How is "Wheel Offset" Measured?

[ctrcbob]

I keep reading about our wheels having a 35mm offset, and that you can use up to a 50mm offset. How is the offset measured?

I picture a straight edge layed across the edge of the outer rim, and then measuring down to the center, between the bolt holes, (or somewhere else close to the boltholes) but I have no idea if I am correct or not.

How is it done?

[Kelsey]

The distance from the mounting pad to the centerline of the wheel.

[rocknrod]

Look at the wheel:
a. The mounting surface of the wheel to the hub (on the car).
b. The rim of the wheel closest to the center of the car (inside rim).
The measurment (distance) between the two is the backspace.
You can order wheels with a different backspace to suit your cars mounting hub to outside and inside clearance. To get the look and clearance you want.
I can send you a page from a catalog (pdf) of you want that shows it.
Or you can go to like:
http://www.wheelvintiques.com/
And search around, they have how to take measurements page there also.

[ctrcbob]

OK, lets see if I got this right.

1. I first measure from outboard rim edge to inboard rid edge, then find the center (midway) between the two and mark it.

2. I then can put a straight edge on the inside edge where the bolt holes are, and touch the straight edge to the center point in #1 above, and mark it.

The offset is the distance between the two marks. Is this correct? Also, are our wheels a plus (+) or minus (-) offset?

[rocknrod]

Don't worry about the outboard side of the rim. Don't worry about the center of the rim.

Call me.

[TooManyFords]

I think the best link I've ever found that explains all of this can be found here:

http://www.rsracing.com/tech-wheel.html

Try this.

Cheers!

john

[SergntMac]

The distance from the mounting pad to the centerline of the wheel.Correct answer.

Look at the wheel:
a. The mounting surface of the wheel to the hub (on the car).
b. The rim of the wheel closest to the center of the car (inside rim).
The measurment (distance) between the two is the backspace.
You can order wheels with a different backspace to suit your cars mounting hub to outside and inside clearance. To get the look and clearance you want.
I can send you a page from a catalog (pdf) of you want that shows it.
Or you can go to like:
http://www.wheelvintiques.com/
And search around, they have how to take measurements page there also.Not the correct answer.

Offset is the distance between the centerline of wheel and the location of the mounting hub. The greater the number (i.e. 50mm vs 35mm), the greater the offset. Look at our wheels, and notice how flat they appear, how the center cap is almost flush with the rim. Then look here...

http://www.mercurymarauder.net/gallery/showphoto.php?photo=6392&cat=500&page=1

You're looking at an 18X9 wheel for the Marauder, with a 35mm offset. Note the almost 1.5" deeper dish. This is a result of moving the mounting pad from 50mm to 35mm. The closer the mounting hub is to the centerline of the wheel, the lower the measurement, which describes that more tire is "offset" towards the outside of the vehicle. Smaller offsets produce wider tracks by pushing the wheels out away from the frame.

Here's a pic of my MM with a 285/45 tires on a 18X9.5" wheel with a 50mm offset. Note the relationship between the outside of the tire and the fender lip.

http://www.mercurymarauder.net/gallery/showphoto.php?photo=6491&cat=500&page=1

This is a 265/35 drag radial on a 18X9 wheel (the one ^ there) with a 35mm offset. Again, note the relationship between the outside of the tire and the fender lip.

http://www.mercurymarauder.net/gallery/showphoto.php?photo=6492&cat=500&page=1

Therefore, offset and backspace are not the same measurements, but both do describe where the outside edges of a particular tire wheel will end up. Offset is the outside of the tire, backspace is the inside of the tire.

When Lidio introduced a wider rear wheel, all 1.5" added on is added inside, so, it's an increase of the backspace, but not a change in offset. I'm running wider tires, but wider on the inside of the car. The relationship between the outside of the tire/wheel and the fender lip is not changed and still matches OEM specifications.

BTW, that's not me in the pic, but I have felt that way on bad days. Oh, the drag radials and MB wheels are for sale too, make an offer to SergntMac@aol.com

Just my .02C...

[TooManyFords]

When Lidio introduced a wider rear wheel, all 1.5" added on is added inside, so, it's an increase of the backspace, but not a change in offset. I'm running wider tires, but wider on the inside of the car. The relationship between the outside of the tire/wheel and the fender lip is not changed and still matches OEM specifications.
You were doing great right up until this point. If you add 1.5" to the -inside- of our factory wheels, you have changed BOTH offset and backspacing.

Backspacing increases by 1.5 inches (the obvious one), but offset increases only .75 inches (19 mm) so the final offset would be 69mm.

Again, check the pictures out at http://www.rsracing.com/tech-wheel.html and scroll down to the section "Measuring Wheel Offset" for a description. Offset is derived from the center line of the wheel and adding 1.5" to the back moves the centerline .75" toward the back too. This creates more positive offset than it started with.

Cheers!

John

[SergntMac]

You were doing great right up until this point. If you add 1.5" to the -inside- of our factory wheels, you have changed BOTH offset and backspacing. Backspacing increases by 1.5 inches (the obvious one), but offset increases only .75 inches (19 mm) so the final offset would be 69mm. Cheers! JohnYep. You're right, John, (again, hehehe) and I apologize for flubbing my example.

I agree that if the precise definition of "offset" is the distance between the mounting hub and the centerline of the wheel, and the centerline changes due to added width on the backside, the offset measurement (technically) increases too.

I overlooked this because I was unduly focused on wheelwell clearance. There's been a flood of "will this fit?" posts lately, I'm starting to feel like a tire salesman. BTW, for future inquiries, y'all can learn more about tire fit right here. Please...Bookmark this?

http://www.discounttire.com/dtcs/infoTireMath.dos

However, when considering placement and clearance of the outside line of the tire/wheel in relationship to the fender lip, offset does not change in this portion of the examples I offer. For the sake of discussion here, that outside line still represents a 50mm offset mounting hub, and the owner can project clearance with the fenderwell when selecting tire size. This is exactly how I knew I could fit 285/45/18 tires on my car way back when I decided to take this 285/45/18 path. Due to "exclusive...Size shared only with production/manfacturers" (I guess if you own a Bently, you don't care about cost), I had to pre-pay my order and I could not afford to be wrong.

I believe it's also how others determined they could stuff the 305/45/18 Nitto drag radial in back there, because the clearance between the tire and the fender lip does not change.

I can rotate my 18X8-255/45 combo front to back, chalk line the outside edge of the tire on the pavement, and rotate back, with no change whatsoever.

So, Lidio's 9.5' wide wheel has a 69mm offset, but larger tires up to 305/45 will fit in the wheelwell, because the mounting hub is still a 50mm offset in it's relationship to the fender lip.

Thanks, John. BTW, I wouldn't try to widen a 35mm offset wheel in this manner, would you?

[ctrcbob]

Wow, great information. I'm bookmarking it as its all too much to take in at the moment.

Chief, Going to the phone now.

[TooManyFords]

Heh-heh.. I guess we all get wrapped up in the new custom rim out there and one mfg specifies it as offset and another as backspacing. We're so used to having these bigger rims that it wasn't like the old days. I used to be able to buy a pair of L60-15's and slap them on a pair of 15x10's and put them under just about any old mustang I had back then and it just worked. Nowadays, thanks to Mercury, we have to be careful.

The point is, everyone, that as long as you stay on an 8" rim, a 35-50mm offset will be comfortable on your ride. Just do the math if you want to go w-i-d-e and don't forget to include the tire section width or you just may rub with all the good intentions we post on here.

Cheers!

John

[rocknrod]

I see I kept talking about Backspace and he asked about Offset:(
So I was in the wrong.
However, if you want info on Backspace etc:
http://www.srbymichael.com/help/front-spacing.pdf