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

[PaulNY]

Can someone help a newbie please? Ok, so next week I am supposed to replace my stock JLU Rubi suspension and wheels/tires with the Mopar 2" lift and the Mopar Beadlock wheels with 35X12.50/17 tires. I have read some threads and am conflicted/confused. So, my stock Rubicon wheels are 17X7.5 with a 44mm offset and 6.25" of bs apparently. Further, the new Mopar Beadlocks are 17X8 with a 12 mm offset. However, I don't know what that equates to in regards to bs. I read on another thread where a forum member stated his accessories dept. said the Beadlock wheels won't work with the Mopar 2" lift. Is there any truth to that? If so, does it have to do with the backspacing or geometry of the 17X8 wheel with only 12mm offset compared to my stock wheels with 44mm? This is really confusing, and I sure would like to know ahead of time if the Mopar Beadlocks won't work with the 2" lift because the dealer is having the Beadlock ring powder coated as we speak. Any help is greatly appreciated.

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[seals02]

you can google "backspacing chart" ...I tried to upload it but it wouldn't load

 

[merilm]

Note to self - just be patient (eta this week) and take your JLUS to the tire / wheel specialist, measure and discuss while you stand there and determine optimal backspacing / offset in the flesh, before you order anything !!!

 

[smuddy]

@PaulNY
I'm told by others in this very thread that backspace can't always be calculated by the formula : backspace = offset + 1/2 rim width
I know to get exact backspacing you'd measure it from wheel mounting surface to the outside lip of the rim. However, it seems that should be extremely close to the formula every time. All you would have to do would be to add the lip width as well. But, maybe wheel manufacturers don't make their wheels exactly the same width as they state. Just like 35" tires aren't exactly 35"...they are usually 34.5 or even less. That being said, there are people in this forum who are running the stock wheels (6"+ bs) with 35s without rubbing the suspension components. They rub inside the wheel wells with the tire tread but that has little if anything to do with the backspacing. That is simply a lack of lift height. It is the absolute truth that you can get away with far more backspacing with the JL than you could with the jk, at least, without a lift. How much is an exercise of people trying different setups but I have a really hard time believing that the wheel spec you mentioned wouldn't work given the stock wheel working in the same manner. However, lifting a vehicle (at least on the jk it did) will shift the axle to one side by an amount in relation to the lift height. Without an adjustable track bar to bring the axle back to center, maybe these wheels will he a problem. So, sorry I can't give you a definitive answer... I can only give info on what I've learned so far.

 

[smuddy]

FYI, at least with the brand of 35" tire he has on, this guy says he had no rubbing at all with the 35s with no lift and no wheel spacers on stock rubi wheels, so that's 6"+ of backspacing! https://www.jlwranglerforums.com/fo...jlur-on-chinaman-gulch.4656/page-5#post-97186

 

[seals02]

FYI, at least with the brand of 35" tire he has on, this guy says he had no rubbing at all with the 35s with no lift and no wheel spacers on stock rubi wheels, so that's 6"+ of backspacing! https://www.jlwranglerforums.com/fo...jlur-on-chinaman-gulch.4656/page-5#post-97186

Barlow offroad peeps posted photos of rubbing with 2" lift and 35s I believe but I guess that only comes an issue when lifted.

 

[smuddy]

Barlow offroad peeps posted photos of rubbing with 2" lift and 35s I believe but I guess that only comes an issue when lifted.

Right, it appears the lift changes the geometry enough to cause a problem. Without the lift, it seems fine.

 

[DudeJeeper]

@PaulNY
I'm told by others in this very thread that backspace can't always be calculated by the formula : backspace = offset + 1/2 rim width
I know to get exact backspacing you'd measure it from wheel mounting surface to the outside lip of the rim. However, it seems that should be extremely close to the formula every time. All you would have to do would be to add the lip width as well. But, maybe wheel manufacturers don't make their wheels exactly the same width as they state. Just like 35" tires aren't exactly 35"...they are usually 34.5 or even less. That being said, there are people in this forum who are running the stock wheels (6"+ bs) with 35s without rubbing the suspension components. They rub inside the wheel wells with the tire tread but that has little if anything to do with the backspacing. That is simply a lack of lift height. It is the absolute truth that you can get away with far more backspacing with the JL than you could with the jk, at least, without a lift. How much is an exercise of people trying different setups but I have a really hard time believing that the wheel spec you mentioned wouldn't work given the stock wheel working in the same manner. However, lifting a vehicle (at least on the jk it did) will shift the axle to one side by an amount in relation to the lift height. Without an adjustable track bar to bring the axle back to center, maybe these wheels will he a problem. So, sorry I can't give you a definitive answer... I can only give info on what I've learned so far.

Well said.

 

[JASPER]

35" tire 4.5" backspacing is fine.

 

[SleepEatJeepRepeat]

I am planning on doing rubicon JL with 18x9 fuel Anza with 5.0 backspace , and 35” stt coopers that are 12.4 wide and 35.08 tall.. I will be running a teraflex st1 1.5” spacer lift with falcon 2.1 shocks.. also have my eye on a dancing hula girl... pray for me people, pray...

 

[Pig-Pen]

i worked in the aftermarket wheel industry for over 13 years. i had friends who designed oem prototype wheels for the big 3. backspacing is 100% standard across the wheel industry.

if you have a 17x9 wheel. the actual, true width of the wheel is 10". if you lay it down on the ground, sans tire, it will always measure 1" wider than what it "is". this is due to the flanges on the ends of the wheel. they are .5" on each end. the measured width of the wheel, for tire purposes, is between the flanges, hence the 9" measurement. BUT, when you measure the backspacing, you measure all the way out to the edge of the flange (the outermost edge).

backspacing is measured from the back of the mounting pad (circular part that sits against the hub) to the edge of the wheel.
lay the wheel on the ground, face down. (no tire on it preferably)
lay a straight edge across the wheel (yardstick)
measure at a 90 degree angle straight down to the backpad. this will tell you the backspacing in inches. you can multiply by 25.4 to get bs in mm.

now, some times a wheel will be a little shy of the actual width. some times the flange is not .5" exact. ive seen this more with forged wheels than cast wheels since cast wheels are made from a mold. most theyre ever off is .25" but that's a lot. typically its around .125" at most, or less.

the conversion from bs to offset is standardized and a simple equation.


wheel is 17x9 with +12mm offset

12/25.4= .47" (offset in inches)

now take the TRUE/actual width of the wheel which is 10" and divide by 2= 5"

backspacing = 5" +.47" = 5.47" it seems these wheel companies just round up since its not critical like it is with sports/muscle cars where you have limited room to work with.




lets try a 17x9.5 wheel with +25mm offset

25/25.4= .98" offset in inches, rounded

the true width of the wheel is 10.5". divide by 2 = 5.5"

5.5" + .98" = 6.23" backspacing


to calculate from backspacing to offset, do the above formula in reverse order. its the same every time, guaranteed.
*i have seen it where a name brand japanese wheel had the backspacing and offset both cast into the back of the wheel and they did not calculate to each other correctly lol. perhaps they do it a little differently but in the US its standardized, unlike tires.

speaking of tires, an 8" wheel with a 285 on it vs an 8" wheel with a 315 on it. you now have sidewall that extends beyond the backspacing. in sports cars you can run into trouble there. with a jeep, you probably have quite a bit of wiggle room.

working with a few wheel manufacturers, everyone uses backspacing. its 10x easier to work with. if a customer/shop were to send in paperwork with the offset on it, i would have to do the conversion to get the backspacing and use that. we didnt even discuss offset lol. offset seemed to be more common in the import scene but those who really knew what they were talking about also knew backspacing.

with backspacing, you can figure out from the back of the wheel/tire to the closest part of the car's suspension/body/etc that will come into contact and go from there. let's say you throw a 10" wheel on the vehicle for test fitment and you measure 2.5" before you hit anything. you can then say, i can add 2.25" of more backspacing to the wheel (wider wheel). nobody would say, i can add more offset lol. this is how its done with custom ordered wheels, where the customer tells the manufacturer what backspacing they want for their custom build. shoot, we did a wide body Pantera that had different backspacing at every single corner due to Italian manufacturing of the body lol. talk about custom and measuring ten times.

hopefully that didnt confuse people any more. just trying to clear up some misinformation i have read in this thread.

 

[alphalife9]

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!

The bolded text is the problem. The factory wheel and the 17x9 have a different width. Although the 1.26" difference from the centerline is accurate, the distance from the hub to the inner edge or lip of the wheel is (backspace) is not 1.26"

If the offset was zero, the stock wheel would have 3.75" measured from the hub to the lip, while the 17x9 would have 4.5inches, meaning that both wheels mounted on your jeep right against the center of the wheel would yield one being closer to the suspension and also sticking out by half of the difference of the wheel width, respectively.

Although a tire will sit just the same unmounted on a 17x9 size wheel regardless of the offset and resultant backspacing, once you put it on the jeep you will see the effect of the backspacing.

Offset is not a good number to go by unless you use it to manually calculate the backspacing using the width of the wheel. Without taking into consideration the width of the wheel, the offset is meaningless.

If you picture severely extreme examples you can easily understand the effect. Picture two 17x9 wheels. one with a mounting hub on the very inner lip of the wheel (super deep dish wheel) and one with a mounting hub o the very outer lip of the wheel (flat looking front wheel like a hubcap).

These two obviously have the opposite offsets, one resulting in 0" backspace and one resulting in 9" backspace. Now picture them mounted on the Jeep and assume that the axle mounting surface is exactly in the middle of the wheel well.

One wheel would have its entire width behind the mounting surface, all going towards the suspension. One wheel would have the entire width in front of the hub, sticking out of the fender a lot.

Both could have the same tires looking the same on the wheel but the tire would be in a very different location in respect to the Jeep once you mounted them.

So now picture 6.25" of backspace (6.25 out of 7.5 inches of the stock wheel width behind the mounting surface, towards suspension).

and then 5" of backspace (5" out of 9" of the 17x9 aftermarket wheel width behind the mounting surface, towards the suspension)

The 5" backspace wheel is 1.25" further away from touching the suspension (6.25-5) so you have more clearance.

In front of the hub and away fro the Jeep, the stock wheel has 1.25 inches of wheel "sticking out," leaving it practically flush with the fender as Jeep wanted.

The 17x9 has 4" in front of the hub and away from the jeep "sticking out" (2.75" more than the stock wheel) leaving those 2.75" outside of the fender assuming the stock one is flush.

The above is true with the same wheel widths with different backspacing. The amount of wheel in front and behind the mounting surface of the Jeep will vary depending on the backspacing which will result in having a tire closer or farther away from the suspension, and closer to or farther out from the end of the fender.


I hope this made some sense. Backspace will tell you how close or far away from the suspension you are (more backspace is closer), and wheel width minus backspace will tell you how far out it pokes out from the fender (how much wheel remains in front of the hub mounting surface).

 

[FTheBS]

Looking to pull trigger on lift; 2 to 2.5 max for 35’s (insert joke now). I have 17*9 rims and looking to run 35x12.50 17... seems 4.50 is ideal backspacing my math puts me at 5.79 backspacing. I don’t want to run spacers. Am I going to be okay or now sell new rims to get rims that have -12mm backspacing???? Looking at metalcloak...

 

[blnewt]

Looking to pull trigger on lift; 2 to 2.5 max for 35’s (insert joke now). I have 17*9 rims and looking to run 35x12.50 17... seems 4.50 is ideal backspacing my math puts me at 5.79 backspacing. I don’t want to run spacers. Am I going to be okay or now sell new rims to get rims that have -12mm backspacing???? Looking at metalcloak...

What is the offset on your new 17x9" wheels?

 

[FTheBS]

What is the offset on your new 17x9" wheels?

+20mm which looks like gives me a 5.79 if math is correct... wonder if on JLUR if anyone kept stock rims and did 2 to 2.5 and had no issues with full flex off road.

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