Looking at a set of KMC XD Monsters. 17x9 or 18x9
B/S is 5.70, but the O/S is -12mm, so the O/S will suck them in another 1/2" , which will leave them sticking out approx 1/2" over stock, Does this sound correct? Also considering H2s and I believe they stick out about 1" over stock. Does all this sound correct? Thanks

 

Use B/S or O/S but not both when comparing. Offset is from the center of the rim, Backspace is from the rear edge of rim.

 

Isn't the O/S added or subtracted from the B/S? So in this case 5.70 B/S then -12 O/S = another 1/2"? Thanks

 

No, offset tells you how far off the centre of the rim it is, backspace tells you how far from the rear of the rim it is.

They are essentially the same thing but from two different reference points. Two different ways of measuring. B/S is the old way it was done, O/S is the new way.

 

Ahhh,O.K. Thank you for that Sir. This leaves me in the same boat though. Not many manufacturers out there making anything that applies. H2s seem like a good bet.

 

I just bought new rims and my dealer explained it that the 12mm is the offset was from stock rim placement. Now that was for rims designed for my truck. 12mm was about 1/2" outward movement, and he was dead on. back spacing is indeed from the inside of the rim. Use one or the other is correct. but i always thought the OS was from center too. Still is confusing.

 

All depends what the stock rim O/S is? It is measured from the center, he may have meant it has an addition 12mm offset more than the stock rims.

 

Exactly, from the stock position. The problem comes when you look at the stock offset is 0, but it is not centered on the rim!

 

If stock offset is zero then it has to be the centre of the rim. I don't have a rim to check myself.

http://www.wheelsnext.com/content/wheel_tech.html

 

Stock is not zero. Stock wheels are 6.25 B/S,, or an O/S of 48mm, which is 1.88 ", which I know doesn't add up to 6.25, but that's getting to the ballpark.

 

Take a look at the chart here. It has a chart of BS vs Offset. I just put a set of 8.5" width, +33mm offset, 6" BS wheels on my truck. This is what they look like on the truck with 285-75x17 Toyo A/T tires.

If you don't want the wheels to stick out beyond the fenders, you need all the positive offset you can find.

Oh yes, if the 9" wheel you reference is really a 5.7" BS, then its a +12 Offset, not a -12 (check the chart). Be sure, there's an inch of difference between plus and minus 12 offset.

 

You guys are getting pretty wrapped up in numbers here....

I don't remember everything I had to learn to pick the right wheels, but this I do know.
Backspacing is the amount between rim mounting face and inside edge of rim.
Offset is the amount the mounting face is off from center line.
Stock wheels have 48mm of offset. That means the rim mounting face is 48mm OUT from the center of the rim. That is a POSITIVE OFFSET.
If you go with a negative offset, you are moving the mounting point INBOARD of the rim centerline. You will change the way the bearings run on the axle. Is that bad? I think so, but I don't know for sure.

So here's what I did. I went with a 1" wider rim. That is 25mm wider. The original offset was 48mm, and the new rim is 25mm wider, so to keep the backspacing kind of close, I subtracted the 25mm from the 48mm originally designed and chose an 18mm offset wheel. (the wheel I wanted came in 18mm and -12mm offset)

If you choose a -12mm offset, you will push your rims out the 48mm of the original wheel, plus the 12mm of the negative offset, plus half the additional width of the new rim. So an inch wider rim at -12mm offset will be 48 + 12 + 12.5mm = 72.5mm or about 3" out farther than stock.

Choosing an 18mm offset on a one inch wider wheel is 48mm - 18mm + 12.5mm = 42.5mm or about 1 3/4" farther out from stock.

Also keep in mind that you may put on wider tires, and if you put on a tire that is 2" (50mm) wider than stock, half of that is inboard and 25mm closer to your frame. My 18mm offset rims measure only about 17.5mm further from my frame (48mm original -18mm new = 30mm outward offset difference - 12.5mm wider rim = 17.5mm inside distance increase from frame). So my wider tires will run about 8mm closer to the frame.

I posted a great online calculator on here once, I'm just going off the top of my head now. It helps if you draw it out to see what you are moving.

Here's a pretty good link where you can enter your stock wheel info and then enter the new wheel and SEE what happens and read every dimension change....
http://www.rimsntires.com/specs.jsp

 

That implies that the stock 8" wheel has about a 6.5" BS.

 

Yeah, that number gets tossed around a lot, it may be 43mm, but we're close here.

I know if you go negative with bigger tires, when you turn you will run into the back of the fender well.

 

Thanks guys, good info. It all confirms that my tire guy steered me right. Good for peace of mind.