Wheel and tire fitment

[VKannan]

Hi all,

I'm planning on going with 15" wheels, I like the Torq-Thrust look with an added knock off spinner cap. The set I'm looking at is:

Rear: US Mags Standard 15x9 / 5" backspace / +1mm offset, 3" lip

Factory Five says that for its 15" setup it uses 15x9 / 5.48" backspace / +13mm offset. By my math the 1/2" of backspace plus roughly 1/2" of offset difference puts me at 1" farther into the wheel well in the rear. This actually works out well because this width only comes in 5x4.75 so I'll be using a 1.25" adapter plate, which means the total difference from the FF recommendation will be that I'm 0.25" further out away from the car. I'll be pairing this with a 275/60/15 Cooper Cobra, and Factory Five recommends a 275/50/15. Is this plan going to pose a problem?



Front: US Mags Standard 15x8 / 4.5" backspace / +1mm offset, 2" lip

Factory Five says that for its 15" setup it uses 15x8 / 5" backspace / +12mm offset. Again, by my math the 1/2" of backspace plus roughly 1/2" of offset difference puts me at 1" farther into the wheel well in the rear. I'll be pairing this with a 235/60/15 Cooper Cobra, which is exactly what Factory Five recommends. Is that going to pose a problem, will I need a spacer?


Thank you!

[SteveHsr]

13 mm offset on a 9 inch wheel equals 5.48 inch backspacing.

[Jeff Kleiner]

Cooper Cobras are not available in a 275/50-15 however they are available in 295/50-15. I have built 2 customer cars using these on the FFR rims and they fit without issue. (By the way, they are Coopers dressed up a Billboards ;) )

https://thefactoryfiveforum.com/attachment.php?attachmentid=175776&d=1669465766

I'm afraid you may run into a problem with the 275/60 due to the tire's diameter; they are 28.2" and the 265/50 is 26.8". 235/60-15 work perfectly in front with the FFR rim.

Jeff

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[CraigS]

Backspace and offset are not added together. They are different ways of expressing the same thing. For your 9 in wide wheel, if it had 4.5 inch BS that would indicate that the tire will be centered over the wheel surface that mates to the hub. Also it's offset would be zero. Here is a better explanation than I can do.
https://www.autoanything.com/resources/wheel-offset-vs-wheel-backspacing-explained/
BTW, I find I can think much better in BS, and I can also measure it on my wheel very easily.

[VKannan]

That was really helpful, thanks! I assumed because they were listed separately I had to take both into account, but I see you can derive one from the other now. Appreciate it!


Backspace and offset are not added together. They are different ways of expressing the same thing. For your 9 in wide wheel, if it had 4.5 inch BS that would indicate that the tire will be centered over the wheel surface that mates to the hub. Also it's offset would be zero. Here is a better explanation than I can do.
https://www.autoanything.com/resources/wheel-offset-vs-wheel-backspacing-explained/
BTW, I find I can think much better in BS, and I can also measure it on my wheel very easily.

[Blue Viking]

That was really helpful, thanks! I assumed because they were listed separately I had to take both into account, but I see you can derive one from the other now. Appreciate it!

Please be advised that what Craig is saying is not entirely correct. Offset (or ET) is the position of the mounting surface measured relative to the center of the wheel, thus being independent of wheel width. Backspace is measured from the very inside of the wheel to the mounting surface. Wheel width is always measured on the inside of the wheel lips. So backspace for a 9" wheel with 0 offset will be 4,5" plus the thickness of the wheel lip, normally approx 1/2". So in the example above, backspace for a 9" wheel with 0 offset will be 5" not 4,5"

[SteveHsr]

Ditto.

[CraigS]

Please be advised that what Craig is saying is not entirely correct. Offset (or ET) is the position of the mounting surface measured relative to the center of the wheel, thus being independent of wheel width. Backspace is measured from the very inside of the wheel to the mounting surface. Wheel width is always measured on the inside of the wheel lips. So backspace for a 9" wheel with 0 offset will be 4,5" plus the thickness of the wheel lip, normally approx 1/2". So in the example above, backspace for a 9" wheel with 0 offset will be 5" not 4,5"

Yes I agree. As I was typing my post I realized I had not gotten into the thickness of the wheel bead. Rather than expanding further I thought illustrations would do the job better than I could. Now however I realize that my link didn't illustrate it either. I would question one spec you list though. I have never seen a wheel w/ 1/2inch thick lip. Most are closer to 1/4inch.

[Blue Viking]

Yes I agree. As I was typing my post I realized I had not gotten into the thickness of the wheel bead. Rather than expanding further I thought illustrations would do the job better than I could. Now however I realize that my link didn't illustrate it either. I would question one spec you list though. I have never seen a wheel w/ 1/2inch thick lip. Most are closer to 1/4inch.

Yes bead thickness varies, that's why I wrote approximately. A 1/2" lip is not uncommon, but I agree it's on the thicker side. The wheels i use have about 3/8" lip, normal for most quality aluminium wheels, while steel wheels are generally closer to your example of 1/4".

[VKannan]

I wonder if I’m missing something again, the wheels that I’m looking at have a lip size that ranges between 1.61–5, I was assuming that is in inches, but that seems light years thicker than what you all are mentioned above. I may units incorrect, or are these wheels very different than most others? I was looking at the 15 X 10 for the rear, which per that table at the bottom says a 5 lip size. Again, I assumed that meant inches.


https://www.americanracing.com/american-racing-vintage-classic-torq-thrust-ii

[Blue Viking]

I wonder if I’m missing something again, the wheels that I’m looking at have a lip size that ranges between 1.61–5, I was assuming that is in inches, but that seems light years thicker than what you all are mentioned above. I may units incorrect, or are these wheels very different than most others? I was looking at the 15 X 10 for the rear, which per that table at the bottom says a 5 lip size. Again, I assumed that meant inches.


https://www.americanracing.com/american-racing-vintage-classic-torq-thrust-ii

No sure but that number varies with wheel width, so i believe that number is the width/ depth of the dish, or depth of the wheel if you will, not the thickness of the lip the tire mounts to.

[VKannan]

I see, so maybe the website uses the word “lip” to refer to what you’re calling “dish”, but there is another measurement for tire mounting that is also called “lip”?


No sure but that number varies with wheel width, so i believe that number is the width/ depth of the dish, or depth of the wheel if you will, not the thickness of the lip the tire mounts to.

[Blue Viking]

I see, so maybe the website uses the word “lip” to refer to what you’re calling “dish”, but there is another measurement for tire mounting that is also called “lip”?

You'll have to excuse me, as English isn't my first language, but you are correct. Sorry for the confusion, I'll try to use the correct terminology in the future��

[VKannan]

Oh no problem at all, I think even in English it seems as though people are using those terms interchangeably, so it’s probably our fault, not yours! Thanks so much for helping me understand.


You'll have to excuse me, as English isn't my first language, but you are correct. Sorry for the confusion, I'll try to use the correct terminology in the future��

[Blue Viking]

Not that it is relevant to the original question, but i got qurious and went out to the garage and measured my American Racing wheels. The wheels are 8" wheels, but the actual total width is 8 15/16", much wider than I thought they were. So the lip/ flange/ bead etc width is actually 15/32", certainly something to take into account when clearances are tight.

[SteveHsr]

The convention is that the edge to edge dimension of a wheel is 1 inch greater than the specified width, I.e. a 9inch wheel is actually 10 inches wide making the backspacing 5inches on a 0mm offset wheel.

[Gordon Levy]

8" wide on a 5" backspace or 9" on a 5.5" backspace. 9" on a 5" will stick out substantially.

[CraigS]

Wow this is interesting. I have never seen (or maybe I just didn't pay attention) a wheel lip spec before. So you guys think they are talking about the red or blue dimension or maybe both added together?
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