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Maximum backspacing on SRW D60?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Shaggy, Oct 31, 2002.

  1. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> OK, so I figured out that in order to have the tires on Hummer wheels in the same position on my D60 as my 15x10s were on my 6 lug D44, I need to have the backspacing changed to 5.38". My questions are -

    With 5.38" backspacing on the 16.5" Hummer wheel, am I going to have interference problems with anything on the axle? The part that comes out for the tie rod to bolt to, etc.

    How about the steering box? The tire would be in the exact same place as my 38s were on the 15x10s, but I'll be running 42s so I'm thinking that I may have problems with the tire contacting the steering box?

    I wouldn't mind having a little less backspacing, but my 38s were pretty much all the way under the fenders, which I kind of like to keep the overall width down a bit and to keep from slinging too much mud onto the windsheild and such. I don't really want the tires to stick out more than 1" or so more than they did before, so that would be 4.38" of backspace. Would that be better? </font color>
     
  2. imiceman44

    imiceman44 1 ton status

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    I think that 4.38 would be better but it's all up to you hee, as for interference with the box or the steering arms you have to check that. just get a thick cardboard cut to 16.5 make holes for the lugs and mount it and if you want to make the measuring a little easier, gat a strip of 5.38" width and tape it to the edge and that way you have something to measure acyrately through te whole motion. For tire clearance you have to estimate from that mock up.
    Hope this helps.
    You will definitely need to readjust your stops and reduce your turning angle, I had to do that for a 35: tire so I guess you will need it with the 42". /forums/images/graemlins/thumb.gif
     
  3. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> I was running 38s with the stops in the stock position and had no rubbing, it would be nice to do the same for 42s but that might be a pipe dream. /forums/images/graemlins/thumb.gif Why oh why can't I resist the urge to keep making this thing bigger?! /forums/images/graemlins/rolleyes.gif</font color>
     
  4. m j

    m j 1/2 ton status

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    have you looked at what the scrub radius will be?
    with tires that large it may be possible to go too far with backspacing and end up with neg scrub radius
     
  5. zakk

    zakk 1/2 ton status

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    i think 5.38 will be too much. Running the standard 3.75 backspacing would be a better idea, IMHO, as it is less of a chance with steering box issues.

    Remember the following:
    the ORD Jimmy had its steering box completely moved from the stock location because of tire interferance.
    Russ' Truggy was sawing off WTOR Redneck Ram lines from its flex.

    So, the moral of the story is: 42's = problems Keep as much as what works with the smaller tires as possible. When i go up to 42's (cuz the 39.5's are looking small/forums/images/graemlins/crazy.gif ) i am keeping the stock BS, and just hacking the fender more. You can 'wheel with less sheetmetal on your fenders, but a busted steering box is no fun
     
  6. XtrmChevy

    XtrmChevy 1/2 ton status

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    Well I have 42/16,5s with 3,75" BS and appr. 9" wide wheels (the wheels are made out of two old sets in order to keep the safety lip and they work very well).

    My truck is still under construktion but I have wheeled it a bit just to test things out and I can say that the steering box will be a problem when I get the crossover to work as it should. As it is for now, there is allmost 1/2" to the stops.
    But even now when the left front is flexed in and the right is out wheels turned to the right I have contact with the SB and the tire /forums/images/graemlins/mad.gif (I dont know how much lift I have but the front springs are absolutely straight now?)

    I dont want to go wider, I allready have to extend the fenders by 2" so I have considered to move the SB exactly above the axle. It would have to turn the other way to work crossover. Anybody know witch SB would be right for this application?

    Janne
     
  7. 6.2Blazer

    6.2Blazer 1/2 ton status

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    See my response for your other post. I figured a different backspace than you did.
     
  8. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Zakk is right, my 42s do actually brush the box at full lock, and that's with roughly 4.125 b.s. and the stops run out a bit. As for scrub radius, mine has almost no scrub radius at all the way it is set up. My tires just turn in place, and the marks left on concrete show the pivot almost dead center...
     
  9. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> Yep, that's right in the right range for a good scrub radius. I figure that the Hummer wheels are 9.75" total width, so BS of 4.875 would be dead center of the wheel, so perfect scrub. 4-4.25 is not too far off from that so it should be about right for me. /forums/images/graemlins/thumb.gif Baddog - what width rims do you run?</font color>
     
  10. m j

    m j 1/2 ton status

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    what is the steering axis inclination on your axle?
    what is distance from knuckle to wheel mount surface?

    in other words could you spell out your scrub radius math?

    just eyeballing mine with Ford factory Accuride wheels with the XLs, it looks like the SAI intersects the ground just at the inside edge of the tire contact patch.
     
  11. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    My rims are only 8" wide which makes the "effective" bs a little more (due to the outer rim edge "sucked in"). One thing about it, with an 8" rim it's nearly impossible to unseat the tire. I was running 5lbs on Upper Helldorado and one front tire folded completely under the rim while turning in a bind. Nobody could believe it didn't roll the bead off! You couldn't even see side wall...
     
  12. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> Haven't measured all that stuff, but in my experience if you try to keep the backspacing around 1/2 the total rim width the tires turn pretty nicely without scrubbing like crazy. I'm building a truck here, not a NASA space probe; I don't really feel the need to sit down and figure out what exact backspacing I need in order to get the perfect scrub radius when it's more important to me to be able to actually fit the tires under the truck.

    My last wheels were 10" wide with 4.25" of backspacing and the outside of the tire scrubbed just a bit, a narrower rim with similar backspacing should have an even better scrub radius. </font color>
     
  13. Charly

    Charly Registered Member

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    Just FYI, Hummer wheels are 8.25" wide, with 7" of BS
     
  14. imiceman44

    imiceman44 1 ton status

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    That's what I always tell people, all you need is narrow wheels and you don't need bead locks.
    I never had problems with my stock 8" wheels.
    Now with the 10" I am affraid to go lower than 15PSI.
    I want to go back to 8".
     
  15. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> There's another thread on this, 9.75" is the actual total width of the Hummer wheel, 8.25 is the technical width from seat to seat. </font color>
     

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