offset calculations

[LBarnes]

Since we are all concerned with offsets at all four corners I decided to share this on the forum.
Rim offsets determine where the rim sets in relationship to the mounting surface of the brake rotor. But we are more concerned with the distances between the tire and the fender or strut.
On a stock 17'' BFG there is 2.96'' out from the mounting surface, towards the fender, and 6.74'' towards the strut. This is calculated using the printed section width in the tire specs and the rim offset.
To calculate any tire location use this:

Section width in inches x 25.4 divided by 2 + the offset of the rim divided by 25.4 = the amount of tire to the inside or strut minus the section width = the amount to the fender.

245x45zr 17 Sec. width 9.7'' stock offset 48mm

9.7 section widthx25.4=246.38mm divided by 2=123.19 plus 48mm offset=171.19 divided by25.4=6.74'' towards strut
9.7 section width minus 6.74= 2.96'' toward fender.

Take any section width of any tire and any rim offset to get your answer.
Remember rim width DOES NOT matter.

[Hockeyman]

I was understanding all up until the math mumbo.

You see...I'm dumb, and when it comes to understanding rim/wheel offsets, I need a video diagram. :shiner:

Nice writeup though...I think ;)

[GroupAwheels.com]

I was understanding all up until the math mumbo.

You see...I'm dumb, and when it comes to understanding rim/wheel offsets, I need a video diagram. :shiner:

Nice writeup though...I think ;)


Ok. Maybe I can help.
First of all, that avatar picture.... there is no words...

As for the offset explanation. I think the above explanation was actually good. The only thing is that the norm in the wheel industry is to talk about offsets strictly relative to the wheels, not using the tires. Especially since tire brands vary size to size.
Basically to describe the offset of the wheel you cut the wheel in half between the inner bead and outer bead of the wheel. If the mounting pad of the wheel was to be level with where the center-line of wheel is, it would be called a 0 offset (0 P). From that point any amount that the pad changes from that point, less or more is the offset, negative or positive.

Example: A 17x8 at 48 positive (Factory GTO wheel)
This wheel has a pad that is 48mm closer to the face of the wheel allowing it to sit sit on the hub 48mm in, past the center of the wheel.

If hypothetically that same wheel had a pad that went 48mm PAST the center-line of the wheel, it would be considered 48mm negative.

Offsets are also relative to the width of the wheel. Most people don't realize that and that is the most important thing.
Example: That same 17x8 48p GTO wheel fits fine on the front of the GTO, but not in a 17x9. A 9 needs to be a different offset

Hopefully this helps. I am certainly not a teacher. It is also easier to explain in person.

Did I mention I like that avatar?

[LBarnes]

The reason for the explanation using tire rather than wheel is that most questions concern, will a 275 fit or 265. If you look up tire specs it will usually give section width which is the widest part of the tire. This is what makes or breaks the fitment. Using this in the formula tells how much tire goes to the inside and outside based on the offset of the rim. A 9'' wide wheel at a 48mm offset in the front will rub with a 265, but with a 40mm it probably won't. Look at it this way:
Stock 17 setup 48mm offset 9.7" section width of the tire 2.96 out & 6.74 inside
17 42mm offset 10.2 section width(255) 3.45 out & 6.75 inside
Should clear the strut and probably the fender. The formula allows you to calculate where the tire will set with in the fender well.

[Hockeyman]

Ok. Maybe I can help.
First of all, that avatar picture.... there is no words...

As for the offset explanation. I think the above explanation was actually good. The only thing is that the norm in the wheel industry is to talk about offsets strictly relative to the wheels, not using the tires. Especially since tire brands vary size to size.
Basically to describe the offset of the wheel you cut the wheel in half between the inner bead and outer bead of the wheel. If the mounting pad of the wheel was to be level with where the center-line of wheel is, it would be called a 0 offset (0 P). From that point any amount that the pad changes from that point, less or more is the offset, negative or positive.

Example: A 17x8 at 48 positive (Factory GTO wheel)
This wheel has a pad that is 48mm closer to the face of the wheel allowing it to sit sit on the hub 48mm in, past the center of the wheel.

If hypothetically that same wheel had a pad that went 48mm PAST the center-line of the wheel, it would be considered 48mm negative.

Offsets are also relative to the width of the wheel. Most people don't realize that and that is the most important thing.
Example: That same 17x8 48p GTO wheel fits fine on the front of the GTO, but not in a 17x9. A 9 needs to be a different offset

Hopefully this helps. I am certainly not a teacher. It is also easier to explain in person.

Did I mention I like that avatar?

I now understand...thanks! This will be saved in my folders for future use.

I'll tell the avatar you said hi :)

Thanks again!

[Captgnarley]

where did the avatar go?

[dkrowner]

check out my signature for the offsets that fikse had me go with when they made my rims, they said thats the best offsets for that size of rims

[Scooter70]

LBarnes just showed a way to calculate backspace. Back before anyone cared about offset, backspacing was the important dimension. When looking at strut rub, too much backspace is your enemy.