Aluminum bumper strength?

I was watching a video about trail armor from Expedition Overland (https://www.youtube.com/watch?v=9n-8Al_lGTQ) and they mentioned that while the front bumper is aluminum, the rear bumper is steel with a swing out. The reasoning they used was the weight the bumper (and swing out) would be carrying. Would an aluminum rear bumper and swing out tire carrier really not be strong enough? Or is there something else about aluminum that I am missing?

 

I'm not a mechanical engineer or a metallurgIst but I will make an uneducated guess Aluminum will work harden if stressed beyond its yield point. Since trucks are designed to carry loads in the back there is less concern about adding weight there. So it's easier to just make a bumper out of steel than to do the calculations required to make sure loads imposed by a tire carrier won't eventually harden that particular aluminum alloy somewhere and end up with a crack or permanent deformation.

 

Pretty good descriptor, as a fabricator, I agree.

Second reason is aluminum breaks before it bends. Iron will bend a long ways before it breaks.

Third, to get the strength you would require, you need to use much thicker aluminum. Now you are at close to the same weight as an iron bumper.

Fourth, not as many shops are set up for aluminum welding.

 

Just so I understand, in case of the front bumper, the weight of the winch at rest in negligible, even over time, compared to the strength of the aluminum bumper. Under load, the load is momentary, so this "work hardening" is limited in its effect.

At the back bumper, the weight is constant (tire, jerry cans, whatever else) and would be amplified as these items "hang" from the bumper, and even more so during sharp bumps or while on something like a corrugated road. This would build up the "work hardening" at a much faster rate?

Sorry, just trying to wrap my head around it.

 

Yea, that's pretty much it. Personally I wouldn't hesitate to build my own aluminum rear bumper and am sure it would hold up over time. Its good stuff, just use the correct alloy and design it right. But I don't have to worry about my reputation as a fabricator.

 

I am a metallurgist! Aluminum does work-harden, but so does steel. In any case, the loads on an aluminum front bumper are unlikely to be a big deal. 10,000lbs from a winch-pull may seem like a lot, but in engineering terms it's really not that much if the winch and bumper are sufficiently thick and well-supported. Work hardening shouldn't really be a factor - it's more of a factor when FORMING aluminum parts. Remember, while aluminum isn't as strong as good steel (not that the basic A36 steel that gets used for stuff like aftermarket bumpers is particularly strong...) it's a hell of a lot lighter, so to meet your strength goals you can always just use thicker sections of aluminum and still end up lighter. If you're going to be bashing against a lot of rocks, steel will be better because the common alloys are more ductile compared to the usual aluminum alloys (6061, 7075) allowing them more "give" before something breaks. But for overlanding, no reason not to go with aluminum.

There's no particular reason you couldn't make a swing-arm rear bumper out of aluminum, except that the single-pivot design puts a lot of bending loads on both the bumper and the arm, so the extra stiffness of steel is a benefit. Steel is about three times as stiff as aluminum alloys. You could just as well make an aluminum one, but the parts would have to be much physically larger to avoid too much flex. Flexing on the swing-arm would be a PITA, leading to wear and misalignment.

 

So basically it would take a hell of a lot of engineering to get it right. Even then it would be a maybe.

What about using a traditional steel bearing kit like: https://www.comp4x4.com/products/ti...-duty-1-1-4-spindle-shaft?variant=25281623235

Then add a bolt on mount for the arm? And a way to add it to the bumper? Seems like a lot of work, but I think it could be done.

I guess the question is, is the flexing concern in the pivot point? Or the arm itself? Or both?

 

You have had me thinking about this quite a bit. I just don't see it being a reasonable job.

How do you attach and iron spindle to an aluminum bumper? How do you attach an aluminum tire arm to an iron spindle? In either aspect, you have to bolt an iron part to an aluminum part. You can't weld it. Now you have just introduced galvanic corrosion to the equation.

Lets look at the spindle. It is a very strong part. It gets attached to the bumper in only 1 place. 3" wide bumper, 4" wide bumper? Look at the tire that is supported by the spindle attached to the bumper. That 100 pound weight is cantilevered beyond the bumper. The stress on the spindle is not straight down. It is vectored 45 degrees or so away from the spindle weld point. Theoretically, you only need to weld the back side of the spindle. The weld on the front has no purpose.

Add going over bumps and the up and down motion of the bumper and you compound the forces that want to pull the tire down and away from the bumper. The carrier is gonna bend or its gonna break. With aluminum, its gonna break, with iron, its just gonna bend and you won't be dropping that tire on the road endangering the people behind you.

I can weld aluminum and do it often, but I just cant find a way to justify even attempting to do a swing away tire carrying bumper out of aluminum.

I've enjoyed the discussion.

Thanks

Kirk

 

I appreciate your responses. It is for information like this that I ask questions. After watching the video I posted I looked around and couldn't find a single aluminum swing out for an off road bumper. I was curious as to why, and I think you have answered all of my questions. Thanks for that!

 

love my aluminum bumper lightweight and strong