How to measure wheel offset?
 

I know there's many different offsets for Fuchs, I just don't understand how to measure. I.e. from what point to what point? In relation to what?

Can anybody help?

http://forums.pelicanparts.com/porsche-911-technical-forum/124791-wheel-info.html

Tom

Offset is the distance from the wheel centerline to the actual hub mounting surface. i.e. an ET-zero wheel puts the center of the tread directly above the hub/wheel mating surface, and ET45 wheel would move the tire inboard 45mm.

If the wheel doesn't have different bead to rim edge dimensions between the inside and outside lip, you can get away with using half of the outer edge-to-edge rim width as the centerline measurement. The offset is the difference between that centerline and the hub-mating surface of the wheel.

EDIT: Hokay, I type too slow...two posts showed up after I finished. In the PP thread link, the Fikse drawing has some odd quirks ( like the wheel isn't shown touching the hub ), use Bill V's picture as it clearly shows what I was describing.

Gracias! I didn't see that one for some reason.

Talk about typing/reading too slow. I said gracias to Tom's posting of the link, but by the time I submited, there were two other responses. :)

I don't know where they got their definition of offset, but it is opposite of the one used by everyone else.

negative o/s moves the tire further outboard and positive o/s moves it further inboard.

the correct calculation

o/s = backspace - half of total width

One thing about measurring o/s and b/s that bugs me is that all wheel manufacturers would need to use the same reference point in order to get the same o/s or b/s. Is the dimension of the last outer most lip(away from the tire bead) on a wheel standardized? It's a little hard to describe but if, for instance, Fikse's outer lip is dimension "x" and BBS's outer lip is "y", then never will you get the correct offset when using one to purchase from the other. Confusing - yes. Should the measurement be taken from the wheel at the "bead lip"? and not on the outer edge of the rim?

The standard dimensions for these values is shown in this diagram.
http://www.pelicanparts.com/MotorCit...AQ-diagram.gif

The wheel show + o/s, if the mounting face was inboard of the center line it would have been - o/s

The standard reference planes for o/s are the wheel centerline and the wheel mounting face

The outer face planes are defined by a line(rigid stick, I have a nice mahogony one but any straight, rigid non marking piece will do))laying across the the furthest extension of the lips, for acuracy lay it across the axle ctr. but as long as the wheel is true where ever is ok.

Bill, your willingness to share and your patience are beyond reproach.

Your diagram illustrates my point rather nicely. If the dimension between the RIM WIDTH(bead) and the OVERALL RIM WIDTH are not standardized by the industry, then the o/s and b/s will differ from one manufacturer to another, based on this outer lip width dimension inconsistancy. Am I wrong in assuming that this would effect the o/s value from one mfg to another?

Sucking up will always get a reply!:)


That is exactly why I needed to start collecting my own data. Its bad enough w/ the different Fuchs but different manufacturers can be as much as .5" off of each other.

I suspect Fikse and Kinesis use the same rim half supplier though. There is just too much consistancy in the #s

Wow, I didn't mean to start a war :) JK
But seriously, thanks for the links and the definitions. The reason I ask is I just left my wheels with Al Reed and am having him put 4 new tires, that I bought, on them. I bought 245 45 16's to go on the 9" Fuchs I have on the rear. I'm just concerned about the offset because my old 225's would rub sometimes. I might have to roll the lip. I just want to check the offset when I get them back to make sure they're not some weird variation.

Again, thanks for the help! and Bill, thanks for the spread sheet. As someone in the other post said, it's good to have as reference. Esp. for those thinking about switching rims.

http://www.pelicanparts.com/support/smileys/pray.gif

I am not worthy, oh great one.

Thanks for clearing that up for me. Not that it will make buying wheels anyless annoying but, hey at least I know, perhaps, as much as the "wheel" guy.