- 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?
Wheel offset question
- Thread starter Darkrider_LS
- Start date
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 30222
View attachment 30222
Attachments
I LOVE to teach people how to fish (and do their own analyses). I'm not so much into just handing over a fish, nowadays. 
- Jan 25, 2013
- 9,332
the roadie 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!Darkrider_LS said:
Except for the little detail of now you have to convert backspacing to offset.
Oh, I'm feeling generous tonight.
View attachment 30223
Attachments
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.
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.
- 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.
djthumper
Administrator
- Jan 25, 2013
- 9,332
That's why I advised everybody show your work. If you can explain it adequately, it's likely right.
djthumper
Administrator
- Jan 25, 2013
- 9,332
djthumper
Administrator
Darkrider_LS said:
16X8 with 25mm offset 265 70R16 presently mounted but have tested with 265 75R16's for clearance.
New Wheels Photos by kc7qjo | Photobucket
navigator
Member
- 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.
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.
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:
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:
djthumper
Administrator
Apparently my math was correct because it worked on my truck.navigator said:that's cheating, you have to show your math work to get full credit
- 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
Member
- Mar 10, 2014
- 70
nieman88 said:
this is how im doing so. I have pictures in front of me to help. haha.
offroadTB.com - View topic - Wheel Spacers/Adapters/Backspacing Information
http://gmtnation.com/attachments/f28/16996-wheel-offset-question-offset.gif
Playsinsnow
Member
- 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)."
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)."
djthumper
Administrator
- Jan 25, 2013
- 9,332
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.Pittdawg said:
paul2005tb
Member
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
- 49,665
paul2005tb
Member
Great news ! Thanks. I assume that adaptors less than 1.5 would come with lug bolts to replace the existing lugs.HARDTRAILZ said:
paul2005tb
Member
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 .
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
Member
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
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
Member
- 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.
Good point on the wheel selection, there are so many more wheels for 6x5.5" pattern.
Wooluf1952
Member
Grinding the edge of the UCA was pretty common on the OS. IDK if anyone ever had a problem doing that.
djthumper
Administrator
Slip on spacers are not recommeneded due to you would not have the studs sticking through the lug nuts.paul2005tb said:
HARDTRAILZ
Moderator
- Nov 18, 2011
- 49,665
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.
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.
Sqrly
Member
Mooseman
Moderator
Then how come most stock wheels come with an offset? Even on stock GMT360 wheels, there is a 48-52mm offset. TBSS have a 44-48mm offset.
www.wheel-size.com
2007 Chevrolet TrailBlazer - Wheel & Tire Sizes, PCD, Offset and Rims specs
Wheel size, PCD, offset, and other specifications such as bolt pattern, thread size (THD), center bore (CB), trim levels for 2007 Chevrolet TrailBlazer. Wheel and tire fitment data. Original equipment and alternative options.
www.wheel-size.com
Sqrly
Member
I should have said "total offset".
Stock wheels will have an positive offset equal to the negative offset of the hub. (front wheels)
After all the math your front tires should pivot on the centerline of the tire.
Wheel spacers are made so you can use wheels not made for your vehicle that have too much positive offset.
Often people use the wrong wheels and/or spacers and end up with "negative total offset" and this is wrong in so many ways.
Stock wheels will have an positive offset equal to the negative offset of the hub. (front wheels)
After all the math your front tires should pivot on the centerline of the tire.
Wheel spacers are made so you can use wheels not made for your vehicle that have too much positive offset.
Often people use the wrong wheels and/or spacers and end up with "negative total offset" and this is wrong in so many ways.
Some More Comprehensive On-Topic Technical Data:
"The offset of a wheel is the distance from its hub mounting surface to the center-line of the wheel and for some ungodly reason is usually measured in mm. the offset can be One of Three Types.
Zero offset - the hub mounting surface is even with the center-line of the wheel.
Positive offset - the hub mounting surface is toward the front or wheel side of the wheel.
Negative offset - the hub mounting surface is toward the back or brake side of the wheels center-line.
Back-spacing of a wheel is the distance from the hub mounting surface to the rear outer most edge of the wheel.
Front-spacing is a term not used too often, but is a term I like when discussing a wheels ability to tuck. It is the distance from the hub mounting surface to the front outer most edge of the wheel.
As an example I’m going to use a wheel that is 20x8.5 with a +19mm offset as this seems to be a pretty common size.
The first thing you have to understand is that actual wheel width is about 1 inch wider than stated, so a wheel that is 20x8.5 in actuality measures 9.50 inches wide. That being said the center-line of the wheel, +0.00mm offset, is the same as 4.75in back spacing/front spacing.
9.50in/ 2 = 4.75
Now to get the offset from metric to a more understandable unit of measure (inches), we are going to divide it by 25.4. This number is a constant as there are 25.4 mm in an inch.
19mm/25.4 = 0.7480314in
For practical reasons, we’ll round up to 0.75in. Since we already established that our center-line is 4.75, and we are dealing with a positive offset, we are going to add 0.75 in to 4.75 to give us a wheel backspacing of 5.50 inches. This in turn will allow us to figure up the wheel front-spacing by simply subtracting the wheel backspacing from the overall wheel width giving us 4.00 inches.
9.50in – 5.50in = 4.00in
A common question I run across is “will (insert wheel) with (insert backspacing) tuck on my truck??” The answer to that question is more that likely yes (provided you are not talking about a zero or negative offset wheel) it’s those GD tires that get in the way. I hope this clears up some of the confusion.
Because I own a 2001, I’ll use the 99+ trucks as an example, but you can apply this to any vehicle you see fit. On the 99+ trucks, the magic number is (about) 4 inches. (this number varies from truck to truck) This is the distance from the hub mounting surface to the edge of an unrolled rear fender.
I chose to use the rear as an example for simplicity as the front wheels camber in as the wheel travels into the fender, and are typically not a problem. As long as the distance from the hub mounting surface of the wheel to the front outer most edge of the tire (not wheel) is less than 4 inches, you will not have tucking issues. We’ll call this Total Front-spacing.
To figure out your total front spacing, you need to know some basic information. What is your wheel size, offset, and tire size. I’ll start with the stock wheels and tires.
Stock wheel size is 16x7 with a backspacing of about 5.25 inches. We must first get the center-line by adding 1 to the width and dividing by 2:
(7 + 1)/2 = 4 inches
Next we subtract the center-line from the backspacing to get the offset:
5.25 – 4 = 1.25 in offset
Now we need to find our tire size. Again, I’ll use my stock tire size…Michelin LTX M/S 255/70/16. According to the Michelin web site, this tire is 30.2 inches tall and 10.3 inches wide on a 7.5 inch wheel. For every half inch you need to get to your wheel size, you add or subtract 0.2 inches from the width of the tire. In other words, my tires are 30.2 in tall and 10.1 in wide on my stock 16x7.
To find to total backspacing/ front-spacing, we are going to apply the same principle we did to find wheel backspacing/ front-spacing. The new center-line of the tire/ wheel combo is 5.05. we found this by dividing 10.1 by 2. Again we are dealing with a positive offset, so you add the 1.25 offset to the center-line to get a total backspacing of 6.3 in.
(10.1/ 2) + 1.25 = 6.3 in
This will let us find our total Front-spacing by subtracting the Back-spacing from the tire width.
10.1 – 6.3 = 3.8 in
Since the 3.8 inch total Front-spacing is less than 4, you should have no problems tucking this wheel/ tire combo. You can of course get around the 4 inch limit by rolling or trimming your fenders.
*note – you should take measurements when possible. these numbers are close, but not necessarily exact."
Source:
www.gmfullsize.com
"The offset of a wheel is the distance from its hub mounting surface to the center-line of the wheel and for some ungodly reason is usually measured in mm. the offset can be One of Three Types.
Zero offset - the hub mounting surface is even with the center-line of the wheel.
Positive offset - the hub mounting surface is toward the front or wheel side of the wheel.
Negative offset - the hub mounting surface is toward the back or brake side of the wheels center-line.
Back-spacing of a wheel is the distance from the hub mounting surface to the rear outer most edge of the wheel.
Front-spacing is a term not used too often, but is a term I like when discussing a wheels ability to tuck. It is the distance from the hub mounting surface to the front outer most edge of the wheel.
As an example I’m going to use a wheel that is 20x8.5 with a +19mm offset as this seems to be a pretty common size.
The first thing you have to understand is that actual wheel width is about 1 inch wider than stated, so a wheel that is 20x8.5 in actuality measures 9.50 inches wide. That being said the center-line of the wheel, +0.00mm offset, is the same as 4.75in back spacing/front spacing.
9.50in/ 2 = 4.75
Now to get the offset from metric to a more understandable unit of measure (inches), we are going to divide it by 25.4. This number is a constant as there are 25.4 mm in an inch.
19mm/25.4 = 0.7480314in
For practical reasons, we’ll round up to 0.75in. Since we already established that our center-line is 4.75, and we are dealing with a positive offset, we are going to add 0.75 in to 4.75 to give us a wheel backspacing of 5.50 inches. This in turn will allow us to figure up the wheel front-spacing by simply subtracting the wheel backspacing from the overall wheel width giving us 4.00 inches.
9.50in – 5.50in = 4.00in
A common question I run across is “will (insert wheel) with (insert backspacing) tuck on my truck??” The answer to that question is more that likely yes (provided you are not talking about a zero or negative offset wheel) it’s those GD tires that get in the way. I hope this clears up some of the confusion.
Because I own a 2001, I’ll use the 99+ trucks as an example, but you can apply this to any vehicle you see fit. On the 99+ trucks, the magic number is (about) 4 inches. (this number varies from truck to truck) This is the distance from the hub mounting surface to the edge of an unrolled rear fender.
I chose to use the rear as an example for simplicity as the front wheels camber in as the wheel travels into the fender, and are typically not a problem. As long as the distance from the hub mounting surface of the wheel to the front outer most edge of the tire (not wheel) is less than 4 inches, you will not have tucking issues. We’ll call this Total Front-spacing.
To figure out your total front spacing, you need to know some basic information. What is your wheel size, offset, and tire size. I’ll start with the stock wheels and tires.
Stock wheel size is 16x7 with a backspacing of about 5.25 inches. We must first get the center-line by adding 1 to the width and dividing by 2:
(7 + 1)/2 = 4 inches
Next we subtract the center-line from the backspacing to get the offset:
5.25 – 4 = 1.25 in offset
Now we need to find our tire size. Again, I’ll use my stock tire size…Michelin LTX M/S 255/70/16. According to the Michelin web site, this tire is 30.2 inches tall and 10.3 inches wide on a 7.5 inch wheel. For every half inch you need to get to your wheel size, you add or subtract 0.2 inches from the width of the tire. In other words, my tires are 30.2 in tall and 10.1 in wide on my stock 16x7.
To find to total backspacing/ front-spacing, we are going to apply the same principle we did to find wheel backspacing/ front-spacing. The new center-line of the tire/ wheel combo is 5.05. we found this by dividing 10.1 by 2. Again we are dealing with a positive offset, so you add the 1.25 offset to the center-line to get a total backspacing of 6.3 in.
(10.1/ 2) + 1.25 = 6.3 in
This will let us find our total Front-spacing by subtracting the Back-spacing from the tire width.
10.1 – 6.3 = 3.8 in
Since the 3.8 inch total Front-spacing is less than 4, you should have no problems tucking this wheel/ tire combo. You can of course get around the 4 inch limit by rolling or trimming your fenders.
*note – you should take measurements when possible. these numbers are close, but not necessarily exact."
Source:
wheel offset explained
www.gmfullsize.com
Last edited: