!!

Screw Backspacing lets do with offset:
As we know offset is the metric measurement of WMS to the center line, negative meaning WMS is further inboard, positive further outboard then center of wheel.
12.5" tire on a 8" 0MM offset wheel:
Lets first convert the offset to Inches so 0MM / 25.4 = 0" (took the easy one :) ); Then we take the center of the tire 12.5" / 2 = 6.25" and add the converted offset --> 6.25" + 0" = 6.25" WMS to inner Tire wall

Now a 12.5" wide tire to a 9" Wheel with 0MM Offset:
(12.5"/ 2) + (0MM/25.4) = 6.25" WMS to inner Tire wall
Sooooo same tire width, same offset, different wheel width but same calculated WMS to inside tire wall result**.

but these damn negative numbers with offset what do I do with those.
Easy, same formula if you look at the inner tire wall
12.5" tire -12 offset wheel --> (12.5" / 2) + (-12/25.4) = 6.25 + - 0.5 rounded off = 5.75"
Hey we now have WMS to inner tire wall dim of our 9" - 4.5" BS Wheel example before :devil:


* Flange width is approx. may vary with wheel design and is a noise factor when using Backspacing in the calculation.
** overall inflated tire width may vary slightly when installed on different wheel width and who knows how true to dimension a given brand/type of tire is.
 

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The wheel width Im looking for is between 8 and 9 inches. Im relatively new to most things about wheels and tires so if this creates a problem with the inner wheel rubbing against anything I would like to know.
The 285/75R16 you mentioned will be fine on either 8" or 9" wheels with 4.5 Backspacing however I'd recommend the 9" wheel if you plan to go to 315/75R16 in the future to avoid interference issues.
Just a friendly recommendation from one with 315/75R16 on 8" 4.5"BS (0MM Offset) wheels :jeep2:
 

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Well offset is also independent of wheel width if you look at it this way because -25mm on a 8" Wheel or on a 9" Wheel is -25mm ;)
It changes only if you compare same Backspacing with the offset number and same would happen if you compare Backspacing with same offset numbers.

Backspacing value without wheel width is fine if you look at the rim only but the scenario changes once you throw a tire into the mix and discuss possible tire interference because the part of a tire that hangs past the outer wheel flanges changes with wheel width and there offset is a "better" value if you understand it

e.g.: 12.5" wide tire on a 8" wide 4.5"BS wheel would go about 1.75" past the inner wheel flange.
(12.5" tire width - 8" Wheel bead width = 4.5" / 2 since we are only looking at one side = 2.25" - .5" for wheel flange width* = 1.75" tire wall past the inner wheel flange)
Final tally 1.75" tire over + 4.5" BS = 6.25" WMS to inner tire wall

So now the same 12.5" wide tire on a 9" wide 4.5"BS wheel would per above calculation just with the 8" replaced with 9" wheel width:
(12.5"- 9")/2 - .5" + 4.5" = 5.75" WMS to inner tire wall
Dang .5" less but it's the same Backspacing ???!!!

Screw Backspacing lets do with offset:
As we know offset is the metric measurement of WMS to the center line, negative meaning WMS is further inboard, positive further outboard then center of wheel.
12.5" tire on a 8" 0MM offset wheel:
Lets first convert the offset to Inches so 0MM / 25.4 = 0" (took the easy one :) ); Then we take the center of the tire 12.5" / 2 = 6.25" and add the converted offset --> 6.25" + 0" = 6.25" WMS to inner Tire wall

Now a 12.5" wide tire to a 9" Wheel with 0MM Offset:
(12.5"/ 2) + (0MM/25.4) = 6.25" WMS to inner Tire wall
Sooooo same tire width, same offset, different wheel width but same calculated WMS to inside tire wall result**.

but these damn negative numbers with offset what do I do with those.
Easy, same formula if you look at the inner tire wall
12.5" tire -12 offset wheel --> (12.5" / 2) + (-12/25.4) = 6.25 + - 0.5 rounded off = 5.75"
Hey we now have WMS to inner tire wall dim of our 9" - 4.5" BS Wheel example before :devil:


* Flange width is approx. may vary with wheel design and is a noise factor when using Backspacing in the calculation.
** overall inflated tire width may vary slightly when installed on different wheel width and who knows how true to dimension a given brand/type of tire is.


You're putting the cart before the horse.

:D

Think of it this way:

If you know the BS, you don't need to know the offset, oR the rim width to know where the inboard edge of the rim is.

You CAN'T make the offset stay the same, if the BS is the same, and you change the rim width.

You DO know where the inboard edge of the rim is if you know the BS, no matter what the rim width is.


To know where the inboard edge of the rim is using ONLY the offset, you MUST know the rim width first.

If you DON'T know the rim width, the offset will NOT tell you where the inboard edge of the rim is.



--------------------------------------


The offset is how much its hub face is offset from the CENTER of the rim...

So 12 mm of offset on one rim is DIFFERENT from 12 mm of offset on another rim (12 mm of offset is NOT = 12 mm of equivalent CLEARANCE for all rims)

----------------------------------------


The BS is how far the inboard edge of the rim is from the hub face....the CLEARANCE you're concerned with.


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So, if I have 2 rims with say 25 mm of offset.....and one is a 7" rim, and one is a 10" rim....they do NOT have the same inboard clearance.


If I have 2 rims with say 4.5" of BS, and one is a 7" rim and one is a 10" rim, they DO have the same inboard clearance.


-----------------------------------------

IE: The BS defines the inboard clearance, and is the only clearance number that is directly relevant.

The offset is more important to the steering geometry, etc.

The offset can be used to CALCULATE the BS/clearance, but, again, the rims' widths will CHANGE what offset gives what clearance.

------------------------------------------


Example:

(We'll call the 25 mm as = 1" to make the math easier...)

The 7" rim with 4.5" of BS and the 10" rim with 4.5" BS both put the inboard edge of the rim at the exact same spot...so clearance is the same.


The 7" rim with 25 mm of offset will therefore have 4" of BS as its ZERO Offset point, with the hub face offset 1" from that.

The 10" rim will have 5.5" of BS as ITS zero offset point, with the hub face offset 1" from THAT.


Obviously, that 25 mm of offset is NOT putting the inboard edge of the rim at the same point, even though BOTH rims have the SAME OFFSET.

----------------------------------------------

So, again, as shown in the diagrams, and explained, the BS is consistent in being able to describe inboard clearance, the same BS = the same clearance, regardless of rim width.

The offset is NOT consistent in describing the inboard clearance, as the same offset will provide DIFFERENT CLEARANCES on different rim widths.

:beer::beer:



To clarify the context of what I am trying to explain:


You CAN tell people a blanket statement, w/o knowing what rim width they'll use, about what BS will give enough clearance for a given tire size, etc....because it defines the inboard edge.

You CANNOT tell people what offset to use, w/o knowing what rim width they'll use....because the offset that WILL give enough clearance will DEPEND UPON the rim width they use.
 

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To clarify the context of what I am trying to explain:

You CAN tell people a blanket statement, w/o knowing what rim width they'll use, about what BS will give enough clearance for a given tire size, etc....because it defines the inboard edge.
Incorrect if we are talking about tire wall interference!
As it was stated before Backspacing without wheel width is a good number if your are considering only clearance to the wheel itself but, as it was also mentioned before, that is generally not an issue with our JK unless we are talking for example about modification like a drag link flip which may generate interference with 16" or smaller wheels.


You CANNOT tell people what offset to use, w/o knowing what rim width they'll use....because the offset that WILL give enough clearance will DEPEND UPON the rim width they use.
Incorrect if we are talking about tire wall interference.
If you would take a look again on the calculation examples I did before you'd see that you don't need wheel width with Offset if we are talking about possible interference of the tire.

It seems you're having tunnel vision on just the bowl itself and you're forgetting the wider piece of rubber that goes centered over the bowl. The amount of rubber that sticks past the bowl will vary with the width of the bowl and that is why the width of the bowl needs to be known if you just have the dimension of the rim of the bowl to the bottom (Backspacing) and don't need to know it if you have the number that tells you where the center of the assembly is in relation to the bowl bottom (Offset) :idea:
 

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Incorrect if we are talking about tire wall interference!
As it was stated before Backspacing without wheel width is a good number if your are considering only clearance to the wheel itself but, as it was also mentioned before, that is generally not an issue with our JK unless we are talking for example about modification like a drag link flip which may generate interference with 16" or smaller wheels.



Incorrect if we are talking about tire wall interference.
If you would take a look again on the calculation examples I did before you'd see that you don't need wheel width with Offset if we are talking about possible interference of the tire.

It seems you're having tunnel vision on just the bowl itself and you're forgetting the wider piece of rubber that goes centered over the bowl. The amount of rubber that sticks past the bowl will vary with the width of the bowl and that is why the width of the bowl needs to be known if you just have the dimension of the rim of the bowl to the bottom (Backspacing) and don't need to know it if you have the number that tells you where the center of the assembly is in relation to the bowl bottom (Offset) :idea:

The part you are leaving out is that the zero offset POINT changes with rim width.


The tire is always centered between the flanges, it has no way of "knowing" where the hub face is.


When you mount a tire on a rim, its safe to assume the tire's center is at the rim's center as far as width goes.


If the rim has ZERO OFFSET, that means the tire's center should be even with the hub face.

I believe we are generally talking about the same thing, but from different angles.

And, yes, as mentioned, changing the offset moves the rim inboard and outboard the same way the BS does, except its measured differently.

And, yes, I am considering the rim's inboard edge, as in my world, that is an issue sometimes. :D


So, the bottom line is that if the question is about inboard clearance, its easier to just use the backspacing you wanted, than it is to take the rim width, calculate the offset to get adequate backspacing, and then use the offset.

CAN you use offset, sure, but, the BS is FOR determining clearance.

---------------------------

As you are only talking about the rubber, and you know your brakes, etc, will fit into the concave side, and the rim won't whack anything on turns, etc....sure, you can just say...OK, the 10" wide TIRE has X" of clearance...

...so the 14" tire will need 2" more room, so I'll offset it to be outboard by 2". (converting to mm to keep the offset nomenclature gods happy of course...)

I am 100% sure that this is what you are talking about.

------------------------------------------


:beer:


Now, if the stock rim has a 6.25" BS....and you want to offset it 2" to move it outboard that far...what's your offset going to be?

You want the hub face to position the center of the tire 2" further outboard than it was on the OEM rim.

--------------------------


Same problem, but just wanting the new BS...2" Less BS = 4.25"...

Easy.

------------------------

So, you have the concept, but the application is just meant for BS.


Also obviously, if you know the offset you start with, you can do the same thing in mm...but, when not talking about zero offset, and converting between different offsets, most people's heads explode.

You are free to do it anyway you want of course, but its just easier to give people the backspacing #'s. Inches are easier to get than mm for most people.

:D
 

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You are free to do it anyway you want of course, but its just easier to give people the backspacing #'s. Inches are easier to get than mm for most people.
:D
No Problem with that, only thing I'm asking is to include the wheel width when mentioning Backspacing since it is needed for possible interference issues.
Lets do a few visuals for those that don't grasp why wheel width value is important.



So same tire width, same off-set will give same WMS to Tire Wall measure on different width wheels.
Same Tire width, same Backspacing will result in different WMS to Tire Wall measure on different width wheels.

Oh btw sorry for the not so good quality of the image, did it on the quick.
 

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No Problem with that, only thing I'm asking is to include the wheel width when mentioning Backspacing since it is needed for possible interference issues.
Lets do a few visuals for those that don't grasp why wheel width value is important.



So same tire width, same off-set will give same WMS to Tire Wall measure on different width wheels.
Same Tire width, same Backspacing will result in different WMS to Tire Wall measure on different width wheels.

Oh btw sorry for the not so good quality of the image, did it on the quick.


I COULD use a larger pic...but, it appears that you are making the TIRE move on the rim with the BS and offset.



The center of the tire itself should be at zero offset, as the tire stays centered in the rim width....moving the hub face doesn't move the tire on the rim...it only moves the rim relative to the hub.

Right now, it LOOKS like you are saying that the 12.5" section width tire is no longer centered on the rim.

The rim width typically just puckers the beads in out, w/o making that much difference in a truck's tire clearance, with a bit of a caveat that a wider rim can make the sidewall straighter up/down, and less pulled in at the rim/bead.

If the section width stays at 12.5", and its inboard bead is in the same place, why would the rest of the tire shift inboard with the bead not moving in?

IE: What's making the tire asymmetrical?




I'll make an extreme example:


If the tire was 2 dimensional, a plane...with its center at the line even with the hub face (Zero offset)....

That would put the inboard edge even with the hub face.

It has ZERO BS AND ZERO OFFSET. :D

OK, so we have our ultimate pizza cutter tire, with its inboard edge at the hub face line.

In your wheel well, that is a clearance measurement point.


Now, lets say we make our rim wider, by say two feet...the rim sticks out into the oncoming lanes of traffic...its wide.

We will keep the same backspacing, so the inboard bead is still even with the hub face....but the outboard bead is 2' outboard.

What did keeping the BS do to move the inboard tire edge FURTHER inboard?


What about our offset?

We had zero offset....but now the rim is 2' wide, so zero offset is now ~ 1' outboard of the hub face....


We kept the same BS, it works, for inboard clearance at least....the same for either rim.

We need to change the offset though...as keeping ZERO offset ALSO puts the INBOARD edge of the tire ~ 1' further inboard than our original set-up.

Obviously, we can KEEP the SAME BS, but, we need to CHANGE THE OFFSET, to keep the same inboard clearance.
 

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you know there is CTRL & + or scroll wheel to make everything bigger on your screen while in your web browser (CMD & + on Mac) and CTRL + 0 to put it back to normal :smokin:
However either your screen has distortions or you need to put on some glasses because that tire is perfectly centered where it's supposed to be centered on, between the bead seats of the wheel. The shots you see comes from a model I put into SW CAD and it is modeled at true dimensions.
There are also nice Center lines there named " Wheel CL" in nice and big letters, Zoom into the picture and take a ruler between those line and the related tires and you'll find out how nicely centered those tires in the picture are :grinpimp:

OK pop quiz, a 10" specified width 5" Backspacing wheel with a 12" wide tire:

a.) What is the approx. overall width of the wheel (outside of inboard flange to outside of outboard flange)?
b.) Is the Hub (WMS) in center or off-center in relation to the wheel width?
c.) what will be the approx. dimension between WMS and inside tire wall?
d.) What is the approx. offset? ( in Inches will be fine for me :) )
 
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