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

[smuddy]

After reading seemingly hundreds of posts about backspacing, offset and rim width, I'm finding that opinions are all over the place! But after playing around with a lot of numbers with this site: http://www.rimsntires.com/specspro.jsp?item=Wheel-Tire size comparer&sw1=285&ar1=70&bd1=17&rw1=7.5&et1=44&text1=Sample 1: Hummer H2 wheel&sw2=315&ar2=70&bd2=17&rw2=9.0&et2=12&text2=Sample 2: Honda Civic wheel

It seems that backspacing doesn't really matter on whether or not a wheel/tire will fit without rubbing the suspension. To me, it seems more about the change in offset coupled with the change in tire width. Rim width doesn't matter one difference. If you have a rim that is 7.5" wide and one that is 9" wide using the same offset of say +12, the 9" rim will have more backspace, and yes, the rim will both stick out further and be closer to suspension than the 7.5" rim, but if you are relating this to the same size tire, say a 315 width, then where the tire is will be in EXACTLY the same location in relation to where it sticks out past the fender and how close it is to the suspension even though one has a backspace of 4.7" (7.5" rim) and the other has a backspace of 5.5" (9" rim).

So why is everyone all over the place with this?

The mopar beadlock wheel on the mojito jeep at the LA autoshow has a +12 offset and it's an 8" wheel with an effective backspace of 5". So it would seem that any rim width would work as long as it has +12 offset even though it will change the backspace for the rim quite a bit since, it won't change the proximity of the tire to the suspension components on the inside of the wheel.

I was looking at a 17x9 wheel with +12 offset to use with 315 70 17s and that site says I'll have more clearance on the inside of the tire by about .5" so I should be fine, even though it has a backspace of 5.5".

Thoughts?

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[on--belay]

My limited experience with this: for me the 4.5" comes from the JK Mopar 2" lift I have, which states in the instructions that they recommend that Backspacing as stock may rub, both in the front, and be an issue with the rear swaybar link attachment hardware (I remember people in a Mopar lift thread somewhere did say they had issues with that and stock backspacing of 6.25").

With the increased axle width, and redesigned suspension of the JL, my question is do we still need the 4.5" backspacing go-to that developed with the JKs. Because going from a 7.5" rim to a 9" rim, and then simultaneously moving from a 6.25" to 4.5" backspacing gains you 3.25" more width on both sides, and I'd kind of like to keep rock throwing M/Ts under the fenders.

I think on the JK the backspacing was the critical component, the offset was more of a result of whatever rim width one picked, and not a critical measurement.

 

[tdubs]

Backspacing is annoying measurement that I had never heard of being referred to before joining the jeep/4x4 community. Backspacing is simply a consequence of rim width and wheel offset. Because offset is typically in millimeters and backspacing is typically described in freedom units, I think it confuses people more than it should.

Apologies if what follows is something you already understand or information you weren't looking for. Skip to the last section if you're just looking for an answer to your question(s).

To say that any wheel with +12 offset will work is incorrect (although it would probably be fine in most cases). To save the internet of bad MS Paint drawings, here are examples of different offsets (side views of a hypothetical wheel cut in half).

0 offset - the mounting pad is directly centered in the middle of the wheel.

In this case backspacing is easy to calculate, it would be wheel width divided by 2 (roughly). For instance a 9.5 inch wide wheel with 0 offset would have a 4.75" backspace. 8" wide -> 4" backspace. etc etc.


Negative offset

In this case backspacing would be wheel width divided by 2 plus (minus because it's negative) offset.


Positive offset

Backspacing calculation would be wheel width divided by 2 plus the offset.


In extreme cases of positive offset, you can probably imagine the wheel would potentially interfere with suspension components or start "rubbing" on the frame/body.

Negative offset helps this situation, but in extreme cases you may be poking out of the fender flares by many inches.

So while Mopar's 8 inch wide wheel may work with +12 offset, a hypothetical 12 inch wide wheel would extend 2 inchs further toward suspension components with the same offset. This is why you can't look at only offset when trying to verify clearance.


To answer your question, I think there's so much misinformation/conflicting opinions on what will "work" simply because no one knows for sure. There are only a few examples out in the wild where people have replaced their wheels/tires. We certainly haven't exhausted all possibilities at this point, so people can only suggest what worked for them. There won't be a definitive "you need X amount of backspace for full clearance/flex" until the community has the opportunity to try different combinations or someone who has one takes a bunch of measurements.

The picture gets more complicated in that backspacing doesn't tell the whole picture either. If your tire is wide enough, it will extend beyond the backspacing measurement even further towards the suspension components. Not by much, but enough to not be able to ignore it in certain situations. Offset being the same, different width rims will cause the tire to have a slightly different "shape" or "bulge" of the sidewall. This may or may not affect clearance, again one of those every situation is different things.

Based on the specs you proposed and having looked at a handful of different wheel/tire combos I'd say you'll probably be alright with 17x9 at 12 offset. But the only way to know for sure would be to measure the clearance between stock wheels and suspension components/frame/body at full turn, then use a tire/wheel calculator to estimate how much further you will be "poking" in. If the additional poke is less than the stock clearance measurement, you should be good.

Hope this helps

 

[on--belay]

So while Mopar's 8 inch wide wheel may work with +12 offset, a hypothetical 12 inch wide wheel would extend 2 inchs further toward suspension components with the same offset. This is why you can't look at only offset when trying to verify clearance.

Which is exactly why backspacing isn't an annoying measurement, but rather has been a defining measurement in terms of what works best with Jeeps. Most compatible rims for Jeeps, especially those lifted, seem to have close to 4.5" backspacing, so then it doesn't matter in terms of rim width or offset with respect to clearance from suspension or hard parts, it just mattered how much wheel you had sticking out past your fenders, so if you went from an 8" wheel to 9" wheel, and maintained your backspacing of 4.5", you stuck out exactly 1" farther, but did nothing to your suspension clearance. Maybe if you were already super tight pushing it out would give you fender or liner issues at full articulation, but in the Jeep world it seems the offset is the least useful measurement, as you picked the backspacing that worked (usually 4.5" in JK world), you picked the rim width you wanted, and then the offset was just a consequence of those 2 choices.

 

[Zoobie]

The Mopar catalog and the wheels they offer have a 12mm (4.4”) wheel backspacing. I'm not sure if that's right... or just carried forward from the JK... but if you look at the wheels on all the JL's they have at the auto shows with larger tires, they all have 12mm (4.4”) wheel backspacing

 

[on--belay]

The Mopar catalog and the wheels they offer have a 12mm (4.4”) wheel backspacing. I'm not sure if that's right... or just carried forward from the JK... but if you look at the wheels on all the JL's they have at the auto shows with larger tires, they all have 12mm (4.4”) wheel backspacing

12mm is just under 1/2", that is the offset. But your point about the backspacing is correct, go to TireRack, or any of the giant online wheel and tire places, and you'll find the vast majority of JK aftermarket wheels have around 4.5" backspacing. Which is what people are throwing on JLs now.

 

[tdubs]

Which is exactly why backspacing isn't an annoying measurement, but rather has been a defining measurement in terms of what works best with Jeeps. Most compatible rims for Jeeps, especially those lifted, seem to have close to 4.5" backspacing, so then it doesn't matter in terms of rim width or offset with respect to clearance from suspension or hard parts, it just mattered how much wheel you had sticking out past your fenders, so if you went from an 8" wheel to 9" wheel, and maintained your backspacing of 4.5", you stuck out exactly 1" farther, but did nothing to your suspension clearance. Maybe if you were already super tight pushing it out would give you fender or liner issues at full articulation, but in the Jeep world it seems the offset is the least useful measurement, as you picked the backspacing that worked (usually 4.5" in JK world), you picked the rim width you wanted, and then the offset was just a consequence of those 2 choices.

Backspacing and offset measure the same thing (position of mounting pad) from two different points on the wheel. Offset is from the center of the wheel, backspacing is from the inside "plane" of the wheel. Wheel manufacturers always (or lets say more often than not) specify offset regardless of application. This is why I choose to look at it from the perspective that backspacing is a consequence of your offset and wheel width, not the other way around.

This is the annoying part. Neither measurement guarantees you clearance, however one is thrown out as the end all be all to clearance, while the other is a "mysterious" number few take them time to apply to their specific application.

 

[on--belay]

This is the annoying part. Neither measurement guarantees you clearance, however one is thrown out as the end all be all to clearance, while the other is a "mysterious" number few take them time to apply to their specific application.

I understand what you are saying. I chose backspacing because manufacturers like Teraflex and Mopar specify 4.5" backspacing when suggesting wheels to accommodate their lifts, and they just don't mention offset at all, since within the confines of "normal" width rims, say 7.5" to 10", the offset doesn't matter assuming their suggested backspacing, and as I mentioned, is just consequential.

 

[tdubs]

I understand what you are saying. I chose backspacing because manufacturers like Teraflex and Mopar specify 4.5" backspacing when suggesting wheels to accommodate their lifts, and they just don't mention offset at all, since within the confines of "normal" width rims, say 7.5" to 10", the offset doesn't matter assuming their suggested backspacing, and as I mentioned, is just consequential.

Fair enough. More of a 6 in one, half a dozen in the other kind of problem. It was just so confusing to me coming from import/domestic muscle/sports car world to see this new measurement I have never seen before (dirty outsider ).

Looking forward to finding a set of wheels I love for my Jeep. I'm sure I'll learn to love backspacing as there will be a lot more examples of people working with backspacing rather than offset.

 

[on--belay]

It was just so confusing to me coming from import/domestic muscle/sports car world to see this new measurement I have never seen before (dirty outsider ).

I can empathize. If I just slapped an extra 3 inches of tire to the outside of my 911, I'd have custom grooved tires, or flared fenders after a few laps. There's just a little more room in the wheelwells to play with on these Jeeps.

 

[Swanny297]

Maybe this will help some - now this is from Ram1500 which has a 4” lift and I run 35” tires. In order to push the inside of my tire far enough a way from my control arms I did the following -

Factory 20x9 wheel - +19mm offset - 5.75” backspace
New wheels - 20x9 - 0mm offset - 5.00” backspace

This small change pushed my tire about 1.5” outside the fender, but allowed a wider tire to clear the control arm supports for my lift. The factory offset of +19 would not allow me to run a 35” and no where close on a 37” wheel without the inner tire wall contacting the control arm area.

The higher the backspace the further into the fender well it will sit.

Positive offset tucks the entire wheel in toward the brakes and suspension.

Negative offset moves the wheel out away from the brakes and suspension.

 

[smuddy]

I understand what you are saying. I chose backspacing because manufacturers like Teraflex and Mopar specify 4.5" backspacing when suggesting wheels to accommodate their lifts, and they just don't mention offset at all, since within the confines of "normal" width rims, say 7.5" to 10", the offset doesn't matter assuming their suggested backspacing, and as I mentioned, is just consequential.

This just doesn't makes sense to me and I think it's wrong. If you have the same offset, for every rim width 7.5, 8, 8.5, 9, 9.5 and 10", the backspace will all be different. However, when you think of the width of the actual tire, that is the part that is sticking out of the wheel well and into the suspension. You'd never have the rim sticking out further than the tire, so it really seems to be that backspacing on the rim has nothing to do with it. For instance,

7.5" rim with 12 mm offset = 4.2" backspace
8" rim with 12 mm offset = 4.5" backspace
9" rim with 12 mm offset = 5" backspace
10" rim with 12 mm offset = 5.5" backspace

Even though the rim is that much closer to your suspension components, you tire is exactly the same distance from the suspension components (barring changes in tire shape because of rim width. The tire may bulge a bit with a narrower rim...but it should be negligible) So, it doesn't matter.

Now, if you have at least 4.5" backspace, you are probably guaranteeing that it won't interfere with suspension but it's simply not needed. If you keep to that 4.5" backspacing requirement then you'll be pushing your tire further and further outside of the fender as you go up in rim width that has nothing to do with the tire width...so you'll be dragging that inside part of the tire out further than necessary along with it.

Look at this screenshot from http://www.rimsntires.com/specspro.jsp?item=Wheel-Tire size comparer&sw1=285&ar1=70&bd1=17&rw1=7.5&et1=44&text1=Sample 1: Hummer H2 wheel&sw2=315&ar2=70&bd2=17&rw2=9.0&et2=12&text2=Sample 2: Honda Civic wheel

It compares the factory rim of 7.5 with 44mm offset and a 285/70/17 tire against a 10" rim with 12mm offset with a 315/70/17 tire. They calculate their offset by adding an inch to the wheel width since it's supposed to be measures to the outside of the flange. Both have the same backspace of 6" (5.5" if you don't include the flange) And yet the 315 will stick out 1.9" further to the outside than the stock tire which is more than the difference in tire width giving you more space on the inside of the tire between it and the suspension components.

 

[smuddy]

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/

 

[on--belay]

This just doesn't makes sense to me and I think it's wrong. If you have the same offset, for every rim width 7.5, 8, 8.5, 9, 9.5 and 10", the backspace will all be different. However, when you think of the width of the actual tire, that is the part that is sticking out of the wheel well and into the suspension. You'd never have the rim sticking out further than the tire, so it really seems to be that backspacing on the rim has nothing to do with it. For instance,

7.5" rim with 12 mm offset = 4.2" backspace
8" rim with 12 mm offset = 4.5" backspace
9" rim with 12 mm offset = 5" backspace
10" rim with 12 mm offset = 5.5" backspace

Even though the rim is that much closer to your suspension components, you tire is exactly the same distance from the suspension components (barring changes in tire shape because of rim width. The tire may bulge a bit with a narrower rim...but it should be negligible) So, it doesn't matter.

But it isn't wrong, you are just making it way more complicated than it has to be, or trying to oversimplify it as offset is all that matters, which is wrong. Tire manufacturer's quote a rim width range, a reference rim width, and resultant section width and tread width. While it is true the rim width won't change the tire section width or tread width a great deal, you missed the point I made. Here is the copied text from one of the Mopar 2" lifts:

IMPORTANT TIRE / WHEEL and CLEARANCE DATA • Maximum tire width for use on factory wheels is 10.50". Factory Wrangler wheels are 17" diameter x 7" wide with 6.25" backspacing. • Minimum aftermarket wheel diameter is 17". • For an 11.50" wide tire, use an 8" to 8.5" wide wheel with 3.25" to 4.75 backspacing. Ideal backspacing is 4.5". • For a 12.50" wide tire, use an 8.5" to 9" wide wheel with 3.25" to 4.75" backspacing. Ideal backspacing is 4.75". With 12.50" wide tires, expect some rear tire scrub at the plastic inner fender liners during full travel / articulation while off-roading; this generates a slight sound, but no damage. • Maximum tire diameter is 35". For adequate clearance of 35" tires during full travel / articulation while off-roading, minor body trimming is required at the front / lower corner of the rear fenderwell openings. The rear ends of factory Rubicon rock rails must also be trimmed; both trimming steps are detailed in these Instructions.

Notice they speak to minimum backspacing, nothing about offset, because how far you position the inner rim surface, and subsequently tire sidewall IS critical, so your examples, the 5.5" backspacing might rub. Your example assumes all sorts of different widths and backspacing, and that is not what I was saying, and does not tie in with the "norm" for JK lifts. As I've stated the bulk of JK aftermarket rims have a 4.5" backspacing, and that is for a reason, so holding that as your primary dimension you just push out the wheel and tire farther and farther as you widen the rim, it's that simple.

Now, if you have at least 4.5" backspace, you are probably guaranteeing that it won't interfere with suspension but it's simply not needed. If you keep to that 4.5" backspacing requirement then you'll be pushing your tire further and further outside of the fender as you go up in rim width that has nothing to do with the tire width...so you'll be dragging that inside part of the tire out further than necessary along with it.

Thus my question about what is "needed" on a JL. On a JK, you are wrong, it IS needed. I mounted my stock Recon rims to see just how close the rear swaybar link hardware was to rim/tire, and it was CLOSE, and under hard cornering I would have had contact, which is exactly what some other owners reported, so what was needed in terms of backspacing depended on the section width and rim width, but something other than stock WAS needed, and somewhere Teraflex, Mopar, and others arrived at 4.5". That number wasn't pulled out of a hat, it gave the best chance of having no clearance issues with most lifts, and most upsized tires.

It compares the factory rim of 7.5 with 44mm offset and a 285/70/17 tire against a 10" rim with 12mm offset with a 315/70/17 tire. They calculate their offset by adding an inch to the wheel width since it's supposed to be measures to the outside of the flange. Both have the same backspace of 6" (5.5" if you don't include the flange) And yet the 315 will stick out 1.9" further to the outside than the stock tire which is more than the difference in tire width giving you more space on the inside of the tire between it and the suspension components.

See the Mopar text above? It says NOTHING about offset. I understand offset and its significance in wheel design, and coming from sports cars, it is critical there, but why do you think none of the lift companies talk about offset with Jeeps and only talk about backspacing? Because you have these massive basically open fender wells on a lifted Jeep, and so bumping things out to create clearance for hard parts, and turning radius consideration doesn't cause the same issues it would on say, a Civic. I am not discounting that pushing things out has its own consequences, and keeping things in as far as possible will benefit the suspension in several ways, so what I want to know is where do we run into clearance issues on a lifted JL, will the new norm for JL wheels actually be the stock 6.25" backspacing, because if it is, and we assume the 9" standard width, then the 12mm will not be the "usual" offset anymore.

 

[on--belay]

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.

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.

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