How to Calculate Offset after Lip and/or Barrel Change

[Han]

Made this a while back...


Before we start, please use Roan's (zeroluxxx) DIY Guide on measuring wheel parts so you get the correct calculation to start out with: [ http://forums.vwvortex.com/zerothread?id=3773369 ]


Maybe this could help some people as I know it can get confusing. Let's start with my wheels as an example:

Fronts:
BBS RS320
17x8 ET43 (7" barrel + .5" center + .5" lip)

Rears:
BBS RS300
17x9 ET53 (7.5" barrel + .5" center + 1" lip)

Adding the center's thickness into the equation only works if your wheels are sandwich mount (lip, face, barrel).

I moved the 1" drop lips from the rear to the front, resulting in:
17x8.5 ET37 (7" barrel + .5" center + 1" lip)

Now how did I get the final offset? Using [ http://www.1010tires.com/WheelOffsetCalculator.asp ]. I input the stock set up (8 ET43) and input the new width (8.5) and got to ET37 to maintain the same inner position because you aren't changing anything except the outer position.

I have noticed for every .5" you go out (pattern), while maintaining the same inner position/clearance, you will lower your offset by ~6mm (6.35mm to be exact (25.4 / 4 = 6.35). I have also noticed the outer position will go out 12mm (using the calculator, it gives you 12mm) (12.7 to be exact; 25.4 / 2 = 12.7)).

Then I got 1.5" lips to replace on the rears resulting in:
17x9.5 ET47 (7.5" barrel + .5" center + 1.5" lip)

Now what about if I changed the barrels up? You'd see the same pattern, but the outer position stays the same, whilst the inner position goes in or out (depending if you get smaller or larger barrels, of course).

Let's use my stock fronts for example (17x8 ET43 with a 7" barrel):

I'll put a 6.5" barrel on it, resulting in: 17x7.5 ET37 (6.5" barrel + .5" center + .5" lip). Notice the pattern again; .5" on that inner position results in 6mm lower offset (again, technically 6.35mm, easier to round down). Same thing with the inner position, it would gain 12mm (12.7 to be exact).

Well, what if I do both [smaller barrels and bigger lips]? You basically use the same formula/input the same things, but one at a time.

For example, lets use the stock fronts again but put .5" smaller barrels and a 1.5" lip on it.

Stock we know is 17x8 ET43 (7" barrel, .5" center + .5" = 8")
.5" smaller barrels we know results in 17x7.5 ET37 (6.5" + .5" + .5" = 7.5)
Now lets add that 1.5" lip to the equation: 6.5 + .5 + 1.5 = 8.5"
Final specs would be: 17x8.5 ET24 (which would gain 12mm inner clearance and outer position would "poke" 26mm more than stock lips).

Hope this helps

[firelizard]

That's a lot of text to essentially say:

Lip increase, converted to MM = net reduction in offset
Barrel increase, converted to MM = net increase in offset

Lip and Barrel changes: net reduction by lip + net increase by barrel = net change in offset (could be negative or positive)

[Ghk420]

^^^^^i was thinking the same

[migid]

Hi! I want to by an e46 m3 rim for my e36 320i. I dont know if the rim will work perfekt without the modification on the fender etc..., on the car. There isnt any e46 m3 rims near here, so i cant try it on. The rims are 8JX18 ET/IS:47 9JX18 ET/IS:26 .

[BLKonBLK98]

That's a lot of text to essentially say:

Lip increase, converted to MM = net reduction in offset
Barrel increase, converted to MM = net increase in offset

Lip and Barrel changes: net reduction by lip + net increase by barrel = net change in offset (could be negative or positive)

changing the width x amount only changes the centerline x/2 which is where offset is measured from. all you ever need to know:

+1" lip = -12.7mm offset
+.5" lip = -6.35mm offset
-.5" lip = +6.35mm offset
-1" lip = +12.7mm offset

+1" barrel = +12.7mm offset
+.5" barrel = +6.35mm offset
-.5" barrel = -6.35mm offset
-1" barrel = -12.7mm offset

[bbaction]

The easiest way to do this is look at a Backspace to offset chart and calculate...

Find your current width and offset then trace directly down to desired width it gives you roughly your new offset (add or subtract accordingly)

[SIN3]

saved.

[fastcar]

That's a lot of text to essentially say:

Lip increase, converted to MM = net reduction in offset
Barrel increase, converted to MM = net increase in offset

Lip and Barrel changes: net reduction by lip + net increase by barrel = net change in offset (could be negative or positive)

I was thinking the same

[indy510]

.
That chart is really helpfull ....... . I am trying to figure out if I can fit a 16x8.5 -6mm wheel

I have 16x8 +12 .. . so according to that chart, if my math is right... I think I need 24mm of spacers to replicate a 16x8.5 -6mm???

.... assuming I will use the same tires is my math correct???,,,,, it's gonna be a really tight fit,, every mm counts

[Francois Dillinger]

That chart is awesome. Anything saving me from doing math gets a thumbs up in my book.

[Ollie]

That's a lot of text to essentially say:

Lip increase, converted to MM = net reduction in offset
Barrel increase, converted to MM = net increase in offset

Lip and Barrel changes: net reduction by lip + net increase by barrel = net change in offset (could be negative or positive)

lol ok but you're wrong. thanks anyway

[Noodle]

i was going to say easiest way is just know the backspacing . If you have a 4" backspace on a 9" wheel and you add a 1/2" wider lip then just figure the offset on a x9.5 with a 4" backspace.

[jamesklass]

I know it can get confusing. Let's start with my wheels as an example

[Capt. Obvious]

[scaredofstance]

nice thread. i suck at math. this will help. lol
i know its old, whatever.