Offset question. - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 9 Old 09-06-2019, 01:53 PM Thread Starter
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Offset question.

I just purchased a set of 35x12.5x18 tires and now looking at rims. I found a pair i like 18' with a 10mm offset. I have a 3" lift and my concern is should I go Negative offset or will I be ok with 10mm Positive offset?
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post #2 of 9 Old 09-06-2019, 03:13 PM
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Quote:
Originally Posted by mttamer View Post
I just purchased a set of 35x12.5x18 tires and now looking at rims. I found a pair i like 18' with a 10mm offset. I have a 3" lift and my concern is should I go Negative offset or will I be ok with 10mm Positive offset?
Depends on how wide your new wheels are.

You want a maximum of 4.75" backspacing for 35s, with <4.5" being preferable in case 37s happen later.

It can get confusing because offset is in mm while backspace is in inches, but it's "easy" to calculate - see below.

Definitions / explanations of wheel width, backspacing, and offset:

  • Wheel width (advertised and "effective" width) is the inside distance between tire mounting flanges.
  • Overall width is +1/2" more per side, or 1" wider total, than the advertised width.
  • Backspacing is the distance (in inches) from wheel mounting flange to innermost edge of the rim.
  • Offset is the measurement (in mm) from the wheel mounting flange to the wheel centerline.
  • Positive offset = more backspacing
  • Negative offset = less backspacing.


Still with me? OK, here's the math:

Calculating backspacing from width + offset information:

(1/2 of wheel width) + (offset dimension) + 0.5" (the inner rim flange width) = backspacing.

<Note: to convert offset dimension in mm to inches, divide by 25.4>

So . . .

18x10" wheel with +10mm offset would have (10/2)+(10/25.4) +1/2" = 5.89" backspacing. NO
18x10" wheel with -10mm offset would have (10/2)+(-10/25.4) +1/2" = 5.10" backspacing. NO

18x9" wheel with +10mm offset would have (9/2)+(10/25.4) +1/2" = 5.39" backspacing. NO
18x9" wheel with -10mm offset would have (9/2)+(-10/25.4) +1/2" = 4.61" backspacing. OK

18x8" wheel with +10mm offset would have (8/2)+(10/25.4) +1/2" = 4.89" backspacing. NO
18x8" wheel with -10mm offset would have (8/2)+(-10/25.4) +1/2" = 4.11" backspacing. OK

18x7" wheel with +10mm offset would have (7/2)+(10/25.4) +1/2" = 4.39" backspacing. OK
18x7" wheel with -10mm offset would have (7/2)+(-10/25.4) +1/2" = 3.61" backspacing. OK

__________________________________________________ ___________

If that^ doesn't answer your question, I'd be genuinely curious as to why
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Last edited by ExWrench; 09-06-2019 at 04:34 PM. Reason: Adding more backspacing calculations (spoonfeeding)
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post #3 of 9 Old 09-06-2019, 05:06 PM
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Huh, I just realized you're the guy that was asking about going from 18s to 17s or 16s in another thread.

Here's another option:

If the 18" wheels you have are good and you just need to decrease backspacing, Spidertrax wheel spacers are a viable solution.

I ran my stock Rubi rims with Spidertrax spacers for a few years with zero issues. Torqued correctly with red Loctite, I trust 'em with my life. Sold 'em used for half the price of new.

There are other brands out there, but I'd definitely go with a reputable manufacturer and wouldn't risk offshore crap - the material certifications could be bogus, and the risk isn't worth it.

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post #4 of 9 Old 09-06-2019, 05:20 PM Thread Starter
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Quote:
Originally Posted by ExWrench View Post
Huh, I just realized you're the guy that was asking about going from 18s to 17s or 16s in another thread.

Here's another option:

If the 18" wheels you have are good and you just need to decrease backspacing, Spidertrax wheel spacers are a viable solution.

I ran my stock Rubi rims with Spidertrax spacers for a few years with zero issues. Torqued correctly with red Loctite, I trust 'em with my life. Sold 'em used for half the price of new.

There are other brands out there, but I'd definitely go with a reputable manufacturer and wouldn't risk offshore crap - the material certifications could be bogus, and the risk isn't worth it.
Yeah i found a really good deal on some 18" tires so I need to find 18" wheels and the company asked what the specs required from my lift kit?
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post #5 of 9 Old 09-06-2019, 05:21 PM Thread Starter
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the wheels I am looking at are 9" wide
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post #6 of 9 Old 09-06-2019, 08:50 PM
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Quote:
Originally Posted by mttamer View Post
the wheels I am looking at are 9" wide
Then you want wheels with a negative offset of at least 6.35mm (to get max. 4.75" BS).




EDIT: the math:

18x9" wheel with -6.35mm offset would have (9/2)+(-6.35/25.4) +1/2" = 4.75" backspacing. OK

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Last edited by ExWrench; 09-06-2019 at 09:01 PM.
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post #7 of 9 Old 09-07-2019, 06:39 AM Thread Starter
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Thank you! It's somewhat confusing, but after watching several videos explaining it I think I am beginning to understand.
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post #8 of 9 Old 09-07-2019, 11:13 AM
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Quote:
Originally Posted by mttamer View Post
Thank you!
You're welcome!


Quote:
Originally Posted by mttamer View Post
It's somewhat confusing, but after watching several videos explaining it I think I am beginning to understand.
I hope all that shit I typed above also contributes to your comprehension.

If not, I believe it'll help the . . . . . . "book learners" among us

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post #9 of 9 Old 09-08-2019, 04:40 AM
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Quote:
Originally Posted by ExWrench View Post
Huh, I just realized you're the guy that was asking about going from 18s to 17s or 16s in another thread.

Here's another option:

If the 18" wheels you have are good and you just need to decrease backspacing, Spidertrax wheel spacers are a viable solution.

I ran my stock Rubi rims with Spidertrax spacers for a few years with zero issues. Torqued correctly with red Loctite, I trust 'em with my life. Sold 'em used for half the price of new.

There are other brands out there, but I'd definitely go with a reputable manufacturer and wouldn't risk offshore crap - the material certifications could be bogus, and the risk isn't worth it.




fwiw, I have a brand- new, never run set of 2 pairs of 1.5" black spidertrax wheel spacers for 07-18 Jeep JK/JKU included in my 'Jeep parts sale" thread right now.
You'd usually spend a couple hundred for 2 sets , but I have them listed for super cheap ...I just am clearing garage space,not looking for big $ 😉


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