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Thread: Understanding Rim Offest

  1. 13th May 2009, 09:28 PM #1
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    Understanding Rim Offest

    First of all please ignore my terminology if it is incorrect, I will try to word this so that it will make sense. I have done lots of research on rim offset and think I have it nutted out but want to check my understanding is correct.
    Please tell me if I am right in my understanding for each section for a 16x7J ROH alloy.


    I have found a post that states the standard rims are stamped with offset 33mm. Assuming this is a positive offset that should make the backspace 134mm.

    On my ROH alloys I have 20P stamped on the edge of the rim which should mean that the offset is 20mm from the centre line of the rim out towards the outside face of the rim. This should make the backspace 121mm.

    Assuming the above are both correct this would mean that my alloy will sit 13mm further out from the hub then a standard alloy or steel wheel.


    I hope what I have is correct as it has really been doing my head in the last couple of days.
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  2. 13th May 2009, 09:46 PM #2
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    Its almost correct.

    The difference is that offset is measured from the inside of the rim flange, where backspace is measured from the outside of the rim flange. An 8" rim for example will probably measure up at around 9" wide or slightly less on the outside, but the tyre is constrained within an 8" area. This is what will cause a discrepency between backspace measurements and offset measurements.

    So for example, an 8" rim with a 0mm offset will probably have a backspace of something around 4.5" or just under.
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    Slunnie


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  3. 13th May 2009, 10:15 PM #3
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    Here is a link to some useful data on wheels and tyres, including offset.

    Car Bibles : The Wheel and Tyre Bible Page 2 of 2

    Hope it helps.
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  4. 13th May 2009, 10:18 PM #4
    klappers Guest
    You need to measure from the inside of the J... But what you are saying is correct, your rim will stick 13mm further out, towards the guard.
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  5. 13th May 2009, 10:28 PM #5
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    Thanks Slunnie and gazk, I have actually looked at that webpage before and it got me to the point I am at now.

    If I am comparing offset with offset though, a 33P offset (can someone confirm wether or not this is the standard offset for a 7 inch rim) will sit 13mm closer to the hub then my 20P offset alloys.

    The main reason for all this is I want to fit 265/75/16 tyres and I want to make sure the offset I have will be alright with just a camel cut.

    Edit: beat me to it klappers
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