Is an aluminium chassis a smart choice?

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Re: Is an aluminium chassis a smart choice?

Postby nbcarey » Tue Jul 26, 2022 2:23 am

gudmund wrote: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.
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Re: Is an aluminium chassis a smart choice?

Postby QueticoBill » Tue Jul 26, 2022 6:11 am

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"
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Re: Is an aluminium chassis a smart choice?

Postby John61CT » Thu Aug 04, 2022 3:54 pm

Yes, you need "bigger" members to get the same strength.

So what?

Yes welding to be avoided as much as possible. Stainless steel fittings where steel is required, ideally all materials marine grade if you go anywhere near the coast, or snow country salted roads.

With good design, using the same weight of materials you will get much higher strength.

Yes higher cost, but little to no corrosion, so longevity without as much periodic maintenance.

To me that's worth paying more.

And it looks cool.

Obviously not for those who need cheap and want easy, makes it a much more costly and complex project.
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Re: Is an aluminium chassis a smart choice?

Postby gudmund » Thu Aug 04, 2022 11:12 pm

"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..........
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Re: Is an aluminium chassis a smart choice?

Postby Tom&Shelly » Fri Aug 05, 2022 5:10 am

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! :D

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.)

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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Mon Mar 06, 2023 9:27 am

Whine
I wrote a big thing, and then I remember this site had a special feature where you get logged out and lose everything -- oh, how I missed that feature! So, I'll try and do it again...

Background
This is really a loaded question, and there was so much misinformation that I felt like I had to chime in. Background: I'm an aerospace structural analyst with 38 years of experience working with and analyzing flight vehicle structure (wide-body, fighters, helicopters, and tilt-rotors). I have utilized aluminum, steel, titanium, fiberglass, and Gr/Ep as well as various sandwich panels. My background is primarily in static analysis though I have done some fatigue analysis and generally understand damage tolerance. I also have some experience in welded aluminum parts, though we don't weld much, and never for primary structure -- but on occasion for systems items like ducts, reservoirs, small tanks etc.

What material?
Many of the materials we regularly use aren't readily available to hobbyists because of cost. The regular hobbyist consumer would typically use Al 6061-T6 as the high end, and for us 6061 T-3 or T-4 is the absolute least capable material we would use with most Al 6061 used would be in the T-6 condition -- anything less than that is decorative! Though it's true Steel can be stronger than aluminum, it's frequently not as most folks out there are working with mild steel. Mild steel has an ultimate strength of around 58 Ksi and a yield strength of 36 Ksi, whereas Al 6061 T-6 has an ultimate allowable of 41 Ksi. and a yield strength of 37 Ksi (Fcy). So, the least capable aluminum we use has a better yield and nearly the same ultimate strength as mild steel. Keep in mind that for most of the aluminum we use (Al 7050, 7075, 2024) they are stronger than mild steel and are only 1/3 the weight! Yes, aluminum has less stiffness, but unless your problem is stiffness or deflection driven, the stiffness doesn't enter into the strength equation (it does come into the deflection equation). When we use steel in aerospace, we use the good stuff, and sometimes the really good stuff, and none of it comes cheap! So, when we are comparing steel to aluminum, we are generally comparing mild steel against Al 6061 T-6 and there is really no comparison on which will come out significantly lighter (I could talk about bikes all day long, but suffice it to say that aluminum wins hands down in the bang-for-the-buck category (yet I ride Gr/Ep because I can!))

Welding
Al 6061 can be welded, whereas the other aluminums I listed cannot be. There are primarily 4 aluminums that can be welded Al 6013, 6061, 5052, and 7005, but none are much available for the hobbyist other than Al 6061, so I'll focus on that one. When you weld aluminum, you do end up with a heat-affected zone (1/4”-1” from the weld) wherein you lose your T-6 temper; however, Al 6061 has a quality that it age-hardens at room temperature; so, after a couple of weeks it goes from the 'W' (post welded, which is a bit better than the 'O' condition) to nearly the T-4 condition. If you can re-heat-treat you can actually get it back to the T-6 condition, which is what is frequently done. However, for a trailer chassis, it would be a real challenge to find an oven big enough to put it in to re-heat-treat; so, for practical purposes, we can use the allowables from the T-4 condition to make our strength calculations right next to the weld. Al 6061 T-4 has an ultimate strength of 26 Ksi, and a yield strength of 16 Ksi (better than wood at least!). Though that isn't great, if we are clever about it (and we are), we can put the weld areas outside of the high stress areas so that the localized loss of heat-treat doesn't affect the overall design integrity.

Fatigue
Yes, for aluminum, fatigue has to be considered, but it isn’t a show stopper. Good design practice avoids high stress concentrations (Kt), and we can also choose to locate fasteners in such a way as to avoid putting them in location where the fatigue will be a problem. Then if we set the stress level below a certain threshold, we can end up with something that pretty much has an infinite life.

Aluminum chassis design coming
I’m not sure this is the place for it, or whether I should start another thread, but I’m in the process of laying out an aluminum trailer chassis for an ultralight teardrop to tow behind a micro car like a 1963 FIAT 500, or a 2004 smart fortwo (I have one of each).

here's the link to the lightweight aluminum chassis design: https://www.tnttt.com/viewtopic.php?f=35&t=75602#p1299706
Last edited by kennyrayandersen on Fri May 05, 2023 12:26 pm, edited 2 times in total.
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Re: Is an aluminium chassis a smart choice?

Postby Tom&Shelly » Mon Mar 06, 2023 9:42 am

kennyrayandersen wrote:Aluminum chassis design coming...I’m in the process of laying out an aluminum trailer chassis for an ultralight teardrop to tow behind a micro car like a 1963 FIAT 500, or a 2004 smart fortwo (I have one of each).


Looking forward to reading it! :thumbsup:

(I Especially enjoy reading about good engineering put into practice!)

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Re: Is an aluminium chassis a smart choice?

Postby aggie79 » Thu Mar 16, 2023 3:16 pm

Tom&Shelly wrote:
kennyrayandersen wrote:Aluminum chassis design coming...I’m in the process of laying out an aluminum trailer chassis for an ultralight teardrop to tow behind a micro car like a 1963 FIAT 500, or a 2004 smart fortwo (I have one of each).


Looking forward to reading it! :thumbsup:

(I Especially enjoy reading about good engineering put into practice!)

Tom


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Re: Is an aluminium chassis a smart choice?

Postby twisted lines » Thu Mar 16, 2023 6:30 pm

Too Much of my first one was Alu, not this one.
Racking up; And Rapin foam
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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Fri Mar 17, 2023 12:23 am

I ran through a sample set of calculations for a lightweight trailer (A-frame-type), but I'm in the airport heading back to the States for a week so I'll try and get it posted when I get back. I think if you can do math with exponents you'll be able to follow it alright and I'll be willing to answer any questions on how to do it. This isn't looking up tables, this is how to calculate the trailer frame strength directly. Sorry for the delay, but it's was tax time plus I had some other urgent paperwork to fill out. I'm clear so, so should be able to get to it shortly. Thanks for the patience.
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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Fri May 05, 2023 12:01 pm

TimC wrote:---


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.
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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Fri May 05, 2023 12:23 pm

gudmund wrote:"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.
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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Fri May 05, 2023 12:42 pm

Tom&Shelly wrote: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! :D

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.
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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Fri May 05, 2023 12:51 pm

QueticoBill wrote: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"


nbcarey wrote:
gudmund wrote: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
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Re: Is an aluminium chassis a smart choice?

Postby kennyrayandersen » Fri May 05, 2023 12:59 pm

John61CT wrote: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.
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