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Wheel Backspacing and Offset Info

Discussion in 'Wheels & Tires' started by tcBob, Jul 17, 2010.

  1. Jul 17, 2010 at 1:37 PM
    #1
    tcBob

    tcBob [OP] Gringo Bandito Thor

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    Alloy Wheel Cross-Section:
    [​IMG]

    Before mounting a set of aftermarket wheels, there are some dynamics involved with fitment that are best to understand first.

    1. Wheel Diameter. This is the diameter of the wheel. Stock wheels are commonly 15 or 16 inch diameter. This dimension is usually in 1 increments (i.e. 15, 16, 17) but some manufacturers offer 16.5, which is rare.

    2. Wheel Width. This is the width of the wheel, measured inside the outer lip of the wheel. This dimension is usually in 1/2 increments (i.e. 7.5, 8)

    3. Wheel Center. This is the center of the wheel in relation to the width.

    4. Offset. The distance from the centerline of the wheel to the face of the mounting surface of the wheel that contacts the hub.

    4.1 Zero Offset. Indicates the mounting surface is at the wheel center.

    4.2. Negative Offset. Indicates the mounting surface is behind (or inboard) the centerline of the rim. This is often found on standard rear-wheel-drive vehicles and on so-called reversed rims. (Diagram below shows negative offset)

    4.3 Positive Offset. Indicates the mounting surface is in front of (or outboard) the centerline of the rim. This is often found on front-wheel-drive vehicles. Stock Tacoma wheels have Positive offset.

    5. Backspacing. The distance from the mounting surface to the inside lip of the wheel. This measurement is closely related to offset (without actually measuring, backspacing is about equal to [Wheel Width / 2] + [Offset] + [about 1/4] ).

    6. Centerbore. The centerbore of a wheel is the size of the machined hole on the back of the wheel that centers the wheel properly on the hub of the car. This hole is machined to exactly match the hub so the wheels are precisely positioned, minimizing the chance of a vibration. This measurement may be critical to clear the hubs of a 4WD.

    7. Bolt Circle. Also known as PCD (Pattern Circle Diameter). The bolt circle represents the diameter of an imaginary circle that goes through the center of the bolt holes. 4x2 Tacoma Bolt Circle: 5 lug on 4.5 PCD; 4x4/Prerunner Tacoma Bolt Circle: 6 lug on 5.5 PCD.
     
  2. Jul 17, 2010 at 1:47 PM
    #2
    Cape Codder

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  3. Jul 17, 2010 at 8:31 PM
    #3
    supremetaco2

    supremetaco2 Well-Known Member

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    +hp, lifted, lights, dif breather mod, debadged, shorty antenna...
    what exactly is the purpose of backspacing? what does it determine? how you mount the tire on the wheel or something like that?
     
  4. Jul 17, 2010 at 8:44 PM
    #4
    pittim

    pittim Play stupid games, win stupid prizes

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    Pink slider.
    backspacing is the distance from where the wheel mounts to the surface of the brake assembly to the outside of the wheel.

    suppose the width of your front axle from brake to brake is 60 inches.

    now, you have two different sets of wheels. one is a 16x8 with 5" bs, the other is a 16x8 with 3" bs.

    when mounting the 5" bs wheels, the distance between the outside of the wheels will be 66". when mounting the 3" bs wheels, the distance between the outside of those wheels will be 70".
     
  5. Jul 19, 2010 at 11:52 AM
    #5
    Ace83

    Ace83 Well-Known Member

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    i used to dig this info on the net before.. nice to have it here
     
  6. Jul 19, 2010 at 11:56 AM
    #6
    Loudpedal

    Loudpedal Mind = Blown

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    Great sticky, bob. Interested to see if it will cut down on the number of "will this fit?" threads.
     
  7. Aug 1, 2010 at 7:08 PM
    #7
    DethTaco

    DethTaco Active Member

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    Ok. /deep breathe
    The scrub radius may/may not change with a 4" backspaced wheel. Right? Depends on how the wheel is made?
    But it will definately change by adding spacers.
    The larger scrub radius will put more stress on steering bits and wear bearings faster.

    So how much is too much and how do I figure out my scrub radius with the wheel/tire combo and spacers?
    Not too certain that I understand the equation in the original post. I have tried... Trust me.
    You said: Mounting surface to mounting surface on the axle is 60".
    16x8 wheel with 5" offset.
    Wheel width= W
    Offset= O
    W / 2 = 4 + O = 9

    Why is the measurement from the outside of each wheel now 66" ? Wouldn't it be 69"?

    I gotta nail this part down before adding in the spacers...
    And how much is too much scrub radius?
     
  8. Aug 1, 2010 at 7:42 PM
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    pittim

    pittim Play stupid games, win stupid prizes

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    Pink slider.
    a 5" offset is HUGE. Look at the diagram again. Offset is from the centerline of the wheel to the mounting surface. Backspace is from the inside edge to the mounting surface. Backspace is typically measured in inches, offset typical in millimeters.

    backspacing is about equal to [Wheel Width / 2] + [Offset] + [about 1/4]

    So,

    BS = (WW/2) + (O) + .25

    For a 8" wide wheel with 5" BS

    5 = 4 + O + .25

    O = .75", or +19mm

    So, the mounting surface is ~.75" in front of the centerline of this particular example.


    If I could draw something up in CAD on my laptop everything would be easier
     
  9. Aug 2, 2010 at 2:17 AM
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    DethTaco

    DethTaco Active Member

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    Ok I think I got it now.
    So what is too much scrub radius?
    I'm looking at a Fabtech 6" IFS suspension lift which calls for a 4" backspace wheel.
    My plan was to use spacers on the front to get some turning radius back once all is installed.
    From what I've been reading, a large scrub radius is correctable by adjusting caster/camber?
    I'm really trying hard to do this lift right. Turning out to be allot for a first time lift.
     
  10. Aug 3, 2010 at 3:33 AM
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    DethTaco

    DethTaco Active Member

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    Bumpt.
     
  11. Aug 3, 2010 at 5:09 AM
    #11
    98tacoma27

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  12. Sep 14, 2010 at 6:55 AM
    #12
    moelov

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    What amount of back spacing would I need on Stock Steelies with 285's 75?

    Thanks.
     
  13. Sep 14, 2010 at 6:57 AM
    #13
    98tacoma27

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  14. Sep 14, 2010 at 7:01 AM
    #14
    moelov

    moelov Well-Known Member

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    Ok, thanks. I though it was 1.5. But rather have the 1.25.

    Now I cant find a thread with some pics with this setup, any leads?

    Finally is it worth it or bite the bullet and get new wheels? Just have not found anything that I preferred.

    Thanks again.
     
  15. Sep 14, 2010 at 7:07 AM
    #15
    98tacoma27

    98tacoma27 :POOPCORN: Thor

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    Well, that's all personal preference. Figure you only need spacers for the front so theres $100. Or if you want to even the look out, $200 for all four corners. But, if you go the new wheel route, you can get 16 x 8 for $78 a piece from wheelers. (cheapest I know of)
     
  16. Oct 3, 2010 at 4:57 PM
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    mainland_newfie

    mainland_newfie Mainland Newfie

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    so we all know, I'm half stupid, how much offset is there on the 07's so I know for judging what kind of new rims I can throw in my truck
     
  17. Oct 16, 2010 at 8:10 PM
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    fsbrain03

    fsbrain03 Well-Known Member

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    Alright. How important is the offset? Someone told me that I need at least 4.5 backspace, and at least 12 as an offset.

    I am looking at wheels with 4.5 backspace, and 0 offset. How important is that offset?
     
  18. Oct 16, 2010 at 9:16 PM
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    pittim

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    [​IMG]
     
  19. Oct 16, 2010 at 9:21 PM
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    fsbrain03

    fsbrain03 Well-Known Member

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    ^yes. I see that it's 0. Is that gonna affect anything on my truck? Why would te guy tell me 4.5 backspace, and 12 offset? Is he just trying to sell some wheels?
     
  20. Oct 16, 2010 at 9:26 PM
    #20
    pittim

    pittim Play stupid games, win stupid prizes

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    You cant have 2 wheels that are the same width and backspacing, but have different offsets.

    Were the wheels you were looking at and the ones hes trying to sell same everything?
     
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