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post #1 of 7 Old 04-29-2013, 07:28 AM Thread Starter
cwadej
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dumb question ?

Maybe this is a dumb question, but so what.

I currently have Mickey Thompson Classic Lock wheels (that i want to get rid of) in 15x8. According to their website they have a -30mm offset with 3-5/16 backspace.

I dont understand how the offset and back space can equal just over 4 inches, but still have a negative offset.

I've looked at all the pictures and stuff online, but still dont understand.


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post #2 of 7 Old 04-29-2013, 09:32 AM
mdm
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You're confusing me. The backspace you state is 3-5/16 inch with a -30mm offset. Then you say the offset is just over 4". Which is it? A negative offset will have a backspace of less than 1/2 the wheel width.

Keep in mind that free advice can frequently be worth less than you pay for it.
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post #3 of 7 Old 04-29-2013, 10:40 AM Thread Starter
cwadej
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that's what I thought!

So the numbers dont add up i guess. Then why give both numbers?

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post #4 of 7 Old 04-29-2013, 11:16 AM Thread Starter
cwadej
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What i mean is, you can find back spacing by subtracting offset from half of the wheel width, and you can find offset by subtracting backspace from wheel width

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post #5 of 7 Old 04-29-2013, 01:13 PM
jp360cj
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Its because of how the measurements are taken. An 8" wide wheel is 8" from bead to bead, not from outer edge to outer edge. Most 8" wheels are right at 9" overall width. The backspacing is measured from the outer edge on the back of the wheel. So when you calculate backspacing from offset you have to 1/2 of the overall with, which is going to be about 4 1/2. Also, you have to add the offset to 1/2 the wheel width, not subtract.

-30mm / 25.4 = -1 3/16 (offset in inches)
4 1/2 + -1 3/16 = 3 5/16 backspacing

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post #6 of 7 Old 04-29-2013, 02:21 PM Thread Starter
cwadej
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adding a negative = subtracting the positive.

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post #7 of 7 Old 04-29-2013, 02:53 PM
jp360cj
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Quote:
Originally Posted by cwadej View Post
adding a negative = subtracting the positive.
True but as general formula its easier and more consistent to add the negative offset. Then the formula stays doesn't change based on whether the offset is positive or negative. But no, you weren't wrong in how you did it

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