# Steel Mounting Bracket Question

1. May 18, 2010

### jsigned

I need a hand choosing the right size, thickness and shape of steel for a bracket to mount an old style 5 gallon Jerry water can on a Jeep. I estimate the weight of the can + water + misc = 50 to 55 pounds. The bracket will be subjected to rough off-road conditions.

In the drawing the red lines represent the load and the black lines the bracket. The 7” side will be welded to a solid mount. The width of the steel used in the mount can be up to 1.5 inches wide by .25 thick. Any commonly available shape can be used, angle, channel, square, rectangular or round tube.

A sturdy mount is the priority, but weight is important too. From an esthetics point of view I’d rather use just a single 24” bar of some sort, but I added the triangle shape just, well just because. If it isn’t needed lets drop it.

I hope this makes sense, what would the recommended shape and thickness of the steel be for this mount? I can clarify further if needed.

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2. Jun 8, 2010

### jsigned

So here's the mostly finished project.

No one replied, but that's OK because there were enough hints and links that I was able to figure out what would work. I ended up using 1"x2" rectangular .120 wall mild steel. It's more than sturdy enough, my 200+ lb per side test produced no significant deflection.

But, the added weight amplified a flaw that was already there and I'm hoping someone will point me in the right direction.

The gate with the tire and jerry cans swings out so the back can be opened. There is a significant sag at the hinge that needs to be rectified. I have sourced a hinge with tighter tolerances and higher load capacity, but I don't know how to calculate the load. (I'm not an M.E.) I know the weight of the various pieces, but what's got me is the water and gas cans. Do I measure the distance from the water can or the center post? The gas and water essentially balance each other on the center post. (Yes I know the gas is about 10 lbs lighter) Any help, links, or pointers would be appreciated.

(btw I made the entire bumper system)

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3. Jun 9, 2010

### pantaz

Is that only a single hinge attached at the bumper? I'd be more worried about the bending moment -- that's a heck of a big lever you've got hanging off of that bumper! I have a mental picture of the hinge ripping out of the bumper.

For the generic "weight rating", I would just go with the total weight of both cans (full), the tire, and the weight of the steel.

4. Jun 9, 2010

### Ranger Mike

.125" thick is plenty..make sure that the can / bracket will not vibrate. use at least grade 5 bolts and loctite the nut or use nylon insert lock nuts...i hate lock washers.

5. Jun 9, 2010

### jsigned

The bending moment is the problem for sure! The current hinge is a 7/8 grade 8 bolt with bronze bushings and thrust washers. The bushings are a "sloppy" fit over the bolt and are the biggest part of the problem. I don't have the tools to make a better hinge so I'm looking at this http://www.daro-ind.com/gatehinges.htm" [Broken] hinge. The 800 lbs hinge is rated as a pair, so I'm not sure if that means it will do 400 lbs as a single. I know hinge strength is not additive, so it goes to figure it's not subtractive as well. But, how big does it need to be?

As for the hinge ripping out, well my engineering skills tend to go to the nuclear overkill zone. "when in doubt about the strength just make it 2 sizes bigger just to be sure." This was built pre-internet and it is Heavy at 3/16 2"x2" tubing, which is part of the problem. I'm considering remaking the whole thing now that I have the ability to look things up and ask simple questions.

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6. Jun 9, 2010

### Ranger Mike

id say you are good to go..even in Ba Ha off road race!
get her done!

7. Jun 10, 2010

### nvn

jsigned: To answer one of your questions, you can measure the distance from the hinge to each water can. Or you measure the distance from the hinge to the center post using the total weight of both water cans. I would use the first method.

It is hard for us to see what the dimensions and design look like from a side view from your attached files. If your first attached file is still correct, the water cans seem severely cantilevered. Therefore, it seems they would severely bounce (oscillate) up and down.

You are correct in saying hinge strength is not simply additive nor subtractive. One hinge is far, far weaker than two hinges (for resisting moment perpendicular to the hinge pin axis). The strength depends on the moment resistance of the system. Two hinges resist a far higher perpendicular moment than one hinge. The information on the cited link seems indeterminate, because they did not list their assumed distance between the two hinges, nor their assumed gate center of gravity location; and they do not explain what they are talking about.

8. Jun 11, 2010

### pantaz

If you look at the older CJ7 Jeeps with the fold-down gate, they hinge the spare tire off of the body (at two points). I would try something like that, reinforcing the hinge mount locations as necessary.

9. Jun 11, 2010

### jsigned

The dimensions are still correct, however the as-built is a bit different. I dropped the angled support and added a support directly under the jerry can center. You can just see the end of it in the pictures just under the cans. The cans end up sitting on a "T" shaped support. The .120 wall tubing seems stiff enough, but if something less than .120 would do the job that would be my preference

I can see what you're saying in my mind's eye. A call to the vendor wasn't much help either. The sales guy only knew it was \$100 and had a 4 week lead time. But, after more googling I found this http://www.rockstomper.com/catalog/pieces/hinge.htm" [Broken] and I think it will do the job just fine. Its more engineering by making things bigger than they need to be, but . . . It's ready made and solves the problem.

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10. Jun 13, 2010

### nvn

If you wish, if you want us to see how your frame is made, you might want to provide a better sketch, showing different views, and showing dimensions (and cross-sectional dimensions) of all parts, as well as joint details, dimensions, and call-outs. Off-road acceleration can be 2.5 g (including gravity), if I recall (?). Or maybe it was 3.5 g. I don't remember the typical value, at the moment. Maybe someone knows.

Good find; that hinge at your second link seems like an excellent concept. Let us know how it goes.

11. Aug 9, 2010

### jsigned

Well, thanks to the help of this excellent forum, here it is, the finished product. 5 gallons of gas and water, tire, shade umbrella and hi-lift jack. I like to explore the desert and this bumper is a critical piece of my survival gear.

The 1 ton trailer axle hinge does a great job of supporting the load. What you can't see are the reinforcing brackets to tie the bumper to the frame.

Now with a chair, shade, water, and food I'm set just in case something breaks. Again, thanks

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12. Aug 10, 2010

### Ranger Mike

looks good except you forgot the beer cooler....

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