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Old 10-19-2010, 07:26 AM   #1
Sir Nuke
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OFFSET and BACKSPACING EXPLAINED

I have seen this question ask many times recently, WHAT IS OFFSET? or BACKSPACING? or the terms used incorrectly, for they are not the same thing....SO with that in mind, here is an explanation of these two terms.

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Wheel Offset

Offset is measured from the centerline of the barrel of the wheel to the hub mounting surface. If that hub mounting surface were in the exact center of the barrel of the wheel the offset would be “0". If the mounting surface is away from the center of the wheel the offset is POSITIVE, if the mounting surface is towards the center, the offset is NEGITIVE. Offset is measured in millimeters. There are 25.4 millimeters to an inch. So if the mounting surface is 12mm from the centerline of the wheel it has a 12mm offset. That would also mean that it is about ½ inch. If the mounting surface is moved away from the vehicle that is a positive offset. The picture shows a wheel with the positive offset. A positive offset will cause the wheel to set in or tuck into the vehicle. Originally you found positive offsets on just front wheel vehicles. Due to that some people refer to positive offset wheels as front wheel drive offsets. Currently there are many rear wheel drive cars and trucks with positive offsets. The higher the positive offset the less it sticks out from the vehicle and it will have a higher Backspace. If the mounting surface is moved in toward the vehicle past the centerline, that is a negative offset. A high negative offset will produce a lower Backspace. Once again the measurement in millimeters is how far away from the centerline the mounting surface is. A -24mm offset means that the mounting surface is located 24mm or 1 inch from the centerline toward the vehicle. The wheel will come out farther from the vehicle and will have that deep dish look.

Width

The width of a wheel is measured inside the beads which are usually ½ inches. If you measure on the outside of the beads an 8 inch wheel will measure 9 inches

Backspace

Back spacing is measured from the inner edge of the wheel to the hub mounting surface. It is a convenient measurement in that, as long as the back spacing remains the same, the clearance to the suspension also remains the same. If you know the width of the wheel and the offset you can compute the backspacing. For example if you have an 8 inch wheel with a +24mm offset. An 8 inch wheel is actually 9 inches wide so if the offset were 0 the mounting surface would be right on the centerline and the backspacing would be 4.5 inches. With a positive offset the mounting surface moves off center 24mm or 1 inch toward the outer edge of the wheel. This will make the inner edge go more inside which will result in a 5.5 inch backspacing.

Centerbore

This is the diameter of the center of the wheel. The centerbore must be equal to or larger than the hub of the vehicle for the wheel to seat. If the wheels centerbore is the same as the hub on the wheel then that wheel is hub-centric and uses the hub to center the wheels. If the centerbore is larger than the hub then the lugs will center the wheel and that is lug-centric. When using a lug-centric wheel be sure and snug the lugs up slowly and tighten them in an opposite or star pattern.

Bolt pattern

A bolt pattern of 5x120 mm means it has 5 lugs and if there were a circle through the center of all the lugs the diameter of that circle would be 120 mm.
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Last edited by Sir Nuke; 10-19-2010 at 09:46 AM.
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Old 10-19-2010, 09:39 PM   #2
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It can't get any clear than this , very simple and clean explanation !!!
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Old 10-19-2010, 11:15 PM   #3
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That was what I was going for......glad you liked it.
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Old 10-20-2010, 10:04 AM   #4
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Let me see if I’m using this terminology correctly.

"The reason we don’t see really deep concave wheels on the new Camaros is because there is too much positive offset."

??
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Old 10-20-2010, 10:23 AM   #5
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actually no, its because the Brembo brake calipers are so big, diameter wise, that they rub on the wheel.
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Old 10-20-2010, 10:23 AM   #6
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Very well written and explained.
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Old 10-20-2010, 12:55 PM   #7
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Thanks

Thanks Sir Nuke - I learned the hardway - testing Cragars - I did all I know to do to get them to fit but the 1LS has 6"+ of backspacing needed to clear the caliper - Cragar is only 4.5". Thanks again for the lesson. JFR1
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Old 10-29-2010, 04:05 AM   #8
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Now all I need to know is what width and backspacing 18" wheel will fit on my RS SS so I can mount stickies with a sidewall.
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Old 10-29-2010, 06:14 AM   #9
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the best thing you can do, is when you see or find a wheel that LOOKS like what you want, ask the people that actually sell them, and if they don't kniow what fits what, they don't know their business and I would to elsewhere.
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Old 11-04-2010, 12:31 PM   #10
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So a negative offset gives you a deeper dish look, with more lip so to speak?
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Old 11-04-2010, 06:01 PM   #11
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yes, you would most likely have more LIP, so to speek
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Old 11-10-2010, 01:33 PM   #12
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you deserve an award for that explaination
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Old 11-12-2010, 10:53 AM   #13
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Great Explanation, thank you!
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Old 11-26-2010, 01:09 AM   #14
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Extremely helpful, thanks Nuke!
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