Backspacing vs. Offset Explained

Discussion in 'Exterior' started by Mattman24, Sep 15, 2011.

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  1. Mattman24

    Mattman24 MAG Autoworks

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    Backspacing vs. Offset Explained(thought this might help some of us)

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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 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.
     
  2. TSINND

    TSINND GMFS Member

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    Nice job, very helpful
     
  3. Cuban.

    Cuban. 2wd-RazorbackChevy

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    what would make this perfect is if there were pics for those who are visual learners
     
  4. Mattman24

    Mattman24 MAG Autoworks

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    can u see the ones up top?
     
  5. GCncsuHD

    GCncsuHD ΜΟΛΩΝ ΛΑΒΕ

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    Oh, is it Thursday already? Guess it was about time for this to come up again. :crazy:
     
  6. CUB

    CUB GMFS Senior Member

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    Most confusing subject ever.
     
  7. bakillawhale

    bakillawhale GMFS Member

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    Possibly sticky worthy
     
  8. Cwinne

    Cwinne GMFS Member

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    So much confusion over this topic to be so easily explained with pictures.

    I vote to stickie
     
  9. tripn88

    tripn88 Incognito

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    :word: :imo
     
  10. GCncsuHD

    GCncsuHD ΜΟΛΩΝ ΛΑΒΕ

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    Lol it's pretty simple to calculate, the issue that most people never understand when trying to calculate the offset from the backspacing or vice versa is that the bead area is not a 0 thickness obviously, and this thickness changes from wheel to wheel, so I usually assume it to be 1/2" including the thickness of the bead/lip and the lip for the wheel weights, although it is generally thinner on steel wheels and forged aluminum wheels, and can be thicker on cast aluminum wheels.

    For example my 17x10 Boyd Coddingtons were exactly 10" between the beads but 10.75" overall, but my 20x10 MB Motoring wheels, also 10" between the beads were about 11.25" overall. This is why you may see a 10" wide wheel with 4.5" backspacing, the offset could be anywhere from -22-30mm and still have the same backspacing depending on that thickness.
     
  11. LaydSierra

    LaydSierra Pebble Pushers

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    [​IMG]
     
  12. CUB

    CUB GMFS Senior Member

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    Yea, srs..I can't grasp this concept. Maybe cuz I suck at math. I'm more artistically inclined.
     
  13. Kindredmsg

    Kindredmsg Resident Metal Head

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    Make it easier
    Look at a rim sideways:

    Backspace = distance to mounting face from the back of the rim
    Offset = distance to mounting face from the centerline of the rim.
     
  14. SICKS.OH

    SICKS.OH GMFS Member

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    nice,

    i think the confusion is from people thinking they are the same thing. when really its two different way of measuring.
     
  15. kelley350x

    kelley350x GMFS Member

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    thanks for the sticky
     
  16. 04 Silvy

    04 Silvy Chevrolet Member

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    so a wheel that is 9 inches wide with a +25mm offset would stick out 1.25" farther than stock right?
     
  17. jdog1992

    jdog1992 GMFS Member

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    I have a new body style Sierra with a 2 inch leveling kit and I have already done my cutting but I ordered 20x9 fuel krank wheels with a +13 offset wrapper with nitto trails and I am just worried that the offset will basically be just like stock wheels. If anyone could help me out on this I would appreciate it and tell me which offset you would go with! Thanks.
     
  18. KKing

    KKing wants a crew cab

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    Ok, since you obviously couldn't search for your answer, I'll answer with questions so you can work through this and learn. What offset are your stock wheels? Take the offset of the stock wheels, then the offset of your new wheels and find the difference. So your math in this case would be
    (factory wheel offset - new wheel offset)= Difference in offset.
    Let's say, for example sake, you get the number 19. So your new wheels will stick out 19mm's more than stock. Hint...it will be a positive number, not always, but I don't want to confuse you.

    Now, an easier approach for you specifically
    Is 13mm ( the offset of your new wheels) a smaller number than your factory wheels offset? Then they stick out more. Is it a bigger number than your factory wheel offset? Then the sit inward more. Don't expect a lot of answers to be handed out around here, especially when your answer is actually in the very first post if you'd study it.


    Disclaimer: For going from negative offset wheels to negative offset wheels it would be just the opposite from my last statement, and going from a positive offset wheel to a negative offset wheel would need to be calculated using the first paragraph. Just trying to learn him specifically

    Sent from my iPad using Tapatalk HD
     
  19. BuckmarkTN

    BuckmarkTN New Member

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    Thanks for posting this, it's a big help for the folks that have trouble understanding it from the other examples and definitions.
     
  20. kp1500z71

    kp1500z71 2KTallChevy

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    Tried to to do the math, and need help. Looking at some older style helo wheels that are 20 x 8.5 with a 18mm offset. Can someone tell me what the backspace would be on these wheels for I am hoping possibly they will work for I am planning on putting these on a 08 Sierra CC 1500. Thanks Kevin
     

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