Aluminum Bumpers?

mrkake · **Posted:** Thu Apr 02, 2009 11:24 pm

I have been talking to a local fab shop that specializes in aluminum fabrication about a tank for my wvo conversion and the guy asked me why I had a custom back bumper but not a front one. I said that there were some production issues from the fabricator but that they have been worked out now and he is gradually ramping up again. Then he asks me if I want him to design and fab custom bumpers for the Liberty along with the tank. He has done some custom work for the Terribles desert racing vehicles here and showed me some of the work he was in the middle of (making and F150 into a full on PreRunner). He does good work but I wonder about the strength and durability of aluminum components. And what about galvanic corrosion/bonding? I think he needs the work; which bothers me less than it should because we could all use more business these days. Cost, of course, is a factor but my first concern is will this work and hold up? Any thoughts?

jsc7002 · **Posted:** Thu Apr 02, 2009 11:51 pm

it would hold up fine, it would just have to be made from thicker metal than your normal steel would need to be, are you wanting it to be a winch mount also?

IIRC TJM made some that were aluminum....or alloy I think is what they called it.

tommudd · **Posted:** Thu Apr 02, 2009 11:52 pm

TJM had a steel frame and aluminum outer if I remember right

mrkake · **Posted:** Fri Apr 03, 2009 12:08 am

tommudd wrote:

TJM had a steel frame and aluminum outer if I remember right

I would be a lot more comfortable with it if it was "skinned" with aluminum but had a steel frame. I still want recovery points and possibly a winch in the future. How much weight do you think it would save me to have it built that way?

Doesn't pipeliner have a TJM that's aluminum?

riptricket · **Posted:** Fri Apr 03, 2009 1:06 am

I agree.
STEEL IS ALWAYS BEST!

jsc7002 · **Posted:** Fri Apr 03, 2009 1:13 am

mrkake wrote:

tommudd wrote:

TJM had a steel frame and aluminum outer if I remember right

I would be a lot more comfortable with it if it was "skinned" with aluminum but had a steel frame. I still want recovery points and possibly a winch in the future. How much weight do you think it would save me to have it built that way?

Doesn't pipeliner have a TJM that's aluminum?

it would depend on how much of a frame there is, what the thickness is and what its made from.

cmohr · **Posted:** Fri Apr 03, 2009 2:10 am

Yes, Pipeliner has The TJM Alloy bar, it has indeed a steel subframe that attaches to the chassis, it allows him a winch, which, he has just had fitted. Looks good, and saves some weight, but, that steel suframe is still very strong and where most of the weight is.

Pics are in the "What did you do to you KJ today " thread, and some in the Aussie section.

Guest · **Posted:** Fri Apr 03, 2009 2:22 am

Aluminum, in general, has a higher strength to weight ratio in many applications. If it is designed to the same weight as steel it is stronger. However if designed to the same standards of strength it is lighter. But the ratio of tensile strength to weight of high grade aluminum is about the same as for good grade steel. Aluminum is generally stronger in bending applications. Aluminum does not rust as easy as steel either but it can corrode in the presence of certain chemicals like mercury. Which is why it is illegal to carry mercury on a plane.

An advantage steel has is that it will deflect more before failure making it more ductile and giving more warning before failure. Another advantage is it has a higher abrasion resistance. Aluminum will deflect less before failure and be more rigid in general. However steel will dent easier than aluminum. Also, because steel is generally heavier in a given application it has the psychological effect of being stronger to the user so that they will feel safer.

My source for much of this information is from here and has lots of useful info about steel versus aluminum in regard to designing boats. I did did some courses in steel and metallurgy when i was in engineering. Of course that was years ago and i've forgoten most of that. So forgive me for any errors. I do remember it was fun to watch steel being stretched apart on a testing machine though. :wink:

Boiler · **Posted:** Fri Apr 03, 2009 4:18 am

I design a lot of machinery out of aluminum (and steel) in my day job. I find both to be useful and adequate in their own right, but they require different considerations for application. If anyone has the mindset that "steel is better" or "aluminum is better" they are wrong, and possibly dangerous. I think an aluminum bumper is the ideal personally, due to its vastly superior weight characteristics. The real problem is deciding what the loads on bumper are. I have no doubt that I could design adequate mounting in aluminum for a winch. Typically these types of items get designed in a garage though, and saying "um, that should be strong enough" is dangerous using any material. There are things to consider though.

Properties (typical, both have alloys that are better / worse):

A36 steel
Yeild Strength (point at which it deforms permanently)>36,000 PSI
Modulus of Elasticity: 29,000,000
Weight per cubic inch: 0.284 lbs

Aluminum 6061-T6
Yeild Strength: 40,000 PSI
Modulus of Elasticity: 10,000,000
Weight per cubic inch: 0.100 lbs

Many steel items, such as "A500" tubes have a higher yeild strength, often around 45-50 PSI. Generally you can consider them to be equals, if you don't know your exact material designation. Aluminum deflects 2.9x as much as steel under a given load, as the modulus of elasticity shows. Steel weights 2.84x as much as aluminum typically.

With steel I usually size members for strength, but my aluminum structures are usually upsized by 1.3 to 1.5x, due to its elasticity, increased deflections, and welding considerations.

WELDING
One very important thing to look at when considering steel vs. aluminum is welding. The strength of heat treated (T6, T5) aluminum is greatly reduced in the "HAZ" (heat affected zone). Tensile strength is reduced 30-50% in this region "as welded". Often people will do the heat treatment after welding, or as in the case with some aluminum scissors I designed for a man-lift, we started with T4 aluminum (solution heat treated) and then welded it, followed by artificial aging heat treatment. This brought yeild strength back within 10-20% of non welded T6 material.

One thing I try to do with my designs is to avoid welding in the maximum load carrying areas as the HAZ is usually only about 3/4" wide for a 1/4" weld.

GALVANIC CORROSION
When building with stainless steel, steel, and aluminum in combination, its generally a good rule to use coatings (paint) between different metals to electrically insulate them. Also, if the more Anodic of the metals is kept much larger than the Cathodic metal, corrosion can be limited. Typically aluminum structures will not be corroded significantly by steel or stainless steel fasteners, for example. On the other hand, aluminum fasteners in a steel structure will corrode quickly. Anodizing aluminum is a good way to electrically insulate it as well (and for heat, try welding anodized aluminum...)

mrkake · **Posted:** Fri Apr 03, 2009 4:30 am

Thanks guys! This is great info. I think I will investigate this further and see where it goes.

JJsTJ · **Posted:** Fri Apr 03, 2009 9:39 am

So you replace your plastic front bumper w/ no inner structure w/ steel or aluminum. If you build it off a good foundation, say a detours backbone type setup, how important, Really, is all of the science behind the materials and super-engineered design?

Sure you want it to stand up to some abuse but nobody really Trys to hit their bumpers while wheeling and in the case of any accident at any normal street driving speed, any bumper regardless of material and design will take some damage and if it doesn't will it not transmit all remaining energy to the rest of the vehicle causing damage to other parts of the vehicle or its occupants? (parts harder to fix/replace than just a bumper)

I'm no expert in any of this nor do I pretend to know WTH I am talking about. it just seems that there is often a lot of "scientific" thought that goes into designing a bomb proof bumper when if it looks good and can withstand a drop onto a rock on a trail & support a winch then it should be just about good enough for trail use and offer plenty of vehicle protection in an accident. Too strong and it just sends impact energy to other vehicle components and passengers.

I agree that designing structural components for other applications that Can't fail, or life and limb are at stake, is important enough to consider all the ramifications of material selection and engineered design but is, or should that be, as important for a bumper skin?

Thoughts on all that drivel? :lol:

Learn me sumpin'.

USAFCOP · **Posted:** Fri Apr 03, 2009 7:33 pm

JJsKJ wrote:

Sure you want it to stand up to some abuse but nobody really Trys to hit their bumpers while wheeling .

You sure about that :roll:

tjkj2002 · **Posted:** Fri Apr 03, 2009 7:37 pm

USAFCOP wrote:

JJsKJ wrote:

Sure you want it to stand up to some abuse but nobody really Trys to hit their bumpers while wheeling .

You sure about that :roll:

Ted are local R&D guy,that's research and destruction :lol:

JJsTJ · **Posted:** Fri Apr 03, 2009 9:25 pm

USAFCOP wrote:

JJsKJ wrote:

Sure you want it to stand up to some abuse but nobody really Trys to hit their bumpers while wheeling .

You sure about that :roll:

well, cept for you.....and look at all the trouble you caused doing so. All these years later and still no bumper due to your prototype destruction!
Nice going!

tjkj2002 · **Posted:** Fri Apr 03, 2009 10:14 pm

JJsKJ wrote:

USAFCOP wrote:

JJsKJ wrote:

Sure you want it to stand up to some abuse but nobody really Trys to hit their bumpers while wheeling .

You sure about that :roll:

well, cept for you.....and look at all the trouble you caused doing so. All these years later and still no bumper due to your prototype destruction!
Nice going!

You'd be amazed at how running into a cliff face in MOAB makes the bumper line up better :wink:

mrkake · **Posted:** Fri Apr 03, 2009 10:44 pm

JJsKJ wrote:

So you replace your plastic front bumper w/ no inner structure w/ steel or aluminum. If you build it off a good foundation, say a detours backbone type setup, how important, Really, is all of the science behind the materials and super-engineered design?

Sure you want it to stand up to some abuse but nobody really Trys to hit their bumpers while wheeling and in the case of any accident at any normal street driving speed, any bumper regardless of material and design will take some damage and if it doesn't will it not transmit all remaining energy to the rest of the vehicle causing damage to other parts of the vehicle or its occupants? (parts harder to fix/replace than just a bumper)

I'm no expert in any of this nor do I pretend to know WTH I am talking about. it just seems that there is often a lot of "scientific" thought that goes into designing a bomb proof bumper when if it looks good and can withstand a drop onto a rock on a trail & support a winch then it should be just about good enough for trail use and offer plenty of vehicle protection in an accident. Too strong and it just sends impact energy to other vehicle components and passengers.

I agree that designing structural components for other applications that Can't fail, or life and limb are at stake, is important enough to consider all the ramifications of material selection and engineered design but is, or should that be, as important for a bumper skin?

Thoughts on all that drivel? :lol:

Learn me sumpin'.

Very thought provoking . . . I never even thought about the transmittal of kinetic energy to the rest of the vehicle. So it seems to me that the very best setup would be to have a strong subframe with a more malleable skin. So that it could deform and take some of the shock from an impact. If you built it right, you could just re-skin it (assuming the the subframe was still straight). I impart no scientific evidence to support this thought and make no claim to technical expertise in these areas. Just thinking out loud and interested to see other's opinions.

JJsTJ · **Posted:** Fri Apr 03, 2009 10:51 pm

^^
Ditto above. only thinking out loud to stir up conversation.

ATXKJ · **Posted:** Fri Apr 03, 2009 11:48 pm

Well - every time I look at an Aluminum wheel -
I remember there is no endurance limit for Aluminum -
if you cycle it enough times - it will fail (the nature of the crystal structure)

not true for steel.
(and not an issue for bumpers)

Boiler · **Posted:** Sat Apr 04, 2009 2:51 am

Your absolutely right JJ. All that discussion was overkill, and your also right that a bulletproof bumper is rather silly. A solid winch mount and subframe out of either material plus an aluminum skin would be ideal. Since aluminum deflects about 3x as much before reaching its yeild strength, it can absorb / cushion more of a load before deflecting permanently. Steel however would absorb more load AFTER deflecting permanently.

I think aluminum might be better when taking small bumps and wanting to not damage the bumper, and steel probably better in protecting the vehichle in larger collisions. You could probably design the aluminum to crumple effectively as well, assuming you made some provisions for it.

Personally I'd do moderately thin aluminum with a hefty winch mount to reduce weight on the suspension and call it a day.

As far as the endurance limit goes, the key is to design the load have a very large safety factor if its cyclical. You can design aluminum items to last several "lifetimes" if you can correctly quantify the load.

Warning: never "hard" anodize aluminum that is under cyclical load. The thinner the anodize layer the better. Thick layers (0.0005+) of aluminum oxide will crack when the part flexes, and will act as stress risers for the rest of the part. Thin layers (0.00002") will protect the metal and reduce the endurance "limit" of the material much less. The linked article is not a very complete study on this, but it illustrates the problems of fatigue and anodizing. I actually worked with the author to dive into this topic deeper and have more info if anyone cares in the slightest :roll:

http://www.experimentalhelo.com/Anodizing&Fatigue.pdf

JJsTJ · **Posted:** Sat Apr 04, 2009 9:09 am

dangit! more i read this thread the more i learn.....can't fit much more in this melon of mine.....ugh. :shock:

Aluminum Bumpers?

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