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post #1 of 21 Old 03-28-2016, 06:34 PM Thread Starter
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Backspacing Confusion

I've been reading for the last 2 days on backspacing and wheel offset which has only led to more confusion.

My 2 brain cells can't handle it so...My Specific Question is:

Can I run a 37x12.50 tire on a 17x9 rim without rubbing given the following:

Wheel: 4.5" backspacing
Jeep: Teraflex 3" Lift
PS Flat Fenders
Aftermarket Front Bumper (ACE Pro-Series)

Thanks for any input I hope I provided enough information for answers.

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post #2 of 21 Old 03-28-2016, 07:05 PM
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Should be no problem. I run that same size wheel same back space on 37-13.5/17s.

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post #3 of 21 Old 03-28-2016, 07:11 PM
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You should be fine.


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post #4 of 21 Old 03-28-2016, 07:41 PM Thread Starter
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Quote:
Originally Posted by Mark915 View Post
Should be no problem. I run that same size wheel same back space on 37-13.5/17s.
Quote:
Originally Posted by marcspaz View Post
You should be fine.
What is the offset on the wheels you guys are running- if any?

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post #5 of 21 Old 03-28-2016, 09:25 PM
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I am running 4.5" of backspacing. I have about 1.75 inches of space between my rim (and side of the tire) and the closest suspension parts, being the rear sway bar links. You will have about 0.5" of space at the closest point with the tires you mentioned.


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Last edited by marcspaz; 03-28-2016 at 09:31 PM.
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post #6 of 21 Old 03-29-2016, 06:37 AM
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Quote:
Originally Posted by LepreCon View Post
What is the offset on the wheels you guys are running- if any?
A 9" wheel with 4.5" of backspacing has an offset (ET) of -13.

A 9" wheel with 0 ET would have 5" of backspacing.

Easiest way to calculate: Take the wheel width, add 1 inch, and divide by 2. That's the backspacing with 0 offset.

For your wheels: (9+1)/2 = 5" of backspacing

Offset is measured in millimeters, and you want to decrease the backspacing by 0.5", or 13mm. So that gets you to -13 ET.

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post #7 of 21 Old 03-29-2016, 06:53 AM Thread Starter
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Thanks for all the input. Making it much easier for me.

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post #8 of 21 Old 03-29-2016, 07:19 AM
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That may not take the confusion out of it for him. LoL

50,000 foot view. You have a flat matting surface on the back side of the wheel that touches/mates with the wheel hub. That flat surface is the starting point for our measurements.

If that flat surface is perfectly aligned with the center of the wheel, you have zero offset. If the mounting point is closer to the back of the wheel, you have a negative offset. If it is closer to the front of the wheel you have a positive offset.

That's it. Its that simple.

Now, with our Jeeps, unless you are buying a wheel that advertises the offset and not the backspacing, the offset doesn't have any real significance. The reality is, regardless of how wide any wheel is, if you don't have 4.5" of backspacing, or numerically lower, something is going to rub.

Use the offset If no backspace info is available. Every 25.4mm is 1". If you are looking at 9" wheels that only have the offset displayed, to get 4.5" of backspacing, you need a 9" wheel with zero offset. 9/2 is 4.5. Your mounting surface being smack in the center of the wheel giving you your 4.5" of backspace. A suitable 9" wheel would have between zero and some negative number.

For another example, If you have a 7" wheel, and you wanted 4.5" of backspace, you would get a wheel with a +26mm offset. If it was an 11" wheel, you would want a -26mm offset. Both examples yield 4.5" of backspace.

Just remember, the further out you push the tire, the more pressure on your bearing and other steering and suspension stuff. Only push it out as far as you need too.


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Last edited by marcspaz; 03-29-2016 at 07:24 AM.
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post #9 of 21 Old 03-29-2016, 08:35 AM
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Quote:
Originally Posted by marcspaz View Post
That may not take the confusion out of it for him. LoL

50,000 foot view. You have a flat matting surface on the back side of the wheel that touches/mates with the wheel hub. That flat surface is the starting point for our measurements.

If that flat surface is perfectly aligned with the center of the wheel, you have zero offset. If the mounting point is closer to the back of the wheel, you have a negative offset. If it is closer to the front of the wheel you have a positive offset.

That's it. Its that simple.

Now, with our Jeeps, unless you are buying a wheel that advertises the offset and not the backspacing, the offset doesn't have any real significance. The reality is, regardless of how wide any wheel is, if you don't have 4.5" of backspacing, or numerically lower, something is going to rub.

Use the offset If no backspace info is available. Every 25.4mm is 1". If you are looking at 9" wheels that only have the offset displayed, to get 4.5" of backspacing, you need a 9" wheel with zero offset. 9/2 is 4.5. Your mounting surface being smack in the center of the wheel giving you your 4.5" of backspace. A suitable 9" wheel would have between zero and some negative number.

For another example, If you have a 7" wheel, and you wanted 4.5" of backspace, you would get a wheel with a +26mm offset. If it was an 11" wheel, you would want a -26mm offset. Both examples yield 4.5" of backspace.

Just remember, the further out you push the tire, the more pressure on your bearing and other steering and suspension stuff. Only push it out as far as you need too.
Not trying to be nit-picky here, but...

A wheel's specified width is the distance between the beads, not the outside-to-outside width. You have to add an inch to the specified width to get the outside-to-outside width, then you can figure the backspacing.

Chase

2013 Billet Silver JKU Sport S, 6-Speed, TF Leveling Kit, Spidertrax 1.5" Spacers, 285/70R17 Firestone Destination M/Ts on OEM Sport wheels.
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post #10 of 21 Old 03-29-2016, 08:46 AM
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There are other things that go into "not rubbing" besides wheel back spacing. Shock length and bump stops will affect the amount of axle rotation turing suspension travel. Driving around on the road a 4.5" on a 9" wheel should be fine but I would expect it to rub if you run disconncted and you wheel hard.

Some build info here:
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post #11 of 21 Old 03-29-2016, 09:50 AM
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post #12 of 21 Old 03-29-2016, 10:33 AM
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Quote:
Originally Posted by COBrien View Post
Not trying to be nit-picky here, but...

A wheel's specified width is the distance between the beads, not the outside-to-outside width. You have to add an inch to the specified width to get the outside-to-outside width, then you can figure the backspacing.
The bead flange is not 1" thick. Maybe 1/2" on the back side. Also, the math you are doing is not industry standard.

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel, measured in millimeters. Backspacing is the distance from the hub mounting surface to the inside lip of the wheel, measured in inches. You don't add an inch to the rim width to get backspacing.


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post #13 of 21 Old 03-29-2016, 10:50 AM
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Quote:
Originally Posted by marcspaz View Post
The bead flange is not 1" thick. Maybe 1/2" on the back side. Also, the math you are doing is not industry standard.

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel, measured in millimeters. Backspacing is the distance from the hub mounting surface to the inside lip of the wheel, measured in inches. You don't add an inch to the rim width to get backspacing.
I understand how it works. Strictly speaking, you are correct. The backspace of any particular wheel is the distance from the mounting flange to the inside lip, regardless of what the manufacturer advertises.

However, if you look at any aftermarket wheel with 0 ET, the backspacing is equivalent to the advertised width plus one inch, divided by 2. It may not be "industry standard," but it's damn close at least 95% of the time.

EDIT: I think we're saying the same thing, just looking at it from different perspectives. When you refer to a 9" wide wheel, you mean 9" wide, measured outside to outside of the lips. When I refer to a 9" wide wheel, I mean a wheel that's advertised as being 9" wide (which, I believe, is what the OP is asking about). When laid down and measured, a wheel that's advertised as being 9" wide will actually measure 10" wide (outside to outside) or damn near it.

Chase

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Last edited by COBrien; 03-29-2016 at 01:46 PM.
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post #14 of 21 Old 03-29-2016, 09:19 PM Thread Starter
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Bit the bullet and finally ordered.

Ended up buying 17x8.5 (4.75" BS) wheels
1.5" Spacers as insurance
37x12.50r17 (KO2 so on the smaller size.)

My current wheels are 16x8 (4" BS). I wanted to be slightly less than this with a taller tire and new setup will end up being a full 3/4" less with spacers.

Aside from trimming the pinch seam, I'm hoping for no rub. I'll get the wheels and tires and play around with bump and clearance issues.

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post #15 of 21 Old 03-30-2016, 03:48 AM
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I'm having a set of 37x13.50-18 mounted to a set of 18x9 wheels with 0 offset next week.

I'm going to try like all get-out to not use the 1.5" spacers, but I suspect that I will be forced to spacer up.


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post #16 of 21 Old 03-30-2016, 05:36 AM
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OP. Sounds good. Good luck. Looking forward to some pics.


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post #17 of 21 Old 04-01-2016, 08:48 PM Thread Starter
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Had the wheel and tires installed by DT today. The ride home (highway) was much smoother and quieter than the 315/75/16 Cooper STT's but somewhat expected and wanted.

The sad part, the tires at 34 psi measured exactly 35" tall.

Anyhow, certainly need to trim the rear pinch seams (both front and back) and based on the eye test, would need to add the spacers. Seems that at full lock and compression, I'd be getting into the frame... and need add some bump. A lot of work for 35's.

Hoping to get the time this weekend to run it through some flex tests and check it all over.
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post #18 of 21 Old 04-01-2016, 11:44 PM
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They look good.


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post #19 of 21 Old 04-04-2016, 03:40 PM
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I have a 2013 Sahara and it had a lift on it when I bought it but I cant tell what it is. It looks to be 3" coils but the dealer painted the underside shocks ect.. but there are not shock extender mounts or anything however it sits a lot higher than a stock one. I am ordering wheels and tires it has stocks on it now, I would like to run a 35 12.50 and try to stay under stock flares as much as possible. I am in PA and they are hard on jeeps with tires sticking out. I was looking at 18 x 9 with zero offset / back space. will these stick out a tone or do you think I will be ok? sorry for the long winded message
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post #20 of 21 Old 04-04-2016, 04:45 PM
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If it helps I was advised by rock krawler to have a back space of 4.5 for my 2.5" lift to be able to use oem fenders with 35's..


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post #21 of 21 Old 04-04-2016, 05:56 PM
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Quote:
Originally Posted by jmor1000 View Post
I have a 2013 Sahara and it had a lift on it when I bought it but I cant tell what it is. It looks to be 3" coils but the dealer painted the underside shocks ect.. but there are not shock extender mounts or anything however it sits a lot higher than a stock one. I am ordering wheels and tires it has stocks on it now, I would like to run a 35 12.50 and try to stay under stock flares as much as possible. I am in PA and they are hard on jeeps with tires sticking out. I was looking at 18 x 9 with zero offset / back space. will these stick out a tone or do you think I will be ok? sorry for the long winded message
You will be fine with a 0 offset.
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