Is an aluminium chassis a smart choice?

[kennyrayandersen]

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Aluminum generally gets a very superficial oxidation on the surface and then the corrosion stops. So, long-term it will do better than steel. When the two are mated, like aluminum-bodied Zagato 750s, for instance, the aluminum body is intact LONG after the steel chassis has rusted away. Aluminum doesn't get brittle... It does fatigue when stressed above the endurance limit, but that can easily be designed for.

 

[kennyrayandersen]

"gudmund" here - I stand by what I said about the stiffness of aluminum...... this is what was explained to me by some of the Boeing engineers I worked with when I was working there in the late 70's/80's on the B767/B747-400 development in their mock-up department (when there still was mock-up - I never worked on assembling "real" airplanes in their factory, just the 'development work' w/engineers & some flight line 'mod' work/ & 2 'AOG' trips I got to go on). I was 'just' a mechanic, thus = 'not' knowing anything about anything.....and as far as bicycles go, am 'a bit' more versed on them - having ridden upwards of 10thou miles on numerous trips in the 80's with them being of the '6061T6 heat treated' Cannondale/Klien aluminum variation's of frames, which I can say = "Lb for Lb" they were 'no' lighter in weight than "any" of the steel frames I had ever owned or ridden = but YES, they were much "stiffer/rigid" due to the mass amount of aluminum used in their construction. I have just stated my opinion here, from what I had learned over my 68+ years of life so far and if I am "wrong"? = so be it............. take care and stay safe..........

Steel has a higher Young's modulus compared to aluminum, but that is only part of the story. Why aren't airplanes made out of mostly steel instead of mostly aluminum? The reason is that more things size the structure besides strength, or stiffness. A skin panel on an airplane, in the thickness that would satisfy the strength requirement would NOT satisfy the stability (buckling) requirement. Similarly the same can be said for the bulkheads, keel beams, fuel tank boundaries etc. Some things ARE sized by strength such as the landing gear, actuator pins, actuators, screws, and fasteners (which are mostly titanium, which has the strength of steel, but 1/2 the weight). What has to be taken into account is the E (Young's modulus), A (area), and, t (thickness) when you are running through all of the criteria that the part has to meet. In the end, due to the requirements, the best material just kind of falls out when you get done running the numbers.

I'm also a cyclist. The lightest bikes are carbon fiber hands down (my last carbon bike frame and for only weighed just over 2 lb - frame AND fork!). Carbon has the strength of steel (kinda), but weighs less than aluminum. However carbon is an expensive material and it's expensive to manufacture -- two strikes! (Interestingly enough, the 787 Fuselage is carbon wrapped around a giant mandrel!) Next is aluminum bikes which, due to stability of the cross-section (as mentioned earlier) is lighter than steel. I had a pretty expensive steel bike made from triple-butted 180 Ksi steel tube that was so thin you could squeeze it between your thumb and forefinger and watch it deflect! Yet, the steel bike was heavier than a moderately-priced aluminum bike and significantly heavier than a high-performance aluminum bike. I'm a little younger than you -- nearly 65, but I've spent nearly 40 years analyzing aerospace structures. The real key to squeezing the best performance out of any material is understanding the strengths of the material while designing around any of the shortcomings.

 

[kennyrayandersen]

Speaking strictly as an electrical engineer, I suspect most of the confusion here comes from the materials engineering definitions of concepts like "stiffness", "flexibility", and "rigidity" and the meaning of things like "Young's modulus", vs our intuitive notions of what we need, and our experience.

As my one and only, single semester, survey class in materials was 38 years ago, I won't attempt to un-muddy the waters. But, if anyone cares to post some basic definitions and explanations, it might be interesting reading, and may even help!

I mean, I surely don't want a trailer that cracks driving it down bumpy roads. Nor do I want one that avoids cracking by bending like a wet noodle, so that the cabin I've mounted on top cracks. Somehow these things get translated into technical language, and into the relative merits of steel vs aluminum (vs wood, pure gold, copper, etc.)

Tom

Young's Modulus is the initial slope of the linear portion of the stress-strain curve. When you go up the linear part of the stress-strain curve and then take the load off you come right back down the curve to the start. This slope is linear up to where the material starts to yield and starts to exhibit plastic deformation. After that happens, when you remove the load the curve comes back down parallel to the linear curve, but it is now offset because plastic deformation has occurred -- you don't get back to the same spot... The Young's modulus is a measure of the material's stiffness -- those two are equivalent. Steel has a steeper curve than aluminum - titanium is in-between.

Flexibility is not only a function of the modulus, but also a function of the geometry (design). If the section is 'weak' then it will be more flexible irrespective of the material (the stiffer material would still help the section be a bit more stiff, but if the geometry is bad, it may result in excessive compliance (springiness) regardless of the material).

As with flexibility, rigidity is determined by the stiffness, the area, and the section properties of the thing being analyzed. A 2" diameter tube compared to a 1" diameter tube of the same area, would be much stiffer in bending because it has a stiffer section, but it would have the same axial stiffness because it has the same area. So, the rigidity, or stiffness of something depends on lots of things -- not just the material, and not even just the area. As mentioned earlier, the key to getting anything to behave, or function, how you want is understanding the strengths and limitations of the material as well as how the part is loaded and how it can fail, combined with any strength and stiffness requirements.

 

[kennyrayandersen]

So a 10 pound aluminum beam cannot be more rigid than a 10 pound steel beam? I would think that 10 pounds of aluminum could be so much deeper that it would be stiffer. "pound for pound"

I 'second' what was just said...... Note, aluminum, 'pound for pound' is more rigid than steel...

Absolutely untrue.

Modulus of elasticity (Young's modulus) for Aluminum is c. 60-70 GPa, depending on alloy.

For mild steel . . . c. 200 GPa

Higher numbers better - so steel is more than 3x stiffer than aluminum.

That's why the tubing used in aluminum bicycle frames is much larger in diameter than steel tubing used in bicycle frames: it has to be.

That requirement for larger sections to achieve the same engineering properties offsets much of the [potential] wraith T savings.

Assuming that both the aluminum and the steel have exactly the same shape the steel will be stiffer; but, if they were the same shape the steel beam would weigh 3 times as much. no? So, it's very likely that for the same weight I can make (design) an aluminum beam that would be stronger and stiffer because it would be more stable for the weight and I could get a deeper beam for the same weight which would make it more stiff (beam depth is a big deal). For the same weight I get 3 times the area with aluminum. That would be fo beam bending. For axial load it would be about the same because you'd have the same E*A. The steel has 3 times the E, but the aluminum has 3 times the A -- it's a wash. But. remember theres more to the stiffness of something than just how it is loaded axially.

The stress is = P/A + Mc/I where P is the axial load, A is the area, M is the bending moment, and I is the section inertia.

It's a little more complicated than just using the E, or the A

 

[kennyrayandersen]

1. Aluminum in itself is 100% better against corrosion long term,

so long as galvanic issues are prevented where there (inevitably) is steel present as well.

The specific alloy selected is key, especially if salts are involved.


2. To get the advantage of optimizing adequate strength for **that** load at minimum weight requires design work by a knowledgeable and experienced mechanical engineer

3. In order for welding to not be the "weak link" factor, it needs to be done by skilled expert with very specialized equipment, and likely re-tempering treatments performed afterwards.

Rather than welding, best to use "lock bolts" e.g. Huck bolts for the structural joins.


So yes it is "possible" to get a far superior result to steel

just as aircon "can" be powered from battery stored energy.

But the expense involved compared to alternatives makes it difficult to justify as a practical project for a lay person DIYer.


However, if you think this

https://www.clcboats.com/shop/products/ ... 0-clc.html

would meet your needs, then it's just a matter of being willing to pay the price.

Do you see how they put all of the bolts through the center of the beams? That's because the stress in the center of the beam is low so it avoids fatigue issues that way. Having said that, the chassis could be much stronger AND lighter with an A-frame configuration. Welding aluminum isn't bad, but you do have to design around it because you lose the heat-treat and re-heat-treating is not easy with something that big! So, you can just put the welds in low stress areas that don't need the full strength of the heat-treat. What you don't want is welds anywhere near where the cabin meets the frame.

 

[kennyrayandersen]

Compass Rose is now 12 years old, the frame is all aluminum, unfortunately they did not use heavy enough aluminum for the tongue. The first one they built (and we owned) failed at the point where it goes under the body fortunately the second owner was going slowly and had it repaired. I had ours reinforced but was still concerned with too much flexing. During rebuild we moved the axel forward and again reinforced the tongue and no more flex.

The problem isn't the material, or even likely the thickness of the material, but the design (since a number of miles got put on the frame in the first place). There are welds right smack dab in the middle of the highest stress area of the tongue. So, you introduce both a stress concentration (where cracks like to start) and you lose most of the heat-treat of the aluminum in the weld heat affected zone. I would let the tongue pass under the first cross member and attach it with an angle which that had two fasteners in the frame and two in the tongue, but all of them going through the center of the respective beams. That way you don't lose the heat-treat, and you don't have a fastener going through a high stress area. Do that and the frame, if sized properly, will never have a fatigue failure. I think even better is to combine the frame and tongue into an A-frame, which potentially solves both of those problems.

 

[ABQ]

Hello KennyRayAndersen, Thank You for the great information & insights, especially highlighting germane equations & principles. Best Regards, ABQ

 

[kennyrayandersen]

Hello KennyRayAndersen, Thank You for the great information & insights, especially highlighting germane equations & principles. Best Regards, ABQ

I hope my input isn't too much. I'm sure on one hand it might sound like I'm a know-it-all, but structural analysis is what I do for a day job and I've been doing it for nearly 40 years. I'm a little slow, but you're bound to learn a thing or two swinging at it for 40 years! I'm hoping to dispel a few myths, had help people make there trailers safer, stronger and lighter -- all of which can be done simultaneously if it's done right. It's been my pleasure to work with some smart people and I've have the opportunity to work with aluminum, composites, and steel. And now I'm working on an airplane which is wood as the primary structure!

In the end the right material is the right material. And each design should take advantage of each material, the design goals, and cost. Everything in the end is a collection of compromises that satisfy the problem your are trying to solve. Aluminum cost is higher, so in my opinion it should be used when saving weight is a priority in the design. Steel is cheaper, and compared to the aluminum available to the home builder, stronger, but the aluminum will be lighter for the same strength. The issue with aluminum is designing for fatigue considerations as well as making sure any welds are in low stress areas. By following good design practices there is no reason an aluminum trailer should last a lifetime!

 

[Tony Latham]

The issue with aluminum is designing for fatigue considerations as well as making sure any welds are in low stress areas. By following good design practices there is no reason an aluminum trailer should last a lifetime!

Any shade-tree welder can design and weld a steel trailer with a bit of help from a mentor. But that same guy can't design or weld an aluminum trailer that will last.

That's the problem with the concept of a DIY aluminum trailer chassis--as I see it.

Tony

 

[kennyrayandersen]

The issue with aluminum is designing for fatigue considerations as well as making sure any welds are in low stress areas. By following good design practices there is no reason an aluminum trailer should last a lifetime!

Any shade-tree welder can design and weld a steel trailer with a bit of help from a mentor. But that same guy can't design or weld an aluminum trailer that will last.

That's the problem with the concept of a DIY aluminum trailer chassis--as I see it.

Tony

I'm trying to help that guy up his game. I wouldn't presume anyone incapable of learning something new -- it'd be like calling them stupid or something...

I also won't accuse anyone of being myopic... That's what these forums are for -- post some ideas, and get help from people that have the knowledge and the experience to help those with a little less 'training'. It's not that complicated - if you want to make a trailer chassis out of aluminum:

1) Don't put holes in the top or bottom of your aluminum members (put them in the middle of the beam)
2) Don't weld in high stress areas i.e. where the tongue meets the cabin (or anywhere near it)
3) You can weld in areas near the coupler, or near the axle attach where the bending is low (see this thread https://www.tnttt.com/viewtopic.php?f=35&t=75602)

It's entirely possible to make an inadequate steel trailer...