03-24-2004, 01:36 PM
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CC Member
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Join Date: Sep 2002
Location: Hickory,
NC
Cobra Make, Engine: ERA 427SC w/427so, ERA GT #2002
Posts: 1,106
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Not Ranked
A tape measure and 2x4 works about as good as anything. All you need to know is wheel width, make the measurement and do the math. Question is do you want backspacing or wheel offset?
Backspacing is measured with the wheel on the floor facing up. Measure from the floor up to the mounting surface of the wheel. More backspacing decreases tread and less increases tread.
Offset is the distance the center of the rim is in or out from the spider mounting surface. So, if the wheel is centered on the wheel, offset is zero. If the rim is move OUT one inch (tread increases double offset change, or two inches), offset is minus one inch according to SAE standards.
To measure offset, lay the wheel on its back with or without the tire mounted, measure from the floor to the mounting surface and record this #. Turn over the wheel and make the same measurement. If they are the same, offset is zero. If they aren't the same, add the two numbers and divide by two. This will give you the center measurement of the wheel. Subtract either of your previous measurements and you've got the offset. If the inside measurement is smaller than the outside, offset is negative and vice versa.
All it takes is a little math. It's always helpful to make a little sketch on which to record your numbers and keep track of where you are in the process.
Simple . . . right. 
__________________
Tom
"If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough HORSEPOWER." Mark Donohue
Last edited by speed220mph; 03-25-2004 at 03:48 AM..
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