I am getting comflicting info on rim offsets when it pertains to positive and neg.

This TGO FAQ site lists the front offset as 0* which is no dispute,

However, it lists; "the rear offset as 16mm(negative - outward) on the rear." Technically it should read -16mm.
https://www.thirdgen.org/newdesign/faq/thirdgen.shtml

Now I cross reference offset in an old book I have (How-to-make-a-car-handle) and it also lists the direction of the mounter surface of the rim being outward from center line width of the rim as being negative offset


Now here's the debate.
Take a look at for instance a site like Skulte adapters and they list our 3rd gen cars as having a +16mm rear offset rim. (not negative, but in fact positive)
http://www.skulte.com/adapterfaq.html
This picture is wrong, yet manufacturers seem to be basing info off this now.

So now I have info on this website pertaining to brake system mounting surface offset and measurements for refence pertaining to a front 0mm offset, and the TGO listed -16mm rear offset (whih is correct to ALL literature I own referencing rim offsets in very sophisticated handling and geometry bibles.

I notice that someone here is now asking for help on rim offset and it shows a 14mm offset, and a 5.5" BSM or backspacing. That should be listed as -14mm offset. What gives? anyone in the rim business that can clear this up. Are they just listing the offset as 14mm whether it is + or - from center and its up to you to differentiate by calculating the bsm to figure out which way it offset to the inward or outward?

If there is question, I can scan and post a picture out of a handling bible to show the opposite of what skulte is showing.

 

I went ahead and scanned the picture out of Fred Puhn's book on wheel offsets. this book has had several different printed editions over the years so any mistakes would have been corrected I would think. This book is highly regarded as one of the best and factual suspension bibles there is.

again, the link to Skutle. Notice that the link shows the "outboard" facing the opposite direction, so don't confuse this because each example is reversed. One shows offset towards outboard as positive (skulte) and one shows offset towards outboard as negative (Puhn)
http://www.skulte.com/adapterfaq.html

 

Apparently there isn't a set spec for what positive and negative offset means. I did a quick search and the first link states that there is no set spec. And that site (USA Compomotive wheels) uses the same +- offset spec as Puhn's book.

Then I did the Tire Rack link and they state the opposite for positive and negative offsets. Yikes. Here are the links:

http://www.usacomp.com/Offset.htm
http://www.tirerack.com/wheels/tech/offset.jsp

To add to the IROC wheel mix there is also a -20mm offset rear wheel in 16 x 8.

RBob.

 

Well isn't that something!

Unless you know what you are doing calculation wise, it looks like the aftermarket rim companies will always manage to not loose too much local business through internet sales from out of state. Most people it appears will have to just drive their cars into shops and have the technicians make rim choice fitments for them. This kind of "non-standard" must hurt companies like Tirerack. I would think they would want to develop an easy to read set standard of charts in the industry.

Heck, Maybe I can start it, we can call it "Dean's Offset"
(Dean as in principle)

 

This is interesting. I also notice that most of the forthgen guys will refer to the wheels at 38mm or 50mm offset, not -50mm. So i guess this confusion is widespread. Who would you need to apply to to get 'Dean's Standard'?

 

That's why I never understood the use of offsets, just use backspacing and you'll be fine.

 

That won't always work either. Take the AR TTII's for example. Their 17x8 with a 4" backspacing lists as -11mm offset. If you measure the backspacing on this wheel is measures out to 4", but if you measure from the mounting surface to the outside lip of the wheel it measures 4 3/4". Go Figure. A 4" bs on an 8" wide wheel should only protrude 4" from the mounting surface... but it doesn't, which makes AR's 17x8 TTII's more like 17x8 3/4. You really have to be careful even when just using bs.

 

Did you measure bead to bead on the wheel by any chance? I'd like to see what your measurement is there. That would help explain what the problem is.

 

Heh… how the hell did that happen…

FWIW, I’ve had exactly this argument with Andris before, I think at least 2x. He swears that the phun book is wrong (but every automotive engineering text that I’ve ever seen agrees with it).

I finally decided that from a chassis engineering perspective that is correct, positive means sticking out, negative means tucked in and that the majority of wheel people are happy enough not caring what the hell the deal is and calling it anything they want. The funny thing is that I know that I’ve heard “positive offset wheels” used to describe wheels that stick out by old timers, so I’m guessing that this is even recent thing.

That brings us to backspacing, which seems almost TOTALLY RANDOM from manufacturer to manufacturer. Some, like American Racing actually just set a standard that they are going to use and use it. They publish a chart in their dealer catalogs, but it can actually just be summed up as take the backspacing that you’d expect for that width rim to have a 0 offset and add .5” and you have their backspacing number (which considering most wheels have a 3/8” or thinner lip that would mean that the backspacing describing a point that is at least 1/8” past the rim). As best as I can figure, centerline and a few others have most of their rim offset vs backspacing numbers off by somewhere between .5 and 1.25”, and then a lot of the more “race” rim manufacturers are just totally random, for example, bogart’s wheels seem like the backspacing is just some vague means of comparison between their line of rims, when trying to fit skinnies on the front of my brother’s car we found that the backspacing for the first rim that we tried was about 1.25” off of what we expected, and the second one was somewhere near 2” off… if I remember right, he ended up using a 15x3 rim with 2.5” backspacing that when mounted on the car stuck out almost to the edge of the fender…

 

here's a scan of ARE's chart out of their 2005 book:

 

I measured from outside lip to outside lip. Total rim width was really close to 9". I didn't measure from bead to bead though.

 

Now that is different from the one I looked at. I don't know if the one I looked at was '03 or '04, but it lists an 8" wide wheel with a 4" bs as having a -11mm offset (sticks out about 4.75" or 5" from mounting surface) and an 8" wide wheel with a 4.75" bs as having a +8mm offset (sticks out about 4" from mounting surface).

 

It was my understanding that a positive offset was moving the mounting surface towards the face of the lip with the rim centerline behind, and a negative offset was moving the mounting surface behind the centerline.

My ZR1' are +50mm offset and sit inwards while my brothers old ZR1's were either +36 or +38 and they sat even with the fender lips while on my car.

 

i was wondering the same thing years ago when i was looking for adapters. i had skulte make my adapters based off his measurements and it worked perfectly. i built by 2 piece billet wheels off the idea that the rear is a +16mm and they worked perfectly. i definately note where your coming from though and that's a great question, fortunately it all worked out for me.