Offset Calculation For Rims.

[CeJay]

Hi all

Just want to check if I've calculated this right. I am currently looking at some rims at a wrecker. The rears are 9.5 inches wide and the fronts are 8.5 inches wide.

Now the rear measure 17cm from the hub of the rim to the rear of the rim. So I'm assuming to get the offset of this rim it would be 2.54 * 9.5 = 24.13

24.13/2 = 12.06

17 -12.06 = 49mm offset?

The fronts measure 15.5cm from the hub of the rim to the rear of the rim so the offset would be:

2.54 * 8.5 = 21.59

21.59/2 = 10.80

15.5 - 10.80 = 47mm offset?

I'm attaching some pics, the only reason im asking is I havent heard of any wheels with such a deep offset so I'm just wondering if I am doing it wrong. Can someone confirm my calculations? Or should the measurement be taken from a bit lower down on the rim? As I'm getting the measurement from the extreme lip of the rim. I was told that these rims came off a skyline and should fit all of them, but with these offsets I don't think they'd fit? As mine is a 32.

pics

anyone able to identify these stern rims as well? I have very similar ones at the moment but mine don't seem to be two piece like these seem to be.

[BHDave]

your calculations are correct, but ideally you need to measure the offset from the centre of the rim. You have assumed the width from lip to lip is 9.5 inches, when it is probably a bit wider than that as the lip flares out.

Measure the total width from lip to lip and sub that for the 9.5 and 8.5 in your initial calculation. It should give a more accurate result.

Offsets like that will not fit an r32, you will need absolute maximum of +40 on the rear and even that will probably require some shaving of the hicas rack end. Fronts need to be closer to +30 to look right.

Quick edit: I have a set of similar wheels also in 17x9 +25 apparently. mine are stern kaisers according to the center caps, but i have 2 different center caps between the 4 wheels so they could be anything.

Edited October 15, 2007 by BHDave

[blind_elk]

Offset = backspace - (width / 2) - which is what you have calculated.

The "width" of the rim is usually the internal rim width (where the tyres sit in). You can only measure the backspace to the outside of the rim. Probably take off 10mm for the thickness of the rim wall.

That should bring it down to 39mm, which is pretty close to a stock 38 offset.

Some rims now have the offset cast into the rim, along with the width and diameter - have a close look.

[CeJay]

Ok thanks for that info. So should the proper measurement for the offset be taken from the inside lip of the rim and not from the top of the lip? See attached pic.

[blind_elk]

It needs to be taken from the edge that the tyre installs against, ie on the "tyre" side of where the arrowhead is pointing.