RX7s

OK....when people talk about rims, the word "offsets" always there. So what is the differences in the 38...40....44...50 ... +....-.....all that . What changes in the wheels when the "offsets" change. Is it refer to how deep the lip of the rim??
Max, or sleepyR1 please explain to me.
Please don't tell me to search cause I did.
Thanks
Mike

Mr. Stock

go read Steve Ciriani's website
Lightning in a world of thunder
Explains all

maxcooper

You know how they make wheel spacers that you can mount between the wheel and the hub? Those are for adjusting what I guess you'd call the "effective" offset. Wheels themselves have different amounts of offset built in. A wheel with zero offset would have that mouting surface in the exact center of the wheel. It would be in the middle of the tread if you were looking down on the wheel from the top. Moving that surface toward the outside of the wheel moves you into positive offset, moving toward the inside would be negative offset. Wheels that have an offset that isn't positive enough (e.g. you have a +20 wheel, but a +50 would fit better) will tend to stick out of the fenders.

Offset is not a measure of the depth of the lip because that isn't a very useful thing to measure precisely -- it is an aesthetic concern rather than an engineering/fitment concern. But, having a less positive offset would tend to increase the depth of the lip. For instance, if you have two of the same wheel except that one is +50 and the other is +25, the +25 wheel is likely to have a deeper lip.

There is a wheel fitment form on http://www.fikse.com/ that shows a bunch of useful wheel dimensions, including offset.

-Max

maxcooper

Also, be careful in converting between backspacing and offset. It is a pretty straightforward calculation, but wheels are always larger than their nominal bead width. For instance, a 10" wide wheel is really anywhere from 10.5" - 11.25" wide. Backspacing is relative to the real width of the wheel.

-Max

SleepR1

Excellent description Max!

ptrhahn

iTrader: (9)

THIS is the really valuable info here. It took a bit for me to catch on to this. It really is a shame there isn't some sort of standardization, so that a 9" wheel is 9" and there'd be no doubt as to how it would fit.

Its almost as silly as the tire size numbers!

RX7s

Thanks Max, for a detailed answer

RETed

Actually, the dimensions are fixed and unchanging.&nbsp Most of the "metric world" uses "offset", which the U.S. still likes to use "backspacing" (especially in NASCAR/circle track/dirt track racing).&nbsp I do have a quick graphic representation on my website:&nbsp http://fc3s-pro.com/TECH/MODS/RT/rims.htm


Actually, it's the inconsistent tire sizes that screw everything up.&nbsp Rim sizes and specs are unchanging, no matter what part of the world you're in; offset and backspacing are easily converted with simple math.&nbsp Due to difference in tire contruction, you get varying answers to rim/tire combos...



-Ted

maxcooper

Ted,

I think he was talking about actual rim width versus bead width. 8.5" wheels are never really 8.5" wide, so it complicates the conversion between backspacing and offset. The thing is that they are standard -- 8.5" wheels have a bead width that is 8.5" wide. That is what matters for mounting tires. Avoiding contact with springs and suspension components is a separate issue. Perhaps the bead width and actual width should both be specified.

Cool diagram. All of the dimensions are very clear on there except PCD. PCD might be a little confusing for patterns with odd numbers of bolts. Measuring between two bolt centers won't give you the right dimensions on a 5-bolt wheel. I realize it is just an illustration, but the potential for confusion is there. That's a minor nit, though. The diagram is great -- much more clear than other ones I have seen.

-Max

ARD T2

iTrader: (22)

Max,

I was going to detail this out on our website and should be up in the next couple weeks. When converting backspacing to printed wheel widths and offsets it can be tricky.

I figured our discrepancy out Max. I used a caliper to measure the width and it was inset a bit in comparison to your measurement of the outer rim lip edge to the edge of the inner rim lip edge.

The diagram is cool and the one most commonly seen. However they do not detail the two points on the wheel from which they measure offset and what the actual wheel width is.

What max is saying is one may have 10" of total spacing and one would assume you can purchase a 10" wheel, but that is not true because the wheels overall width will actually be a good inch larger edge to edge. So this must be taken into consideration and subtracted from the front and backspacing to arrive at what wheel size and offset you should buy.

I'll will post the formula tommorrow for how I convert total "real" spacing to "printed" wheel widths and offsets. I'm trying to use lamen's terms as much as possible.

Gotta hang with the woman now. Not supposed to be on the computer.

Rishie