Wheel offset question

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
Ok figured i would ask this since im looking at wheels again. I am looking at wheels with a 25 mm offset. Would i require spacers to run a tire taller then 30.5 on these wheels?
 

The_Roadie

Founding Member
Administrator
Nov 19, 2011
9,957
Portland, OR
You still have to do the math, and also realize every tire is slightly different in terms of diameter (nominals aren't exact) and the edge roundness. Offset isn't enough to fully define the geometry of the relationship of the tire to the upper ball joint. You also have to know the rim width compared to stock. See if you can find out more about the rim width of the proposed wheel, and while you're doing that research, find out what the OEM wheel rim width and offset is. I haven't led anybody through this in quite a while so it will be useful to do it again for the archives here on GMTN.

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Darkrider_LS

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
the roadie said:
You still have to do the math, and also realize every tire is slightly different in terms of diameter (nominals aren't exact) and the edge roundness. Offset isn't enough to fully define the geometry of the relationship of the tire to the upper ball joint. You also have to know the rim width compared to stock. See if you can find out more about the rim width of the proposed wheel, and while you're doing that research, find out what the OEM wheel rim width and offset is. I haven't led anybody through this in quite a while so it will be useful to do it again for the archives here on GMTN.

View attachment 16996

Great info thanks! The planned wheels are 17x8 with a 25 mm offset. From what I have read the oe wheels are 17x7 with 6" backspacing.
 

The_Roadie

Founding Member
Administrator
Nov 19, 2011
9,957
Portland, OR
Darkrider_LS said:
Great info thanks! The planned wheels are 17x8 with a 25 mm offset. From what I have read the oe wheels are 17x7 with 6" backspacing.
You got it in one!

Except for the little detail of now you have to convert backspacing to offset. :wink:

Oh, I'm feeling generous tonight.

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The_Roadie

Founding Member
Administrator
Nov 19, 2011
9,957
Portland, OR
So see if you can do the math (and show your work).

You need to measure the OEM tire to see how much it can grow in diameter and not hit the ball joint. And see how much room there is horizontally.

The new tire and rim are going to put the centerline of the wheel at some location, but the new rim is wider, so half the rim width increase is going to go towards the center of the vehicle and half is going to grow outwards. So the NEW location for the tire edge is going to move horizontally some amount (increase in tire width plus movement due to the wheel offset) and vertically some amount (diameter increase divided by two because what we're interested in is radius, not diameter.)

Anybody who skipped math because they thought it would never have real-world application can be excused from the classroom now. :hissyfit:
 
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Darkrider_LS

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
Awesome info Roadie! If i am following the chart correctly the new wheels have a 5.5" backspacing. But due to the fact that said rims are also an inch wider then stock and since half of said added width would be on each side of the center line of the rims. The new 17x8 rims would have the inner edge roughly at the same point as the OE 7". Thus answering my question about relation to the ubj. All i really did was gain a wider rim but no outward movement towards the outside of the truck. Thus if i want to go taller then that max 30.5-30.6 tire height i will need to run spacers.
 
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djthumper

Administrator
Nov 20, 2011
14,940
North Las Vegas
Darkrider_LS said:
Awesome info Roadie! If i am following the chart correctly the new wheels have a 5.5" backspacing. But due to the fact that said rims are also an inch wider then stock and since half of said added width would be on each side of the center line of the rims. The new 17x8 rims would have the inner edge roughly at the same point as the OE 7". Thus answering my question about relation to the ubj. All i really did was gain a wider rim but no outward movement towards the outside of the truck. Thus if i want to go taller then that max 30.5-30.6 tire height i will need to run spacers.

Actually you gained .5" of clearance...
 
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Darkrider_LS

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
djthumper said:
Actually you gained .5" of clearance...

Oh really??
 

djthumper

Administrator
Nov 20, 2011
14,940
North Las Vegas
Darkrider_LS said:
Oh really??
Yep.

the roadie said:
That's why I advised everybody show your work. If you can explain it adequately, it's likely right.
I need to show my work as well?
 
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Darkrider_LS

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
djthumper said:
Yep.


I need to show my work as well?

Please do so!
 

djthumper

Administrator
Nov 20, 2011
14,940
North Las Vegas

navigator

Hobbyist
Dec 3, 2011
504
Larry is right though, you do gain .5"
For me, backspace is easier to calculate than offset.
The OEM rim has 6" backspace.
The new rim has 5.5" of backspace.
Backspace doesn't care how wide the rim is. If you take the tire width out of the equation, a 12" wide rim with 5.5 backspace has the same ball joint clearance as a 7" rim with 5.5 backspace. The 12 inch rim will just stick out from the fender further.

Since you gained .5" in backspace that will be additional clearance. You'll have to measure to see if that .5" is enough clearance for the tire you want to run.
 

The_Roadie

Founding Member
Administrator
Nov 19, 2011
9,957
Portland, OR
0.5" is what I get too. The rim width is compensated for in the backspacing to offset chart, so it doesn't need to be in the equation twice.

That said, tire variability can exceed 1/2". The design of the tire might include side lugs for rock crawling that make for a more square corner, or it could be rounded. They can also vary larger or smaller from the "nominal" metric size given in all the tire size calculators.

Now this thread can be the link we gently/snarkily give anybody in the future who repeats the question without searching first. :wootwoot:
 

jimmyjam

Guru
Nov 18, 2011
1,634
the roadie said:
I'm not so much into just handing over a fish, nowadays. :rotfl:
I'd like my seabass served with a nice mango chutney, thank you :thumbsup:
 

djthumper

Administrator
Nov 20, 2011
14,940
North Las Vegas
navigator said:
that's cheating, you have to show your math work to get full credit :smile:
Apparently my math was correct because it worked on my truck. :raspberry:
 
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Darkrider_LS

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
Great info in here for sure. Guess I misunderstood the relation of the new wheels width vs offset. But at least all the above proves I can at least rock a tire taller then 30.5 as long as it is round shouldered.
 

nieman88

Registered Member
Mar 10, 2014
70
The_Roadie said:
You still have to do the math, and also realize every tire is slightly different in terms of diameter (nominals aren't exact) and the edge roundness. Offset isn't enough to fully define the geometry of the relationship of the tire to the upper ball joint. You also have to know the rim width compared to stock. See if you can find out more about the rim width of the proposed wheel, and while you're doing that research, find out what the OEM wheel rim width and offset is. I haven't led anybody through this in quite a while so it will be useful to do it again for the archives here on GMTN.

View attachment 16996


....you lost me (but thats not hard to do). And I searched here and other sites. Im just very confused.

im actually face palming trying to understand offset and backspacing...still.
Dumb it down for me. ...im going nuckin futz
 

nieman88

Registered Member
Mar 10, 2014
70
Nov 17, 2012
9,727
The following is an excerpt from Tirerack.com

Hope it helps.




"The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*

Zero Offset

The hub mounting surface is even with the centerline of the wheel.

Positive

The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative

The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.

*Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches)."
 

Pittdawg

Hobbyist
Dec 5, 2011
538
What about a 25 mm offset wheel with 6x5 to 6x5.5 adapters? If the adapters are 1.5 inches thick, does that mean I would gain 2 inches of offset (that's too much!)?
 
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Darkrider_LS

Darkrider_LS

Original poster
Moderator
Jan 25, 2013
9,332
Pittdawg said:
What about a 25 mm offset wheel with 6x5 to 6x5.5 adapters? If the adapters are 1.5 inches thick, does that mean I would gain 2 inches of offset (that's too much!)?
Depends on the specs of the proposed new rims. Let's take the wheels I was looking at for example with their 5.5" back space. With adapters they go down to 4" backspace which as you stated gives you around 2" of clearance. Which seems to be where most of us are after changing rims to aftermarket fsc application wheels. Iirc kyle aka Hardtrailz runs a set up that has him around 3.5" backspace on his wheels. The wheels I'm looking at start at 6 3/16" backspace which with adapters puts me between 4.5 and 4.75 backspace. Or around 1.25" clearance at minimum.
 

paul2005tb

Active Member
Nov 26, 2014
260
Massachusetts
question: can I purchase shorter lug bolts to mount to the hub so that when I install my 1.5in adapters there will be no need to grind down my existing lugs ?
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,658
paul2005tb said:
question: can I purchase shorter lug bolts to mount to the hub so that when I install my 1.5in adapters there will be no need to grind down my existing lugs ?
1.5 inch adaptors need no grinding....
 
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paul2005tb

Active Member
Nov 26, 2014
260
Massachusetts
HARDTRAILZ said:
1.5 inch adaptors need no grinding....
Great news ! Thanks. I assume that adaptors less than 1.5 would come with lug bolts to replace the existing lugs.
 

paul2005tb

Active Member
Nov 26, 2014
260
Massachusetts
Im considering a low budget spacer solution:

1) 1/4 in slip-on spacers
2) Grind 1/8in off of the upper ball joint knuckle/seat .

That should get me to almost a 31.0in dia. at 245 with a tame tread pattern .
 

Tiggerr

Hobbyist
Jun 6, 2013
1,324
Perrysburg, OH
I don't believe i'd go grinding the balljoint seat.
Don't think I'd do slip on spacer either without using longer wheel studs. Doesn't seem too safe.
Most do the adapter/spacers. Then they bolt down and you use the spacer's studs for wheels. Think it changes to Chevy truck pattern too. Way easier to find wheels too.

Better to spend the $ and be safe than shortcut something that crucial. IMHO
 

BlazingTrails

Banned
Apr 27, 2014
19,409
IMO changing the studs out and using a "slip on" spacer would be stronger and less to go wrong.I would never grind anything on my suspension, (???)

Good point on the wheel selection, there are so many more wheels for 6x5.5" pattern.
 
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Wooluf1952

Guru
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Grinding the edge of the UCA was pretty common on the OS. IDK if anyone ever had a problem doing that.
 
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djthumper

Administrator
Nov 20, 2011
14,940
North Las Vegas
paul2005tb said:
Im considering a low budget spacer solution:

1) 1/4 in slip-on spacers
2) Grind 1/8in off of the upper ball joint knuckle/seat .

That should get me to almost a 31.0in dia. at 245 with a tame tread pattern .
Slip on spacers are not recommeneded due to you would not have the studs sticking through the lug nuts.
 
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HARDTRAILZ

Moderator
Nov 18, 2011
49,658
Tons of people have ground the balljoint without issue.

I ran 1/4 inch spacer on a camaro and had no issues. Check to make sure after 1/4 inch you still have adequate thread engagement and all will good.
 
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