Backspace and offset dont seem to make sense, US rambler mags

Tires and Wheels (Mopar Hubcaps Too)

  1. purplehazenils

    purplehazenils Well-Known Member

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    Hi,
    I have been looking at the US rambler mags wheels for my car but I cant figure out if they will fit or not. Link to the wheels:

    Rambler - U111 - US MAGS

    It says that they are ex. 8in wide with 4.5in backspace and an offset of 1mm. How does this work? Should the offset not be +1/2in or +12.7mm?

    From my understanding offset is a measure from the wheels centre line to the mounting surface and backspace is a measure from the inside lip of the wheel to the mounting surface.

    If an 8in wheel has a backspace of 4.5in this puts the mounting surface 1/2in from the centre line of the wheel, meaning the offset has to be +1/2in or 12.7mm not 1mm. Where am I going wrong here?
     
  2. oldkimmer

    oldkimmer FABO Gold Member FABO Gold Member

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    The mounting surface is so wide also. I think w that's what's ur missing
     
  3. 72bluNblu

    72bluNblu FABO Gold Member FABO Gold Member

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    Backspace is measured from the outside lip of the rim to the wheel mounting surface, so it includes the 1/2" of extra width outside of the tire mounting surface. An 8" rim is actually 9" wide outside lip to outside lip, it's 8" from tire mounting surface to tire mounting surface. So an 8" wide rim with a 0 or 1mm offset has a 4.5" backspace, an 8" wide rim with a +13mm offset would have a 5" backspace.

    offset.gif
     
    • Thanks! Thanks! x 2
    • famous bob

      famous bob mopar misfit

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      both of the above is true, but you order 8" rims from jegs or summit or anyone else, and they measure from the mounting surface to the inside of the wheel where the bead seats. at least that's the way I`ve bought them for 40 some thing yrs.
       
    • 72bluNblu

      72bluNblu FABO Gold Member FABO Gold Member

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      Well, I hate to be the bearer of bad news but unless they've changed how backspace was measured in the last 10 years or so, you've been buying them wrong for 40 something years.

      Backspace is measured from the outside lip to the wheel mounting surface. I've bought rims from Summit, I've bought rims from Enkei, I've bought rims from Aspec on eBay. The advertised backspace is measured from the outside lip, just like how you measure it in your garage. I'm quite certain of this, because none of the rims I just mentioned would have actually fit on my car if they'd been off by a 1/2" on the backspace, I don't have a 1/2" of clearance to give.

      Proof you say? Here's the rims I bought from Summit, Legend 5's. These are 17x9's with 5" of backspace (0 offset) from my Challenger.

      The measurement- You can see the straight edge resting on the outside lip at the top left, and the ruler showing 5" of backspace (well, a smidge over, but the camera angle isn't straight out from the ruler, had to be slightly higher to show the outer edge, the result is a tiny fraction more than 5" in showing in the picture)
      IMG_3645.jpg
      The rim, really is a Legend 5, and you can see the tire's a 275/40/17 so it had better be at least 17x9
      IMG_3646.jpg
      And the specs cast into the rim, yup, 17x9.
      IMG_3647.jpg

      And the ad right from Summit, link here https://www.summitracing.com/parts/vsw-141h7965gm0/overview/
      Screen Shot 2016-10-03 at 9.23.33 PM.png
       
    • famous bob

      famous bob mopar misfit

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      ---funny, they have always fit. outside measurements are for people that are not sure of their clearance and need to be tutored.--jmo u understand.
       
    • purplehazenils

      purplehazenils Well-Known Member

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      Wow, feels like my whole world of wheel understanding has just been turned upside-down.
      I just assumed 8in wheels were, well, 8in in total width. This clears up alot and is good news for the fitment of most wheels, thanks alot!
       
    • 72bluNblu

      72bluNblu FABO Gold Member FABO Gold Member

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      Just remember that if you're factually wrong, it's not an opinion anymore. You're just wrong. Opinions are for things that can't be supported directly with facts. Backspace is measured from the outside lip of the rim to the wheel mounting surface. That's a fact, that IS the definition.

      The fact that your wheels have fit in the past is pretty much just dumb luck, and has more to do with the size of the wheel combination you used and the extra space you obviously had than your understanding of backspace, because you were wrong.

      No problem! Glad I could help! :D
       
    • famous bob

      famous bob mopar misfit

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      well, opinions are like a--holes-everybodies got one , some have 2
       
    • cosgig

      cosgig MoBro Inc. FABO Gold Member

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      Well Bob, I also beg to differ! The backspacing of a rim has nothing to do with the inside bead lip where the tire seats, it is always measured exactly as shown above, from the rim edge to the mounting surface! Why would the inside of the bead lip have anything to do with the backspacing of the rim?? They don't take into consideration the sidewall bulge of the tire either, with has a bigger effect on the clearance issue of backspacing than the bead lip could!! That's because the rim manufacturers are giving you the actual clearance numbers of the rim, and that has nothing to do with the tire!
       
    • Bulldozer

      Bulldozer Well-Known Member

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      Back space is ALWAYS measured from the mounting surface to the outside lip. What I can't stand is this millimeter offset bullshit way of measuring it. Just give me the damn measurements in inches. and yes a 10 inch wide wheel is actually 11 inches total width.
       
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      • famous bob

        famous bob mopar misfit

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        yea the out side always is more , understandably. but if u call jegs or summit and tell them u need 8" wheels, guess where their going to give u measurements from . I`ve ordered 14" cragars, 10" tellstars, 2 sets of 8" prostars-w/ diff offsets and all of them have come w/ the 14-10-8" measurement inside wheel lip to inside wheel lip. local sellers talk about outside measurements around here too. think that's for the street crowd that don`t have , or know how to make room or measure stuff. jmo
         
      • 72bluNblu

        72bluNblu FABO Gold Member FABO Gold Member

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        Uh, you're talking about something totally different. Rim width is always measured inside to inside, where the tire seats. A 15x8" rim is actually 9" wide measured outside to outside. That has nothing to do with measuring backspace.

        Backspace is measured from the outside lip to the wheel mounting surface. That's the definition. Anyone that does it differently than that is wrong, not of a different opinion. Just wrong. And you can see from the specs I listed above and the measurements I showed that Summit does not measure backspace from the inside lip to the wheel mount surface. If they did, my measurements wouldn't match their specs. But my measurements match their advertised specs exactly.

        It's all right here in this picture, which is not mine, by the way, I didn't make this up. And yes I know I already posted it, but apparently no one actually looked at it, so here it is again. A 7" wide rim is shown. It has a 5" backspace, a 3" front space, a 25.4mm offset, and an overall width of 8". If you measure a rim some other way than this, it's wrong. These are the definitions, they are the facts. If your opinion does not match the diagram, then your opinion is incorrect.
        offset-gif.1714976626.gif
         
        Last edited: Nov 2, 2016
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        • famous bob

          famous bob mopar misfit

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          I may have miss understood. think we`re on the same page here! I don`t care for the mm type measurements either.
           
        • toolmanmike

          toolmanmike FABO Staff Staff Member FABO Gold Member

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          Backspace is the only measurement you need when describing wheels in a catalog.
           
        • 72bluNblu

          72bluNblu FABO Gold Member FABO Gold Member

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          Unless they're only listed in offset. :D

          It's just what you're used to. Offset is nice because it doesn't change with respect to the width and the centerline of the rim. Ie, if you have a 17x8 and a 17x9 that both have a 0mm offset, the centerline of the rim is in the same place when it's mounted on the car, the extra width is split equally between the inside and outside of the wheel (but the backspace has changed from 4.5" to 5"). If you have a 17x8 with a 4.5" backspace and a 17x9" rim with a 4.5" backspace, the location of the centerline of the rim has changed and the wheel will not be centered in the wheel well anymore. All the extra rim and tire went to the outside, so the clearance to the fender changed by an inch while the clearance to the suspension remained the same.

          Both backspace and offset are important to know. If you change the width of the rim and keep the same offset the wheel will still be centered, but you need to make sure you still have enough clearance on both sides because the extra width is added to both sides. If you change the width of the rim and keep the backspace the same you know it will clear the suspension, but all of the extra width goes to the outside so the fender clearance has changed dramatically.
           
        • famous bob

          famous bob mopar misfit

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          disagree. an 8" wheel w/ 3.5" offset will not be centered like a 7" wheel w/ a 3.5" offset. go figure !
           
        • 72bluNblu

          72bluNblu FABO Gold Member FABO Gold Member

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          Again, I think you misunderstand. Mostly because you're using inches to refer to offset, which is typically listed in mm.

          Yes, the centerline of a 8" wheel with a -25mm offset is in exactly the same place as a 7" rim with a -25mm offset. I used the proper terminology, a -25mm offset on an 8" wide rim is equivalent to a 3.5" backspace. However, a 7" rim with a -25mm offset does not have a 3.5" backspace anymore, it has a 3" backspace. That's why it's important to use the correct terminology. Backspace and offset are not the same. Keeping the offset the same keeps the centerline of the wheel in the same place regardless of width, keeping the backspace the same does not keep the centerline of the rim in the same place if you change the width of the rim.

          But don't take my word for it, here's the math. Same offset as your example, -25mm or 3.5" for an 8" rim. Same tires. Notice the clearance. The centerline of the wheel hasn't moved, so all the clearances are the same. The backspace however has changed for the 7" rim, even though the offset is the same.
          Screen Shot 2016-11-03 at 11.12.06 AM.png

          And now, the same exact combination, but with the backspace held constant at 3.5" for both. Notice the offset is now different for the 7" rim, and the centerline of the wheel has moved (the wheel is now closer to the suspension for the 7" rim).
          Screen Shot 2016-11-03 at 11.17.00 AM.png
           
        • famous bob

          famous bob mopar misfit

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          this is getting stupid. the diagram shows what I am saying. end of conversation for me.
           
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