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Discussion Starter #1 (Edited)
We following along with a few others on fsb I decided to go ahead and start my rr bumper...heres a few pics as I go...

started with this...with the hitch real poor departure angle.

rr bumper removed.

shot of rr bracket...using factory holes. (grade 8 bolts)

shot of hitch I mounted between brackets. class 3

bumper mounted back on bko...
Now I just need to start on spare tire rack...plan on mounting tire/hi lift/spare gas can/and shovel to the rack.
more photos to follow...
 

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Past Bronco Owner
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:thumbup Looks good.

that's next on my list once the front one is done.
 

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Lick my balls
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Looks real good
 

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Looks good, but be aware that those factory boltholes are only good for about 9k per side in a dead straight, smooth pull before they'll block shear right out of the frame. You can expect a lot less capacity if you're pulling upward or downward. Just keep a very close eye on it if you plan on hooking a recovery strap up in that receiver hitch. :eek:
 

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Discussion Starter #7
thanks for info...one other thing i was planning on doing is running a brace along the bottom of the frame and welding to bracket similar to how the hitch was attached...for extra strength.
 

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That should help a bit. I went a slightly different route, eliminated my #5 crossmember and went up the inside of the framerails:



It's still only about half finished in that shot, and half the fasteners aren't in yet. Did some more work on it last weekend.
 

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Chuck said:
Looks good, but be aware that those factory boltholes are only good for about 9k per side in a dead straight, smooth pull before they'll block shear right out of the frame. You can expect a lot less capacity if you're pulling upward or downward. Just keep a very close eye on it if you plan on hooking a recovery strap up in that receiver hitch. :eek:
I guess I'd like to see the calculations or documentation proving that.
 

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ToddACimer said:
I guess I'd like to see the calculations or documentation proving that.
Let me see if I still have the calcs here, or took 'em home. If they're at home, I still have the dimensions of the mounting holes, so I'll just re-do the calcs. :thumbup I may have been more conservative when I checked the numbers before, since I was getting things together fairly quickly, so we'll see.

I'm also doing some more hunting on the frame material. It's SAE 1018, problem is the SAE spec doesn't give requirements for mechanical properties, only composition. Actual mechanical properties seem to vary widely, but I've managed to track down a fairly consistent set of properties that also indicate this steel is pretty comparable to ASTM A-36, which I've also heard.
 

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Alright, I sat down over my lunch break and re-did the calcs. The bumper illustrated was my initial shot at a design, which is identical to the current bumper pictured, minus the 2" extensions to forward of the shackle points. The frame dimensions are based on measurements I took a couple of months ago, I do have an AutoCAD drawing of the back 1' or so of the frame if anyone else ever needs it. The bumper bolt and shackle bracket rivet holes are located accurately, the shackle bracket itself is only sketched in for reference.

Measurements needed for block shear calc (pentagonal block shown rips straight out of back of frame):


Measurements needed for rotation calc (vertical force applied at shackle eye on bumper causes the bumper to rotate upward around the top bolt, with the bottom two bolts tearing a similarly shaped block out of the back of the frame -- I wanted to be able to lift the vehicle off the shackles if I needed to):


Calcs (I can email you a clearer PDF if you want, this is a screenshot of a PDF, and my handwriting sucks either way :goodfinge ):


Notice that these numbers are with no factor of safety, this is the actual expected failure load, not the safe load you can apply. I was conservative on my initial back-of-envelope number for a straight pullout, but it was already too low for what I needed so I left it alone and redesigned. I hadn't actually calculated the rotational case, and it turned out to be even lower than I expected ... wow. At angles between straight and 90*, you can expect the strength to vary from the 21k down to the 4k shown.

I typically use a factor of safety of 3 for horizontal winching (which is pretty standard), closer to 5 if you plan on overhead hoisting, and you normally want a factor of safety of around 10 if you're actually using a hoist to lift people. In this case, 3 is plenty. These factors are a good bit higher than those used for normal structural design, most buildings are only designed with a factor of about 1.6 ... but you have a lot more control over the forces on a building than a winch line.

I wanted to be able to either use a 12k winch at the center of the bumper running a double line pull back to one shackle point (unfactored force on bumper end = 12/2 + 12 or 18k), or a 9k winch at the center with a triple line pull to a snatch block on the shackle point (9/2 + 2*9 = 22.5k for the attachment to resist). The 9k winch obviously governs, and with the factor of safety that means you have an ultimate load (maximum the structure must be able to withstand) of 22.5*3 = 67.5k. This is far beyond the 21k you get even with a straight pull, hence why I had to go beefier on my attachment. :thumbup
 

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Chuck said:
I do have an AutoCAD drawing of the back 1' or so of the frame if anyone else ever needs it. The bumper bolt and shackle bracket rivet holes are located accurately
Could you e-mail me what you have in CAD chuck... I've been wanting to get in CAD and build a couple of models before I start building my new bumper.

You can e-mail it to me at [email protected]

I have AutoCAD 2000.

Thanks,
John
 

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Discussion Starter #13
Chuck...what if I was to guesset the inside of the rails about 3 feet back...basically sandwiching the frame rails between to the bumper bracket and 1/4 piece of steel?
 

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BigDadddy said:
Could you e-mail me what you have in CAD chuck...
Done. :thumbup Lemme know if you have any trouble with it.

redwagon said:
Chuck...what if I was to guesset the inside of the rails about 3 feet back...basically sandwiching the frame rails between to the bumper bracket and 1/4 piece of steel?
That would be another solution, though how well it works will depend a lot on how well the gusset is welded or bolted in. If you make the back of the gusset conform to the shape of the back of the frame, you need to make sure you have at least about 1/2" of meat above and below the top and bottom boltholes -- that will help force the gusset to have the same failure pattern as the rest of the frame. If you can do that, and the gusset is welded or bolted in well enough to develop the full strength, then you increase those strengths above proportional to the total thickness of gusset plus frame to the original frame thickness. For instance, adding 1/4" plate behind and properly welded to the frame would give you about 2-1/3 times the tear-out strength for both pullout and rotating up.

The main problem is, the shackle bracket rivets get in the way of running a simple rectangular gusset all the way forward to where you need. You can neck down your plate like I ended up doing with mine (you can see the inner 1/2" plates are trimmed down to let the bumper slide right up between the rivets), or you could go what might be the simpler route and chisel off the shackle hanger rivets, weld in a clean, straight plate, and drill matching holes in the gusset and replace the shackle rivets with good Gr8 bolts that bolt through the frame and the gusset. If the plate is 5" tall through it's full length, and you get about a good 1/10" or so weld bead (I figure it might be difficult to get into such a weird corner and get a good bead), you'll need at least about 6" worth of good weld top and bottom (ahead of the bumper bolt holes) to get that full 2-1/3 strength gain.

The third solution is to weld on a small reinforcement plate on the inside about the same dimensions as what I'm showing on the drawings above for my first try at a bumper attachment. Doing it this way, if you get full 1/4" weld beads all the way around, should up your pullout resistance to about 48k and your upward pull (if your shackle or winch fairlead is the same distance from out from the bolthole as mine, about 8" or so) to around 8.4k. Again, no factors of safety, just the failure load. I'd divide the pullout by 3 for a pretty safe winch load, or the uplift by 5 if you're ever considering using the bumper to lift the vehicle so you can crawl under it and work.

Lots of options ... there are a few other ways you can go about it too. Having gone with a slimmed and trimmed gusset, I think knocking out the shackle bracket rivets and running a new full height gusset all the way forward is probably the best way to go.
 

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Discussion Starter #15
Chuck said:
Lots of options ... there are a few other ways you can go about it too. Having gone with a slimmed and trimmed gusset, I think knocking out the shackle bracket rivets and running a new full height gusset all the way forward is probably the best way to go.
Ya I think I like this option...due to I want to yank my gas tank anyways to fabricate a skid plate for it also...looking at it at lunch time looks like I can get a gusset about 2 feet along the frame rails...install new bolts for the shackles and fabricate a new rr crossmember out of some 2/2x.250wall square tube that I have.
 

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If you can extend the bumper you're building forward along the frame any, you can actually gain a lot in the upwards pull department, too. The small three-bolt pattern the stocker used is decent for horizontal forces, but if you're trying to help pull someone up a hill and you're on the level, it's just asking for trouble. A solid extension of the bumper forward would help a lot, even extensions up the bottom of the framerail like some of the hitch receivers use would probably be helpful if going inside isn't a good option ...

Two feet up the inside of the framerail is plenty, as far as the inside gusset goes. You may even want to go heavier than 1/4", since with that much length your strength will pretty much increase proportional to the total thickness...
 

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redwagon said:
shot of hitch I mounted between brackets. class 3

bumper mounted back on bko...
Now I just need to start on spare tire rack...plan on mounting tire/hi lift/spare gas can/and shovel to the rack.
more photos to follow...
Looks good man, is it just me though or is the hitch location slightly off-center? To the left?
 

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Discussion Starter #20
BigDadddy said:
Looks good man, is it just me though or is the hitch location slightly off-center? To the left?
centered it between the bumper brackets...doesnt mean that my body isnt on crooked though!!!
 
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