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Old 09-21-2004, 03:12 PM
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Default wheel offset vs. backspacing???

OK I'm really confused here. What is the difference between offset and backspacing of wheels. I'm thinking stock sammy wheel is 3.5 inch backspacing which would be 88.9 mm offset? Are these wheel companies just converting the inch to mm or is it a totally different way of measuring backspacing. Most rims I see are 42mm offset. That would mean that they are 1.6 inch backspaced and that number seems much to low to me. can someone clarify?
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Old 09-21-2004, 04:01 PM
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Default Re: wheel offset vs. backspacing???

zuku26:

I learned a bit while researching your question.

The relative position of the inside face of the center of a wheel to the rest of the wheel is measured two ways.

The offset measurement refers to the distance away -- either outside or inside measurement from the center plane of a wheel. This offset is usually a small number and may be negative or positive depending on the direction of the offset -- towards the outside face or inside face of the wheel.

The backspace measurement is the distance from the inside face of the wheels' center to the edge of the rim (or at the bead) on the inside of the wheel.

An excellent picture and text description can be found at:
http://www.tirerack.com/wheels/tech/offset.htm

Hope this helps!
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Old 09-21-2004, 10:54 PM
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Default Re: wheel offset vs. backspacing???

Thank you for the info. That is a wealth of info. The problem still at hand is trying to figure out which offset I need to run a 7 inch wide wheel and keep the original backspacing. Or which offset with keeping the original backspacing with a 1 inch spacer/adapter. The info from the site requires you to have the rim in hand to do measurements.
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Old 09-22-2004, 09:09 AM
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Default Re: wheel offset vs. backspacing???

My head is starting to hurt from trying to figure out what you are trying to accomplish. When you refer to stock backspacing (which is 3.75" on a samurai), you can get a 7" wide rim with 3.75 backspacing and have stock backspacing. The offset is irrelevant.

Are you actually asking how to get the out egde of the wheel to end up in the stock location while using 7" rims?

Give or take the thickness of the metal the rim is made from, isn't the backspacing just half of the rim width +/- the offset? if a stock sami wheel is 5.5" wide and has 3.75" BS then it must have 1" offset

3.75-(5.5/2)=1

For a 7" wheel to have 3.75 BS the offset should be .25"

3.75-(7/2)=.25

Does any of that help? [img]images/graemlins/confused.gif[/img]

~daxe
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Old 09-22-2004, 04:23 PM
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Default Re: wheel offset vs. backspacing???

Ok Daxe, Here is what I'm trying to accomplish. I'm trying to fit a particular wheel on my 2wd sammy that only comes with a 5 on 4.5 inch bolt pattern. The wheel is 17x7. The bolt pattern adaptor that will be ordered is also 1 inch wide. All the companies that I have tried to order from said I need to know the offset and that they can not figure it out if I tell them the backspacing. Thank you for the correction on the 3.75 BS. I was having a brain Fart. So I need to figure out what offset is needed to fit a 7 inch wide wheel with a 1 inch adaptor and not stick out as much as possible and not rub the springs on turning. I could deal with the wheel sticking out and 1 inch to 1.5 inch's. So basically what does 3.75 inchs backspaced equal in offset. I hope this all makes sense.
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Old 09-22-2004, 06:41 PM
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Default Re: wheel offset vs. backspacing???

3.75" backspacing on a 7" wheel has to equal .25" offset, right? If the backspacing were' 3.5" then the wheel center would be in the center of the rim, yes? So if you move the center of the wheel outwards .25 inches you will have 3.75" backspacing. If you bolt this wheel to a 1" spacer on the hub you could say that you then had 2.75" backspacing on the wheel (if you consider the spacer to be part of the wheel) in in that process it would have changed the offset from +.25" to -.75" (or the other way around, I dunno which is pos or neg offset)

Does that help? I'm not an expert on this, in fact I never even thought about it before, but it's just simple math once you wrap your head around the parts involved.

On the other hand, if they need to know what the offset is to give you the spacers, just make something up. A one inch spacer is a one inch spacer.

Another way to look at it is this: to get the front surface of a 7" rim to be aligned with the front surface of a 5.5 on the same truck, you need to compensate by .75" more backspacing. If you then add in the 1" spacer, you would need to compensate for that as well by adding a total of 1.75 more backspacing or 5.5" backspacing. I dont think 7" rims come in 5.5" BS, which would have to be 2" offset.

someone check my math please? I know there's engineering types in here and I didnt even go to college.

~daxe
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