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Is the wheel offset the ultimate metric

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2.2K views 7 replies 5 participants last post by  66coupe289  
#1 ·
I want to confirm my understanding of the tire charts listed on this forum. I have researched the issue of tire fitment ad-nauseam, and I thank everyone for bearing with me on an issue that has been beat to death. I understand that there are factors for each individual car that affect tire clearance such as suspension changes due to age or intentional modifications to suspension and fenders. However, in general, with a stock car, are there multiple configurations that give the perfect fit other than stock? I define perfect fit as not rubbing under any conditions. Are there different wheel and tire combinations that provide the exact and perfect fit? Is it the wheel offset that is the ultimate number for a perfect wheel fit and the different combinations of wheel diameter and wheel width that derive this number?

For example, apparently, a 15 x 7 wheel with a backspace of 4.25 is a perfect fit for a wheel to have no rubbing whatsoever with a stock suspension and fender configuration. The embedded tire chart in the forum sticky shows this combination of wheel diameter and wheel width as having a positive offset of 6. Now if the ultimate number is the offset, in this case positive 6, this would indicate I need to find a combination of wheel diameter and wheel width that equals a positive offset of 6. If this is true, this will help narrow down my choices. Reading all of the threads on this issue, I can see that I am not the only one grasping to have firm parameters that will provide a concrete number to ensure a wheel fit that has no rubbing issues at all.

As my example, I really like the polished torq thrust VN515. However, I can only get these wheels in a 15 x 6 with a 3.26 backspace, or 15 x 7 with a 3.76 backspace. Looking at the tire chart, a 15 x 6 with a 3.26 backspace is a negative 6 offset, and a 15 x 7 with a 3.76 backspace is also a negative 6 offset. Am I correct in my assumption that this does not fit within the "safe harbor" parameters listed on the tire chart? Meaning, it may or may not work? I am willing to go with 205/60-15 all the way around even though I wanted 215/60-15 if this means no chance of rubbing. The ultimate question, is there a concrete wheel diameter and width, other than stock 14 inch wheels, that will provide a completely no issue fit and if so is this number a positive offset of 6? Again, thanks for the input on an issue that has so many confused.
 
#2 · (Edited)
There is no "one" ultimate metric. Offset or back spacing are two ways to express the same general thing, the location of the mounting surface of the wheel. If you have been looking at charts you should have noticed that necessary offset AND backspacing changes depending on the width of the rim. Ideal offset for a 6" wheel may not be ideal for a 7 or 8" wheel. Why? From this diagram:

Image


You will see that if you leave the offset the same, and make the rim wider you will push the outside edge of the rim outward. At some point it will likely cause interference between the tire and the wheel opening.

It's all a balancing act. You must balance the outboard tire location, which is governed by the width and height of the tire AND the outer rim location and (generally) the inboard rim location which is governed by backspacing and rim diameter. One must consider rim width, diameter, backspacing (offset) and tire width/height to know if the tire/wheel combo will work. No one variable is more important than the others as they all contribute.

Add to all of this that these cars were never produced as a precise instrument means that saying definitively that that a certain size WILL fit when getting near the max size is impossible. From the factory some cars required no shims to align and some may have required a stack 3/8" tall. Additionally fenders could be installed in or out a bit to make body lines match. Clearances from one car to another can vary enough that what works on one car will not come close to working on another.
 
#3 · (Edited)
Inside rim profile makes a difference too.
Internal diameter of same size wheel can vary as well.
There is a “drop” to consider too for brake clearance as well. Designed into every wheel to mount the tire. Inside drop and outside drop can make a huge difference on some setups.
785815
785816
 
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#4 ·
Alright, I will have to digest all of this information, but confirms my fears, not one precise number other than stock. I dont know why American Racing does not make a 15 x 7 with 4.25 backspace. Anyone seen any very similar in polished aluminum, that have those parameters? As I noted in an earlier discussion thread, id be willing to pay more to get exactly what I wanted. Well, not the extreme price for custom from American Racing, like 500 each.
 
#6 ·
gregorywilliams100 said:
I understand that there are factors for each individual car that affect tire clearance such as suspension changes due to age or intentional modifications to suspension and fenders. However, in general, with a stock car, are there multiple configurations that give the perfect fit other than stock? I define perfect fit as not rubbing under any conditions. Are there different wheel and tire combinations that provide the exact and perfect fit? Is it the wheel offset that is the ultimate number for a perfect wheel fit and the different combinations of wheel diameter and wheel width that derive this number?
Click to expand...
If your definition of "perfect fit is not rubbing under any conditions," then the answer is yes that all sorts of combinations will fit. After all, Ford offered several options of both wheel and tire size.
The problem arises when you try to maximize the tire width. As @66coupe289 's chart shows, back space and set off are actually two ways of measuring the same thing. A zero offset means that the wheel rim and tire are perfectly centered inside/outside the wheel's mounting surface. With zero offset, the wider the wheel the more it will extend both inward (backspace) and outward. Similarly, if you take a wheel and only increase the width outward the back space doesn't change but the offset does.
If you can picture this in your mind's eye (not everyone can) it makes it easier to understand.

Here's a good link to try different tire and wheel sizes:

Custom rims, wheel tire packages

Custom rims, wheel tire packages. Browse hundreds custom aftermarket rim-tire packages for your car, truck or SUV. Online fitment guides - See what fits.
www.rimsntires.com

And here's a chart comparing offset and back space. As an example, if you keep zero offset and go up one inch in rim width the back space increases 0.5 inches (the rim width increases 0.5" both inward and outward.

785821
 
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