Offsets and et numbers - help I'm confused

[richiep]

I'm trying to decide upon which wheels to fit to my bug and have found conflicting information on the 'net about how the offset or et of a wheel is derived. Some say that a positive offset moves the face of the wheel that attaches it to the hub out towards the outside rim of the wheel (larger offset thus tucking the wheel under the wheel arch more). Some say the it's the other way 'round. I understand that a beetle has a standard offset of 23mm. The dropped spindles and disk kit I am using increase the track by 10mm per side. To keep everything under the arches am I looking at an offset of approx 40mm if I intend to use a 5.5inch wide wheel. Tyre size I would like to use is 175 55 15 as found on the rear of a smart car. Anyone got any experience with these?

Any help given will be very greatfully received as my brain feels like it is going to melt soon!

Cheers
:doh:

[vujade]

a positive offset wheel is a wheel where most of the wheel is under the arches.
a negative offset wheel is a wheel that looks like a deep dish

if you are running a 4.5" wheel now with a 23mm offset & you are
changing to a 5.5" wheel with a 40mm offset, this setup will give you 4mm more clearance then you had previously with the old setup.

Now taking the 4mm extra clearance & subtracting it from the 10mm extra your track is going to stick out because of your mods, this will make your car an extra 6mm (about 1/4") wider per side.


To figure backspacing:

Backspacing is figured by dividing the wheel width in 1/2, then applying the offset to that number.

For instance, lets say you have a 7" wide wheel with a 52mm offset.
7 divided by 2 = 3.5"
Now take the 52mm (about 2") & add that to the 3.5"
2" plus 3.5" = 5.5"
So you end up with 5.5" of backspacing on a 7" rim

[richiep]

That has made it a lot clearer. I had read a post on here that mentioned backspacing before, but couldn't find it again. If that was put up by you, sorry for making you repeat yourself!

All I need to do know is find some 5.5in wheels with a 40mm(ish) offset that I like the look of!

Anyone know the offset of those Mahle alloys that were fitted to 914s?(Not very GL I know) I think that I could source a set of them easier than a set of 5.5in Fuchs's 'cos there's a 914 specialist near where I live.

I have asked them but they don't know the offset????

"Bolt straight onto a beetle these will" is the best they could offer!

Cheers

[vujade]

try Wheel Enhancement

email them & they will be able to tell you the offset of those Porsche 914wheels. I bought my wheels from them. They are very helpful.

[bren]

sorry guys, youre wrong! the commonly quoted ET numbers are in fact for NEGATIVE offset! for example, a wheel with an offset of ET35 has its mounting face 35mm outside of the rims centreline, meaning the wheel sits 35mm further in towards the centre of the car than a wheel with zero offset.. positive offset rims tend to stick way out beyond the arch and have a deep-dish look to them.

sorry to put out the fire, but felt i needed to clear up some misinformation!

[zen]

actually bren, vujade is correct. positive is when the mounting surface it outboard of the rim centerline. negative is when it is inboard of the centerline. i am 100% "positive" of this.

offset is often confused with backspacing. backspacing is the measurement from the back of the rim to the mounting surface.

[hot66]

bren is correct about the 'et' situation. A 7x 17 et 52 porsche wheel will sit further under the wing than a 7 x 17 et 35 porsche wheel. Bren s discription of working from the centreline of the wheel is correct. This applies for europe, don't know if its different in the states

[vujade]

Bren-s description of a Porsche wheel with a high ET sticking under the car is correct, but I dont think he is correct about the terminology.

[hot66]

just read vujade's post, & he is also correct in how to work out 'back spacing', or mounting face to inner wheel lip.

I think the confusion lies in what people actually call positive or negative. I don't think this really matters, as long as you know the 'et' & the rim width then you can work out if the wheel will fit.

Just make sure the company you buy your wheels off is talking about the same dimensions

[bren]

maybe this is another USA -vs- the world thing? in europe, ET numbers denote negative offset (ie how far under the arch the wheel sits) in the states it may mean a different thing.

vujades backspacing explanation is correct, although again, "backspacing" is solely used in teh US, nowhere else as far as i know, uses backspacing as a measurement.

tomato, tomato, lets call the whole thing off.

[vujade]

you may be right Bren as far as the US vs Europe thing, or I could be completely off my rocker...:silly:

[richiep]

I agree with Hot66.

After the help I have recieved off this forum and from a local alloy wheel shop, I'm pretty sure how it all works now. Its people's interpretation of what is a positive or negative offset that led me to start this thread in the first place. Kinda comforting to know that other people have differing views on it. Anyway, after talknig to the wheel shop, I will have a go at explaining the whole positive-negative thing. Positive refers to the et number being a positive number, et25 for example. Negative refers to the et being a negative value, eg. et -25.

How this relates to the dimensions of the wheel has been explained before in this thread but I'll have a go aswell (just to make sure I fully understand it now!)


Imagine you have a 6inch alloy wheel with no tyre on it sitting in front of you with the shiny side to your right and the crappy brake dust covered side to your left . If the wheel had a zero offset (et0) then the mounting face of the wheel would be bang on in the centre, 3 inches from either rim edge. Now if the mounting edge is moved (offset) right (towards the shiny side of the wheel) by 20mm, the wheel is deemed to have a positive offset of 20mm (et 20). If the mounting face is moved to the left of the wheel by 20mm from centre - (towards the crappy side), then the wheel is said to have a negative offset of 20mm (et -20). I don't know why offsetting toward the shiny side is known as positive and dirty side is known as negative - it just is.

So a wheel with a positive offset will tuck under the wheel arch (fender) more than a wheel with a negative offset if they both have the same width.

I understand that positive sounds like sticky outy and negative should mean tucky undery - but this is wrong.

Oh, btw I have chosen to go for 5.5 x 15 fuchs alloys with a 42mm offset. Should just get 175 55 15 on the front with those.

If I can't source any of these, Pedrini's or Mahle alloys from a 914 have the same offset but I will have to change my disks (rotors) to standard VW pattern to fit these.

[vujade]

richiep

thanks for the info...thats what I have alway believed myself.

IN regards to what you said about not knowing why a positive offset wheel is called just that, seeing how it sticks under the arch which is infact moving the wheel in a negative direction.

I believe it is called a positive offset, not because of where it moves the wheel (negative) under the arch, but where it moves the face of the wheel (positive) to the outside, away from the car.

[richiep]

Here's a table of some factory fitted porsche wheels with their widths and offsets (and weights!).

Might need to zoom in a bit to read it!

[hot66]

just to add to this......remember the stated width of a wheel (eg 7") is not always the actual width. I know with later cup's a 7" wheel is actually 8" between outside edges. The 7" refers to the tyre mounting surface.

This is only important if you are really trying to squeeze a fat rim into a small place