offset measurement
Safety Car
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From: St Marys GA
Offset is really hard to measure directly, but it's very easy to measure backspacing, from which you can calculate offset. Measure the distance from the inside of the lip of the rim to where the wheel mounts to the hub. This is your backspacing, 7.5" for example. To get offset, you subtract half of the width of the wheel from this number, 5.5" for example (for 11" wheels). This is your offset in inches. Just convert to mm, which is what offset is normally measured in, and you're done. 25.6 mm per inch, IIRC.
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Thread Starter
Race Director
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From: Lake Wylie SC
I found that an 11" wheel with a 50 mm offset doesn't work real well with my 92 vert, not enough clearance between the inside of the rim and the bump stop and brake hose, if you put a tire on it the clearance would be rubbing in turns. The ZR1 (OEM for 92) rim with an offset of 36mm works. Thanks guys.
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Thanks
I found that an 11" wheel with a 50 mm offset doesn't work real well with my 92 vert, not enough clearance between the inside of the rim and the bump stop and brake hose, if you put a tire on it the clearance would be rubbing in turns. The ZR1 (OEM for 92) rim with an offset of 36mm works. Thanks guys.
I found that an 11" wheel with a 50 mm offset doesn't work real well with my 92 vert, not enough clearance between the inside of the rim and the bump stop and brake hose, if you put a tire on it the clearance would be rubbing in turns. The ZR1 (OEM for 92) rim with an offset of 36mm works. Thanks guys.
is that because yours came with thinner wheels in the front on yours, so the bump stops are different?
someday I want 315's on all corners
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St. Jude Donor '09-'10-'11
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I thought the 36's were for zr1 or early c4's
I love your wheels, I can't wait to get some
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Offset is really hard to measure directly, but it's very easy to measure backspacing, from which you can calculate offset. Measure the distance from the inside of the lip of the rim to where the wheel mounts to the hub. This is your backspacing, 7.5" for example. To get offset, you subtract half of the width of the wheel from this number, 5.5" for example (for 11" wheels). This is your offset in inches. Just convert to mm, which is what offset is normally measured in, and you're done. 25.6 mm per inch, IIRC.
When converting "back spacing" to offset you need to use "total wheel width" NOT advertised. It's generally accepted to use the "advertised" width + 1". The formula is sometimes related as:
[back spacing - (width +1) / (2) X 25.4 = offset]
example: 11" wheel 50 mm offset is very close to an 8" back spacing depending on "brand". Most of my 11" wheels are 11 7/8 - 11 15/16 in total width and the back spacing is 7 7/8 +.
Here's a simple calculation using "even" numbers reversing that:
8 - (12/2) X 25.4 = 50.8 mm [8 - 6 (2) X 25.4 = 50.8]
Substituting the actual measured width of the wheel your result would be much closer to the 50mm "advertised"!
Back spacing is most easily measured by placing a straight edge across the "back side" edge of a wheel with NO TIRE mounted and measuring from the mating surface of the hub to the straight edge.
Last edited by WVZR-1; Jul 16, 2009 at 09:49 PM.
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St. Jude Donor '06-'08-'10
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Play here: http://www.rims-n-tires.com/rt_specs.jsp
There's a big plus to using "your tools"! You can measure a "tire/wheel" combination that's mounted lying on the ground and confirm for yourself using simple math the fitment of that tire/wheel to your car if you've established the fit of a tire/wheel combination for starters! Some calculators are interesting and the one you refered to seems quite reliable to use for comparisons. Before reaching in "your pocket" it's good to "understand" a tire/wheel combination! It's NOT all in the wheel!!!!
Last edited by WVZR-1; Jul 16, 2009 at 10:00 PM.
Team Owner
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St. Jude Donor '06-'08-'10
I like the site I listed because it makes it very easy to compare the existing wheel / tire setup and a new combination that's being considered. Makes it easy to visualize the differences.
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From: Overland Park Kansas
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FYI: Anyone doing calculations to figure offset should realize you can have positive or negative offset. That is determined whether backspacing is greater or less than the wheel centerline.
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Yes you certainly gain clearance on the inside of the wheel/tire to suspension/chassis but it seems you certainly pay a price with the wheel/tire that's 14mm extended "outboard" in regards to exterior body panels.
