Rear Bumper/Spare Tire Mount Thoughts

[Steve83]

Did some more work on the bumper. And need your thoughts, please.

First, Here's the new "bumper", and it is 4" x 7" x 3/16" tubing. That size makes it essentially the same size front/rear as the top of the old bumper, but it doesn't have the piece that extends under the tailgate to prevent yuk from coming up there, so may need to have a piece added. The top back/right corner of the tubing is in the same spot as the original bumper, but it extends down 1 1/2" from what the original bumper did - you'll see why in a bit.

Also, I've increased the height of the tow eyes to 3 3/8" so their bottoms will be flush with the bottom of the frame. That allows me to use pieces of 2 1/2 x2 1/2 x 1/4" angle welded to the bottom of the eyes as well as to the bottom of the hitch cross piece (in blue). And the angle will bolt into the front two holes in the bottom of the frame, the red lines, where my hitch currently bolts. All in all, that significantly strengthens the hitch and the bumper.

Then comes the receiver part of the hitch - the tan bit added in this illustration. It has a piece of 3/16" plate (dark gray) bent and welded to the front of the cross piece and to the bottom of the receiver. And it has a piece of 2 1/2 x2 1/2 x 1/4" angle (dark gray) welded to the rear of the cross piece and to the top of the receiver. That's exactly how the hitch that is on the truck was made, so should be plenty solid.

An issue I need to consider is the height of the tow eyes. I don't see any reason to carry the 3 3/8" height all the way through the bumper, but it may be easier to leave them that size instead of cutting them down.

Another idea is to raise the bumper 3/16" to get gap above the tow eye and below the receiver to be equal. But that depends on the height of the tow eye where it comes through the bumper, so that will have to wait until I decide on the tow eye height.

But, for the moment I can turn my thoughts to the spare tire carrier and await your collective input.

Why is the bumper beam a tube? It looks too small to put anything useful in, or to get into it if you did; but bigger than necessary to bump. And why wouldn't you extend the green angle back to the bumper beam, to be welded there, too? As it is, if you hit something with the bottom rear outboard corner of the bumper (the most-likely place for a hit), it would roll under & bend at the recovery eye bar. But if the angle came back with the bar, the bumper would be substantially stronger. Another advantage would be that the hitch cross bar could come down in front of the hitch, giving you a more-useable space between the fuel tank crossmember, bumper, & recovery bars.

[Gary Lewis]

Why is the bumper beam a tube? It looks too small to put anything useful in, or to get into it if you did; but bigger than necessary to bump. And why wouldn't you extend the green angle back to the bumper beam, to be welded there, too? As it is, if you hit something with the bottom rear outboard corner of the bumper (the most-likely place for a hit), it would roll under & bend at the recovery eye bar. But if the angle came back with the bar, the bumper would be substantially stronger. Another advantage would be that the hitch cross bar could come down in front of the hitch, giving you a more-useable space between the fuel tank crossmember, bumper, & recovery bars.

Steve - Good questions. The bumper is a tube because I was working on the tow eye and hitch plans and wanted to see how a tube would work. I thought that once I got the infrastructure worked out I could then explore different bumpers. But I'm starting to see that these aren't independent decisions.

Yesterday, before drawing the above, I briefly looked at using a piece of channel, but the thickness of the edge and the placement of the tow eyes and receiver were such that it required a very tall bumper so we didn't have to cut the angled side. But maybe once I work out how to get those pieces pretty much in line with each other a piece of channel would be an option. But, channel wouldn't provide the 4" top surface that makes standing on the bumper to get into the bed easy/comfortable.

On the angle, I see what you mean about taking it back to the bumper to provide extra support - for a tube. But, if I go with a piece of channel, I'm not sure that will be necessary, especially if I get things pretty much centered up in the channel?

As for the hitch cross bar coming down in front of the receiver, I think I see what you mean. But, wouldn't another option be to take the receiver up instead of bringing the cross bar down, bringing the receiver and tow eyes closer to being in line? That would allow the bumper to be shorter, and increase the angle-of-departure, although that assumes that the gas tank doesn't become the limiting factor.

I realize that taking the receiver up uses the space between the fuel tank crossmember, bumper, and tow eyes. But I'm still on the fence about creating storage there, so I'm just exploring options.

Anyway, thanks for the input. Please keep it coming.

 

[Gary Lewis]

Steve - Good questions. The bumper is a tube because I was working on the tow eye and hitch plans and wanted to see how a tube would work. I thought that once I got the infrastructure worked out I could then explore different bumpers. But I'm starting to see that these aren't independent decisions.

Yesterday, before drawing the above, I briefly looked at using a piece of channel, but the thickness of the edge and the placement of the tow eyes and receiver were such that it required a very tall bumper so we didn't have to cut the angled side. But maybe once I work out how to get those pieces pretty much in line with each other a piece of channel would be an option. But, channel wouldn't provide the 4" top surface that makes standing on the bumper to get into the bed easy/comfortable.

On the angle, I see what you mean about taking it back to the bumper to provide extra support - for a tube. But, if I go with a piece of channel, I'm not sure that will be necessary, especially if I get things pretty much centered up in the channel?

As for the hitch cross bar coming down in front of the receiver, I think I see what you mean. But, wouldn't another option be to take the receiver up instead of bringing the cross bar down, bringing the receiver and tow eyes closer to being in line? That would allow the bumper to be shorter, and increase the angle-of-departure, although that assumes that the gas tank doesn't become the limiting factor.

I realize that taking the receiver up uses the space between the fuel tank crossmember, bumper, and tow eyes. But I'm still on the fence about creating storage there, so I'm just exploring options.

Anyway, thanks for the input. Please keep it coming.

Here's the next iteration, but the bumper isn't done by any stretch. I'm just trying to incorporate most of the suggestions to this point. However, in this pass I'm still using a tube for the bumper, although it has been reduced 1" to a 4" x 6".

Also, I've reduced the height of the tow eyes to 2 1/2" as they were originally. And I've extended the angle to the bumper. Last, I've added fishplates for the cross piece/tow eye interface.

And, here's the receiver added. (Yes, the tow eye is still there.) I've raised the receiver to the height of the cross piece and added fishplates for the cross piece/receiver interface, top and bottom.

At this point I think the evolution of this approach with a closed tube bumper is pretty well done - unless y'all have further refinements, to which I'm fully open. It looks really strong to me and at 6" high the rear bumper is only 1/2" lower than the original bumper.

And, I think it is stout. What do you think?

[Gary Lewis]

Here's the next iteration, but the bumper isn't done by any stretch. I'm just trying to incorporate most of the suggestions to this point. However, in this pass I'm still using a tube for the bumper, although it has been reduced 1" to a 4" x 6".

Also, I've reduced the height of the tow eyes to 2 1/2" as they were originally. And I've extended the angle to the bumper. Last, I've added fishplates for the cross piece/tow eye interface.

And, here's the receiver added. (Yes, the tow eye is still there.) I've raised the receiver to the height of the cross piece and added fishplates for the cross piece/receiver interface, top and bottom.

At this point I think the evolution of this approach with a closed tube bumper is pretty well done - unless y'all have further refinements, to which I'm fully open. It looks really strong to me and at 6" high the rear bumper is only 1/2" lower than the original bumper.

And, I think it is stout. What do you think?

Next, I need input on "angle of departure". The drawing below represents my feeble understanding of the subject.

The upper red line is off of the differential and the lower line is off the tire. My understanding is that the angle of departure is the angle of the lower line. In this setup the bumper and the receiver hit about the same time, but the bumper hits first. So, I think this says that if I make the bumper shorter, meaning that the bottom of the bumper goes up, then the receiver will be the limiting factor. Right?

As for the upper red line, my thinking is that anything that doesn't hang down past that line is fine, although it may well need a skid plate. And I've drawn in the 38 gallon fuel tank that I have. It looks like it'll be fine with a skid plate as it is within the line that a rock that clears the differential would take. Do I understand that correctly?

[Steve83]

But, channel wouldn't provide the 4" top surface...

My bumpers started with channel, and they have flat surfaces to stand on.

https://supermotors.net/getfile/1043883/thumbnail/rearbumper.jpg

...I'm not sure that will be necessary, especially if I get things pretty much centered up in the channel?

I don't see the advantage to having their centers aligned. It certainly won't strengthen the bumper, or prevent the bending that I described before. The strongest simple arrangement is with the forces (impact & vehicle's CG/momentum) in line with the structure (bumper & its connections) between those forces. That would mean the bumper ABOVE its attaching structure (assuming the impacts come from lower, like rocks & most cars); i.e.: receiver & tow eyes at the BOTTOM of the bumper face.

But the strongest for the weight is with half the structure at the top of the bumper face, and half at the bottom. That resists impact anywhere, and torque FAR better than having all the attachment centered (like a hinge, which resists NO torque). That's why there's an advantage (especially for F-series) to add structure below the frame - you don't have access to the top of the frame like Broncos do, due to the bed's overhang.

...bringing the receiver and tow eyes closer to being in line?

I don't see any advantage to that, either. You'll never be pulling or pushing between the receiver & eyes, so having them aligned doesn't affect strength or functionality.

That would allow the bumper to be shorter...

In which direction?

..increase the angle-of-departure...

Assuming your required angle is that precise, but you could also accomplish a greater angle by moving the receiver forward into the bumper, or moving the axle back. What angle do you need?

...the line that a rock that clears the differential would take.

No, a rock will not follow a line, unless the truck is on level ground, in which case "angle of departure" is meaningless. A rock will follow some curving, irregular path relative to the moving truck, depending on its position, and the truck's suspension. It's really "MAXIMUM angle of departure", meaning the greatest angle between 2 flat surfaces that the vehicle can drive squarely through. Obstructions that stick up from either surface, or driving at an angle across the break between the surfaces is too unpredictable & complex to calculate or design for.

This is a cleaned-up version of a Ford diagram:

https://supermotors.net/getfile/846565/thumbnail/eblinedimensions.jpg

And to some degree, those numbers are moot. My truck doesn't have the approach or departure angles to take many of the trails I've driven successfully. Look at the first 2 pics & last 2 videos in this album:

https://supermotors.net/getfile/96528/thumbnail/plow2.jpg

BTW

I think your fish plates & gussets are largely dead-weight. My bumper has none, and it's almost strong enough to lift the 3-ton truck. The forward tubes for my receiver, and those under the frame are 1/8"-wall.

https://supermotors.net/getfile/292107/thumbnail/rear-seethru-s.jpg

[Gary Lewis]

But, channel wouldn't provide the 4" top surface...

My bumpers started with channel, and they have flat surfaces to stand on.

https://supermotors.net/getfile/1043883/thumbnail/rearbumper.jpg

...I'm not sure that will be necessary, especially if I get things pretty much centered up in the channel?

I don't see the advantage to having their centers aligned. It certainly won't strengthen the bumper, or prevent the bending that I described before. The strongest simple arrangement is with the forces (impact & vehicle's CG/momentum) in line with the structure (bumper & its connections) between those forces. That would mean the bumper ABOVE its attaching structure (assuming the impacts come from lower, like rocks & most cars); i.e.: receiver & tow eyes at the BOTTOM of the bumper face.

But the strongest for the weight is with half the structure at the top of the bumper face, and half at the bottom. That resists impact anywhere, and torque FAR better than having all the attachment centered (like a hinge, which resists NO torque). That's why there's an advantage (especially for F-series) to add structure below the frame - you don't have access to the top of the frame like Broncos do, due to the bed's overhang.

...bringing the receiver and tow eyes closer to being in line?

I don't see any advantage to that, either. You'll never be pulling or pushing between the receiver & eyes, so having them aligned doesn't affect strength or functionality.

That would allow the bumper to be shorter...

In which direction?

..increase the angle-of-departure...

Assuming your required angle is that precise, but you could also accomplish a greater angle by moving the receiver forward into the bumper, or moving the axle back. What angle do you need?

...the line that a rock that clears the differential would take.

No, a rock will not follow a line, unless the truck is on level ground, in which case "angle of departure" is meaningless. A rock will follow some curving, irregular path relative to the moving truck, depending on its position, and the truck's suspension. It's really "MAXIMUM angle of departure", meaning the greatest angle between 2 flat surfaces that the vehicle can drive squarely through. Obstructions that stick up from either surface, or driving at an angle across the break between the surfaces is too unpredictable & complex to calculate or design for.

This is a cleaned-up version of a Ford diagram:

https://supermotors.net/getfile/846565/thumbnail/eblinedimensions.jpg

And to some degree, those numbers are moot. My truck doesn't have the approach or departure angles to take many of the trails I've driven successfully. Look at the first 2 pics & last 2 videos in this album:

https://supermotors.net/getfile/96528/thumbnail/plow2.jpg

BTW

I think your fish plates & gussets are largely dead-weight. My bumper has none, and it's almost strong enough to lift the 3-ton truck. The forward tubes for my receiver, and those under the frame are 1/8"-wall.

https://supermotors.net/getfile/292107/thumbnail/rear-seethru-s.jpg

So many thoughts, where to start. (And I'm sure I won't address all of them.)

On the placement of the tow eyes and receiver for the strongest bumper, with your recommendation the angle is already extended back to the bumper at essentially the bottom of the bumper, and in line with the frame. So there's as much strength at the bottom as we are likely to get, and it is close to the corners.

And, the receiver was moved up to gain departure angle. Not that there is a magic departure angle, but the current receiver is 6" lower than in this plan, so this is much better. I don't expect Big Blue to be like a Bronco, but I might as well make the angle as great as is reasonably possible. And I think we are there.

As for aligning the centers, it was just what happened when I raised the receiver up like you suggested to tee with the cross piece. I really like that idea since it makes strengthening the welded joints much easier by using fishplates top and bottom. And, I like the fishplates because I like overkill. A piece of strap 2 1/2 x 2 1/2 x 1/4" doesn't add much dead weight, but it will add a lot of piece of mind.

The bumper could be shorter up/down by an inch, making it 4" wide and 5" high. But, to keep the angles coming back to the bumper the reduction in height would need to be at the top. However, that would let you see into the innards under the bed, and I don't like that idea. So I'm happy with the 4 x 6 size tube - if I stay with a tube.

As for the rock, I take your point about it being complex. And maybe I'm not explaining my thinking very well. But the idea was that if you were to start a climb with a rock in the middle of the road then if the rock clears the differential it'll clear the gas tank. Does that make sense?

But I think I'm ready to move on to the spare tire mount. No, I'm not saying I'm done with things to this point but, as I've found, working on the next part frequently changes the previous parts. So I need to take the next step and see what happens.

Last, this is going to be a slow process. My friend at church, the one with the metal fab shop, has a form of cancer. He's going to be undergoing his third round of chemo/radiation, and this one doesn't have a finish date. He was gung ho to help on this, but I suspect his attention will, and rightly so, be elsewhere. Given that, I'm not in a hurry to get the design done.

Also, I think now is the time to turn to 3D CAD. So I need to make the existing parts three dimensional and get them into the right position with respect to the z-axis. And that is going to take some time.

Having said that, y'all please don't stop sending ideas. I really appreciate them.

[Steve83]

So I need to make the existing parts three dimensional and get them into the right position with respect to the z-axis.

I can say from experience that it's far more important to get the TRUCK drawn first, than any of the parts. If you don't have the truck's shape accurately, no bumper you design will fit the way you want. So spend some time re-re-measuring the truck, and then comparing it to your drawing.

When I drew mine, I found I needed to draw the truck past the closest body mount bolt on the top of the frame, and almost to the axle on the bottom.

[Gary Lewis]

So I need to make the existing parts three dimensional and get them into the right position with respect to the z-axis.

I can say from experience that it's far more important to get the TRUCK drawn first, than any of the parts. If you don't have the truck's shape accurately, no bumper you design will fit the way you want. So spend some time re-re-measuring the truck, and then comparing it to your drawing.

When I drew mine, I found I needed to draw the truck past the closest body mount bolt on the top of the frame, and almost to the axle on the bottom.

That's good advice. I've done a bunch of measuring, but I know it needs to be dead-on, and surely I've missed some things. So when I draw it up in 3D I'll remeasure.

But, is it important to draw the truck forward where things are going to attach? I can understand measuring to ensure that the frame rails are parallel, but why does it need to be drawn?

The reason I ask is that the frame isn't straight. It is only straight for maybe 18" before it bends up, and than comes back down farther forward. So drawing it accurately past the first bend is going to be a pain. And, since nothing I'm doing on the bumper will connect up there, I'm trying to understand the need to do so. But if it is needed I'll do it.

[Nothing Special]

.... As for the rock, I take your point about it being complex. And maybe I'm not explaining my thinking very well. But the idea was that if you were to start a climb with a rock in the middle of the road then if the rock clears the differential it'll clear the gas tank. Does that make sense?....

The gas tank is pretty well protected, but I can imagine driving off a ledge with a high spot between the tires and having that high spot hit the fuel tank before it gets hung up on the bumper.

And getting hung up on the bumper will happen if you get serious enough about what trails you run. I've had both back tires of both my CJ5 and my early Bronco hanging in the air with the bumper sitting on the rock the tires just drove off. And those vehicles will drag their tail less often than an F-250.

So if you can, I'd suggest a skid plate under the fuel tank. But if not, you'll be OK on most of the trails you are likely to do.

 

[Gary Lewis]

.... As for the rock, I take your point about it being complex. And maybe I'm not explaining my thinking very well. But the idea was that if you were to start a climb with a rock in the middle of the road then if the rock clears the differential it'll clear the gas tank. Does that make sense?....

The gas tank is pretty well protected, but I can imagine driving off a ledge with a high spot between the tires and having that high spot hit the fuel tank before it gets hung up on the bumper.

And getting hung up on the bumper will happen if you get serious enough about what trails you run. I've had both back tires of both my CJ5 and my early Bronco hanging in the air with the bumper sitting on the rock the tires just drove off. And those vehicles will drag their tail less often than an F-250.

So if you can, I'd suggest a skid plate under the fuel tank. But if not, you'll be OK on most of the trails you are likely to do.

I do plan to have a skid plate. I have the factory one off Dad's truck, but if I remember correctly it is a flat plate. Instead, I think I need the skid plate from the Bronco with a 32 gallon tank, which is D8TZ 9A147-A. Then I can shim it off the frame a bit to clear the 38 gallon tank.

Bronco Graveyard has it: https://shop.broncograveyard.com/1978-1979-Ford-Bronco-Skid-Plate-33-Gallon-Only/productinfo/20254/. But I don't know if it will fit a pickup. Does anyone know?

Another option would be to shim the one from Dad's truck.

Thoughts?