1970 dart reared question

Mopar General Discussions

  1. lamont 71duster

    lamont 71duster Well-Known Member

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    So I’ve been trying to see what is the biggest sized tire I could fit under the car and a lot of things I’ve read everyone says something about putting a B body rear end in it to fit wider tire under it.can anyone shed some light on this to help me and explain what u need to do to make it work
     
  2. Alaskan_TA

    Alaskan_TA Well-Known Member

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    Backspacing, research back spacing.

    Even if you move the leaf springs in, you still need to understand wheel back spacing.

    Your limitation is the stock wheel well & what will fit inside it, so regardless of differential type, knowing the wheel back spacing is a major factor for stock tubs.
     
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    • diymirage

      diymirage HP@idle > hondaHP@redline

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      as 72blunblu mentioned, the earlier B axles do fit but they are a a little wider then the stock A body 8 3/4

      moving the leaf springs in does relatively little too, so really, if you want to run a wide tire you're looking at mini tubbing it
      (of course, then you're still moving the leafs, but my point is, by ONLY moving the leafs you only gain an inch or so, and the next limiting factor is the factory wheel well, as ATA mentioned)
       
      Last edited: Mar 27, 2020
    • 72bluNblu

      72bluNblu FABO Gold Member FABO Gold Member

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      The amount of tire that will fit in the rear wheel tub isn’t really changed by the width of the rear axle as long as the backspace on the wheel centers the tire in the wheel well. The B-body rear axles are just cheaper than A body rears, they don’t let you run a wider tire. The wider rear axle does work better if you want to run a 17 or 18” wheel diameter, but that’s only because most of those wheels come with more backspace off the shelf. On the flip side more 15” rims require a custom backspace for a B-body rear, you’re more likely to fit an off the shelf 15” rim with an A-body 8 3/4.

      On a Duster, with the stock spring locations most cars will fit a 275mm wide tire without any rubbing as long as the backspace is correct. The stock wheel tubs are capable of taking a 285 if the springs aren’t in the way, and a 295 if you’re willing to work on the quarter lip a little. That’s possible with a 1/2” spring offset.

      If you want more than that, you need to do a full 3” spring relocation AND a mini-tub. The full spring relocation still won’t let you go wider than the 1/2” spring offset does because the stock tub is the limiting factor at that point already.

      So the real question is, what size tire do you want to run and what rear axle do you have now?

      All the B-body rears are wider, not narrower. And a 1/2” offset and a little quarter lip work allowing you to run 295’s is pretty significant. A leaf spring relocation and mini-tub is a fair amount of work, and a 295 is a lot of tire for anything less than 600 hp.

      Here’s the data on rear axle widths. Even the 62-64 B rears are wider (they’re tapered axles so not a good choice anyway though).


      8 3/4" Rear end widths, drum-to-drum:

      A-BODY
      • '66-'72= 57 1/8” (or 57 13/16” for BBP axles and brakes)
        • BBP 8.25/7.25= 56 3/4”
      B-BODY
      • '62-'63= 58 1/2" (And '64 Max Wedge)
      • '64 = 60 7/8" (Exc.Max Wedge)
      • '65-'67= 59 1/2"
      • '68-'70= 60 1/8"
      • '71-'74= 63"
      • '71-'73 wagon= 64 3/8"
       
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      • diymirage

        diymirage HP@idle > hondaHP@redline

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        yes, you are correct, i had that backwards
        thanks for pointing that out
         
      • lamont 71duster

        lamont 71duster Well-Known Member

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        I should of said this in my first post but I’m switching to big bolt axles and wanna buy some weld wheels but some people say a 15x8 will fit and some say only a 15x7 will fit just don’t know how to tell what will go under it and fit without problems but like I said someone said about the B body rearend
         
      • TrailBeast

        TrailBeast Slightly Twisted Member

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        Keep in mind a B axle is going to require a huge backspace if it's not shortened.
        This means you can fit a big tire but your wheel center is going to be far to the outside.
        Would you be ok with the wheels looking like that, or would you rather have a nice dish on the outside?

        This is an example, as this wheel has 5.5 backspace, but is and 8 inch wheel.
        Note how skinny it makes the wheel look from the outside. 8159058350_L1600_3bb6d596-f776-42f7-90d9-d6d8afbf2f1b.jpg
         
      • Lelo Dart

        Lelo Dart * FABO Gold Member

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        The narrower the rear end the more space you will have. Going with a wider rear will leave less space for wheels. If you have a A body 8 3/4 you should stick with that. I didn't see what type of rear end you had. If you are switching from a 7 1/4" I could understand using a B body rear.
         
      • lamont 71duster

        lamont 71duster Well-Known Member

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        It has a 8 3/4 in it now but have b&e body rear ends laying around wasn’t sure what my best avenue was to take
         
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