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How to measure the offset of a rim? 

 

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 Post subject: How to measure the offset of a rim?
Posted: Tue Dec 02, 2008 10:53 am 
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Hi all,

I'm interested in a set of mags that are currently on a really old ford...

They are 18inches, but I don't know if the old fords suspension would hit on post AU rims or not (wheras E series definately will) so my question...

With a tape measure, how can you tell for sure what the offset is? I have to talk the owner though the process over the phone.

ta

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Frank
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Posted: Tue Dec 02, 2008 10:56 am 
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Measure where the mounting face is in relation to the centreline of the rim.
If the face is more towards the rear of the rim in relation to centreline, it's negative offset.
If it's more towards the face, it's positive.
If it's bang dead on in the centreline it's 0

 

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Posted: Tue Dec 02, 2008 11:22 am 
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its hard to measure the offset if the tyre is still on the rim.

if the tyre is off the rim, then you measure the total width of the wheel including the lip. Halve that number & write that figure down (a)

...... then run a straight edge across the back of the wheel and measure from that to the hub mounting face inside the wheel. Write that figure down (b)

Offset is (b) minus (a) equals offset..... most will be positive.

Or if the tyre is still on the rim, just run a straight edge across the back of the wheel and measure from that to the hub mounting face inside the wheel. For example a 6" wide e-series stockie will have 98mm backspacing, so a 6" wide AU stockie will have roughly 130mm backspacing.

Backspacing will change the wider the wheel, so assuming every extra inch is shared on both sides of the wheel, add 13mm to the backspacing figure..... so for example if a 6" wide e-series stockie has 98mm backspacing, then a 7" wide e-series mag wheel has 111mm backspacing, an 8" wide e-series mag wheel might have 124mm backspacing
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Posted: Tue Dec 02, 2008 11:50 am 
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Thanks guys,

All I really need to be able to do is determine if they are pre AU or post AU... Other than that I don't really care....

So basically I get the width of the rim and from that half it to get the centreline.. then measure the distance from the inside of the mounting face to the back edge of the rim. then use those measurements to work out where the inside face is in reference to the centerline of the rim?

So pre AU are negative offset and AU positive?
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Posted: Tue Dec 02, 2008 11:53 am 
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pre au is +6mm..... post AU is 36mm+

btw reason i mentioned its hard to do it with the tyre on is cause a 6" wide wheel is not physically 6" wide when measuring lip to lip.
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Posted: Tue Dec 02, 2008 1:24 pm 
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sweet, so if he gets the center point of the rim.. if it's the right sort, the mounting plate will be very near the center.. if it's towards the front by approx 3cm.. then is AU...

not too hard..

I didn't think it mattered how wide the rim was as long as you measure and get the middle.
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Posted: Tue Dec 02, 2008 1:34 pm 
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frankieh wrote:

I didn't think it mattered how wide the rim was as long as you measure and get the middle.


how do you get the middle, if you dont know exactly how wide it is? :wink:
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Posted: Tue Dec 02, 2008 2:07 pm 
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the middle of the tyre would be the same as the middle of the rim wouldn't it?

close enough so that 3cm difference in offset would be noticeable anyway I'd imagine..
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Posted: Tue Dec 02, 2008 2:16 pm 
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frankieh wrote:
the middle of the tyre would be the same as the middle of the rim wouldn't it?

close enough so that 3cm difference in offset would be noticeable anyway I'd imagine..

Yeah close enough I'd reckon.
You can just about eyeball the 30odd mm difference without measuring.

 

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Posted: Tue Dec 02, 2008 2:24 pm 
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sweet...

I suspect they must be pre AU 18 inch rims if they fit on a early XW falcon but figured it can't hurt to check.
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Posted: Tue Dec 02, 2008 4:42 pm 
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yes it would be pre AU to fit on a XW/XY without spacers.

 

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Posted: Wed Dec 03, 2008 1:01 am 
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the offset should also be stamped on the back of the rim somewhere I believe?

look at a photo of a BA rim compared to an E series rim and you will be able to see straight out the difference. AU/BA the mounting face is only just behind the spokes, 3 series have a noticeable protrusion back inwards.

 

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Posted: Wed Dec 03, 2008 7:06 am 
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Not all rims have it stamped.
But the ones that do will have "ET+30" or something similar to denote a positive offset of 30mm.

 

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Posted: Wed Dec 03, 2008 9:43 am 
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he said the mounting plate was very near dead center..so I'm gonna buy them.
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