Off Roading Forums banner
1 - 9 of 9 Posts
G

·
Discussion Starter · #1 ·
This was posted on a NG by someone:

"Offset is measured from the centerline of the rim to the mounting face.
Negative offset means the mounting face is inside the centerline,
positive offset means the mountinbg face is outside the centerline.
Backspacing is measured from the inside bead to the mounting face. "

(Offset * -1) Backspacing = 1/2 * rim width

C/L
|
|
Outside\_ | _/ Inside
\_________/
| |
offset<->|<--> Backspacing
|
|
|
mounting
face

So that means a smaller backspacing and high positive offset would push the
rubber out away from the tub, correct?

I am running tires that are a bit too wide, and need to push the rubber away
from the inside frame. I poked a small hole in the tire when I turned sharp and
the rubber rubbed. Only it wasn't that sharp.

Spacers won't push the wheel away far enough. So I thought I'd get rims
that set deep, does that make sense? Setting the rubber further away from
the tub?

jason thomas france

www.jasonthomasfrance.com/jeepin
 
G

·
Discussion Starter · #3 ·
Correct, negative offset and a small amount of backspacing will push the tires further out from the tub. I was looking at your diagrams and cant really make them out though. Remember offset and backspacing are two different measurements. Backspacing is measured to the mounting face from the inside lip and will vary depending on offset and wheel width. Offset does not depend on wheel width. If offset is zero then backspacing will be half of wheel width.
If we have a ten inch wide wheel and offset is zero then backspacing will be five inches. Zero offset means the mounting face is the exact middle of the rim. If the offset is zero and the wheel width is eight inches, then the backspacing will be four inches. If the offset is negative zero then the mounting face is one inch toward the inside lip of the wheel. It will be a negative one inch whether it is a eight or ten inch wide wheel. However, backspacing will be three inches on the eight inch wheel and four inches on a ten inch wheel. Hope this helps.
 
G

·
Discussion Starter · #4 ·
I wish people would forget they ever heard the term "off set". It is a name for something that doesn't have any revelance in everyday usage. Let's get down to reality. If the tire rubs in turns, then you have to move the tire and wheel out from the veh. To compare two different wheels, you compare "back space" not "off set". Back spacing is the measurement from the place where the wheel bolts onto the Jeep (the hub, the brake drum, whatever) to the inside edge of the rim. Whatever is on the other side of the wheel mounting has no effect on the rubbing against the Jeep itself. A goofy example: Suppose your tire was 5 feet wide. Only the inside of the tire rubs when the wheel is turned. Doesn't make any difference how much is sticking out from under the fender flare. If the 5 foot wide tire rubs just a little, you move it away from the Jeep an inch. The rubbing stops. You have decreased the back spacing by one inch and moved the tire and wheel out away from the Jeep by one inch. The "off set" in this case would be huge, but irrelevent, meaningless. Suppose you change that 5 foot wide tire for a 2 foot wide tire and install it on the proper wheel. The "off set" has changed dramatically. The back spacing??? No change.
Off set is kinda like the difference in the number of fingers on your hands. If you have them all, you have a zero "finger off-set". If you lost a finger in the winch on your Jeep, if it was on the right hand. you now have a positive 1 finger off set. On the left hand would be a negative 1 finger off set. So What. You still have all the fingers on your right hand. You can still pick your nose, pull the trigger, give the one finger salute, etc. Same with back spacing. If a 5 inch back spacing works, doesn't matter what is on the other side of the rim, 4, 9, 22 or 3 miles. Doesn't matter what the off set is either. Off set is a function of overall wheel width.
Seems like we need to name everything these days. Someone has named all the grooves in a tire, surely. Let's say you are running a regular street tire. Single groove in the middle, and a groove on each side. The center groove is the prime groove. Used to be called the center groove, but with the advent of tire size name changes the center groove was renamed, prime groove. The groove nearest the vehicle is called the, and now we have a name that makes sense, the vehicle side groove. Most people just say "vehicle groove". The outside groove was called that for years. It was also called the "WT" groove by insiders in the tire industry. Why, you ask? Good question. Dr. B. Wilson Taylor, a tire researcher and designer working for Goodyear is the one who came up with the idea to name the grooves in the tires, so those "in the know" started refering to the outside groove as the WTG or WT Groove or Wilson Taylor Groove depending on their position on the circle of those in the inner circle. Anyway, when the tire sizes were renamed, the WT groove became the street side groove because it was opposite the vehicle side groove. Again, most refer to this groove as the "street groove". With all this tire history, you can do some quick calculations and come up with some meaningful information that is applicable to all tires with definable grooves. On all normal production type tires, not speciality items like tires designed for a particular race track or some other specific use, the distance from the prime grove to the street side groove is the same as the distance from the prime groove to the vehicle side groove. On a tire of this type if you know the distance from the furthermost outside edge of the tire to the center of the street groove and know the distance from the prime groove to either of the other grooves, you can calculate the width of the tire by simple addition........................OR you can look on the side of the tire where it states what size the tire is in big one inch numbers. Such is the usefulness of "off-set".

Doug '97 TJ
My Web Site
 
G

·
Discussion Starter · #6 ·
Doug,
Offset IS NOT irrelevant. An axle and suspension of any vehicle is designed to run optimally with a set amount of offset. It places the wheel in the right relation to the rest of the suspension. If you go more negative than the stock setting, you are increasing the amount of stress on the hub, spindle and wheel bearing. This is ok to a point, but dont go overboard. It WILL make wheel bearings go sooner and affect the handling of the vehicle. The less you drive on the road the less important it is. On cars it is extremely important to get the correct offset. When getting custom wheels for a car you only go as wide as the correct offset will allow. It ruins the handling of the car if you mess with offset too much. It is less important on a solid front axle vehicle, but cannot be ignored. You must take both offset and backspacing into account when buying the proper wheels although for the purposes of four wheeling offset is not AS important. You can fudge on offset a little, but not too much.

 
G

·
Discussion Starter · #7 ·
Sorry, gotta disagree. What you say is true, but you can get it all done properly never knowing what "off set" is or using the info it provides.

Here are two 15" wheels. Both will fit your Jeep. One has a 1.25" off set, the other a zero off set. Which one do you pick? You can't 'cause just knowing the off set doesn't help you.

Another two wheels. One has a 5.75" back space and the other a 4.5" back space. Now you can pick a wheel. The 5.75" will cause the steering wheels to hit the veh in a turn because the tires are sitting in too close to the veh. The 4.5" will clear, and you still don't know what the off set is and cannot determine it with just the information I gave you. Give you some more information. One wheel is a 10" and the other an 8". With all those numbers you can see quite easily that any combination will not be harmful to the axles or bearings, BUT you still can't tell me what the off set is of either wheel.

One final look. Suppose the 10" wheel has the 4.5" back spacing. The wheel sits "X" distance away from the from the vehicle. Now suppose the 8" wheel has the 4.5" back spacing. How far does it sit away from the vehicle? The same "X" distance??? Yep, sure does. Is the off set the same on both wheels? No. Is it the off set that makes the difference or is it the overall width of the wheel that makes the difference of where the wheel sits in relation to the axle and rest of the vehicle? Neither. It is the back spacing that makes the difference where the wheel fits in relation to the vehicle. Whichever wheel has the 5.75" back spacing will sit 1.25 inches closer to the vehicle. Doesn't make any difference whether it is the 8" wheel or the 10" wheel.

Off set is just a name for a measurement. That measurement has a meaning when and only when it is combined with another measurement - the wheel width, and when you have these two measurements, you can compute a usable value - the wheel's back spacing. Without the width of the wheel, off set is a meaningless number. Back space on the other hand has a useful and practical meaning by itself. You can make decisions based on the measurement of back space. It isn't the out of bounds off set that causes the problems you are referring to. It is the width of the wheel itself. With a specified min/max of the back space and a specified min/max of the wheel width for any vehicle, you can do all your wheel and tire shopping you ever need to do and never ever consider the off set.

Doug '97 TJ
My Web Site
 
G

·
Discussion Starter · #8 ·
Doug,
I dont want to start any type of argument here because it will not solve anything, but you said:

"With a specified min/max of the back space and a specified min/max of the wheel width for any vehicle, you can do all your wheel and tire shopping you ever need to do and never ever consider the off set."

You're right! YOU do not need to consider the offset because the manufacturer or whoever set those min/max numbers has already done it for you. Offset is a product of the wheel width and the backspacing, so by giving a min and max for backspacing and a min and max for wheel width, they are giving you a min and max for the offset, they are just not stating it that way. If they say that 4.5" of backspacing will work for a particular vehicle and that wheel width should be between 8" and 10" then they are saying that you cannot have an offset greater than negative .5" or less than positive .5". So you are just using different measurements to get the same answer.

You see, you cannot choose a wheel on any one measurement. You need any two of the three (backspacing, offset, wheel width) to make a proper choice.

If you were to go wheel shopping for a car, they use the wheel width and offset and never refer to backspacing (it is a little more confusing because offset is usually referred to in mm while width is referred to in inches), while shopping for truck wheels they refer to wheel width and backspacing. This is because with the complex independent suspensions of cars, offset is a more important factor, while with the (usually) straight axle suspensions of 4 wheelers backspacing can be a more important factor to consider.

So as you can see, none of the measurements are worthless, they all contribute to choosing the correct wheel for your vehicle. It is just done in different ways, and I encourage anyone to use whichever way makes the most sense to them. I for one come from a car tuning backround, so I can use offset and wheel width to determine the backspacing and know if it will work. You are obviously more comfortable with using backspacing. Neither of us are wrong, like wheel manufacturers we just use different measurements to determine the same thing, whether a wheel will work or not.

I have greatly enjoyed our little discussion, thank you, and I hope we have not confused the hell out of everyone else. :) How do you do those little smiley faces?
Jim





 
G

·
Discussion Starter · #9 ·
You and I understand what we are talking about, and am not wanting to start an argument either. My only point is that the term "off-set" and what it represents is confusing to a lot of people, and that confusion is not necessary.

Here's the confusing part. You say, "If they say a 4.5" back spacing will work for a particular vehicle and that the wheel width should be between 8" and 10" then they are saying that you cannot have an off set greater than negative .5" or less than positive .5". So you are just using different measurements to get the same answer." Not quite.

For the record, I can't keep up with which is negative and which is positive as far as off set is concerned, so if the following in incorrect in that regard, the premise will be correct if you substitute the correct term. Another reason off set is confusing.

Here's the point. You just said, "cannot have an off set greater than..." Will use 2 off sets to make the point, neg .5, and pos .5 - both within stated limits. Armed with the info that his Jeep can live with anything between neg .5 and pos .5 off set and knowing that he has the choice of 8" or 10" wheels (also within specs) our newbie goes to xxx tire and wheel shop and finds some beautiful black steel wheels. They are 8" wide, and he has the choice of 2 sets. One has a neg .5 off set and the other a pos .5 off set. Knowing that both are within specs and knowing that the 8" is also within specs, he just points to the closest set and says, "Put 'em on."

Knowing from experience that newbies (speaking for myself here) will ALWAYS make the wrong choice the first time - it's a rule of Jeeps, I think - Anyway our newbie picks the set that actually has a back spacing of 3.5". Yep, they will work. But because the tires now stick out an extra inch, the flares will no longer cover them.........oh well, the plan was for new Xenons at some point in the future anyway. If he had chosen the 10" set instead of the 8" set and had the same choice of 2 sets, he would be in real trouble by picking the wrong one. The 5.5 inch back space causes real clearance problems with those 35 inch Swampers he picked to put on his D35/D30 axle setup. /wwwthreads_images/icons/tongue.gif /wwwthreads_images/icons/shocked.gif (You make moon guys by using the square brackets on your key board with the correct word between them, tongue and shocked in this case)

WHEREAS: had our newbie been armed with 4.5" back space and 8" or 10" wheels, that's all the info he would need. He could have picked the right set to begin with and not been confused by pos vs neg off set. If you know the off set limits and wheel width limits, you still must do some math to figure out which back spacing to pick. You know 2 numbers and have to compute the third number to make a choice. If you are armed with back space and wheel width limits, nothing else is needed. You don't have to compute the off set. Why bother? If you maintain proper back spacing and stay within wheel width limits, you never have to deal with off set. Any wheel you pick will work. If you stay within off-set limits and wheel width limits and DO NOT compute the back space, you can get wheels that will not work. Why use a demension that can cause the wrong wheel to be picked if you don't do extra calculations and/or can't keep up with which is neg and which is pos?

Doug '97 TJ
My Web Site
 
1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top