Benjamin T

iTrader: (2)

i am looking at a set of rays wheels that do not know the offset for. the wheels are old, and the labels on the inside of the rim are unreadable.

is there a way to physically measure out the wheel and find it's offset?

GSXOTIC

iTrader: (6)

some JDM manufacturers "stamped" that information in the bore (hub) of the wheel between each stud hole. so you can check that first if the wheels are off the car. P.S. some of them were actually on the spokes (if the spokes are big enough). Hope this helps.

Benjamin T

iTrader: (2)

the width and diameter are cast into the hub, but no offset.

bb430

iTrader: (1)

The offset(ET) is calculated by the width of the wheel's center point being "0". xmm to the outside of the wheel is +. xmm to the inside of the wheel is -.
So if you had a 10" wide wheel, the center(5") is 0 offset(ET0). +25 offset means the hub is out side of the center line by 25mm. And -25 offset means the hub is inside of the center line by 25mm.
I think Tire Rack website has a picture and explaination of this as well.
Here http://www.tirerack.com/wheels/tech/...jsp?techid=101 you go...

mitsuguy

iTrader: (2)

good info bb430, but here's the practical way to measure it...

lay the wheel (no tire) on the ground, face up... measure from the ground to the face of the hub... that's the backspacing... then measure the entire width of the wheel, from the ground to the topmost part of the wheel - thats the total wheel width (not advertised, advertised wheel width is inside the bead seat)...

change everything to MM (multiply inches x 25.4), 'cause it's easier.... then divide the total wheel width in two... subtract half the wheel width from the backspace and this is your true offset (though it may vary slightly from advertised)

edit: if the tires are on the wheels still, a straight edge from lip to lip across the back will give you the same info...

Benjamin T

iTrader: (2)

thanks for the responses everyone. i'm going to give mitsuguy's instructions a go first...

SoCalSC4

You guys always forget one key thing when calculating this way:

A 10" wheel is not a 10" wheel!
10" is the bead-seat to bead-seat measurement,
not the overall width. When calculating for offset, you MUST add the two 1/2"
bead seats into the equation.

If you forget this, you run the risk of having clearance issues:

Example: 20x10" wheel with 7" backspacing

Wrong way:
10" divide by 2 = 5" centerline
7" backspacing minus 5" centerline = 2" offset (2" x 25.4 = 50.8mm offset)

Right way:
10" + 1" (wheel bead seat area) = 11" total width divide by 2 = 5.5" centerline
7" backspacing minus 5.5" centerline = 1.5" offset (1.5" x 25.4 = 38.1mm offset)

rominl

iTrader: (4)

bob i am also curious, is this the same case for all wheels including jdm, etc...?

mitsuguy

iTrader: (2)

It's not 1/2" per side, it's 1/2" total... and thats when doing thoeretical mathematical calculations, not measuring in the way I showed...

The way I showed took into account the lips of the wheel when measuring both backspace and offset because we technically don't know either...

SoCalSC4

No, no no! It's 1/2" per bead! 1" total.

I see where you are going with this, but to find the centerline, you
must know the actual total width. If you base the calculation on the
overall measured width, then you're okay, but my formula stands.

I suppose if you mean my example of a 20x10" wheel is really a
20x9 (technically speaking), then you're right. I intended the 20x10
measurement as the manufacturer's stated width.

My 20x11.5" wheels are 12.5" overall. All of our wheels are this way,
as are all US-made three piece wheels and most one-piece wheels
(certainly all that I know about).

No worries... I just do this for a living.

mitsuguy

iTrader: (2)

I do this for a living as well, have been for over 7 years, and don't know of any wheels that need or have a half inch thick bead seat... I just ran out to measure mine, and they are advertised as 9.5" wide, and they measure 10.125" (10 1/8") My fronts that are 8" wide measure at 8.75" There should be no need for a half inch bead seat on ANY wheel - certainly some wheels will have thinner and some will have thicker, but to add a full inch as a general rule is crazy... a steel wheel may have only 3/8" total added to it's advertised width... I am sure these variances is why certain wheel manufacturers offsets don't always seem in line with what it actually looks like on the car...

My calculations were based on the total measured width, as I already stated...

SoCalSC4

There certainly will be variations between wheel manufacturers,
and even within styles those manufacturers offer.

I just went out and looked at a 19x9.5 BBS LM-R and it indeed
is 10.5" overall. The RS-GT I looked at was 19x8.5 and it's
9.5" overall. The CV; same way.

There was an old BBS RC wheel which was labeled 7.5" and really 8 3/8" overall.

On the modular wheels, the beads are always 1/2" (as far I I know). This helps
increase surface area and therefore strength (to a degee, anyway).

The measurements you made were on your TSWs? That would explain things.
Different wheel manufacturers use different standards and tolerances.
Once upon a time, CEC made a wheel that was 18x8.25"... sort of odd!

If you have the actual wheel with the tire dismounted, basing your
calculations from the actual measured wheel width is always a good idea.

If a customer calls and says "I want a wheel like my X-brand, 19x10 +45,
I assume a 1/2" bead seat on either side to ensure I don't encounter
interference problems on the inside dimension of the new wheels.

Granted, my decade of experience is strictly in modular wheels,
not cast one-piece.

mitsuguy

iTrader: (2)

Well, I guess that means we are both right and it does vary per manufacturer...

sucks... it's just like tires though I guess... certain brands are definitely narrower or wider than others too...