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 Post subject: Aluminum Frame?
PostPosted: Sat Dec 03, 2011 2:21 pm 
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Has anybody ever made a frame using Aluminum yet? If so, what size tubing and wall thickness? Also, is there any benefit to going with a round profile as opposed to square/rectangular? I am new to the forum, just getting into the planning stages of a build with my 15 year old sons. My parameters for drivetrain are simple 1.3 to 2.5, carbureted 4 cylinder, 5-speed(obviuosly) and probably a live rear axle to keep this basic. Horsepower will not exceed 120. I am a ticketed Aluminum TIG welder with almost 20 years experience so welding is not my issue, just wanted to know if it had been done yet. Thanks in advance for any answers I get and thanks to all of you that have completed builds and are willing to share your knowledge. What an awesome forum! :D

Wayne


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 Post subject: Re: Aluminum Frame?
PostPosted: Sat Dec 03, 2011 3:18 pm 
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The voice of reason
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Sure it can be done. I know of someone with an aluminum frame 7, but I don't know how their experience has been. I've never seen it.

A basic issue is understanding the fatigue life of the frame. Since there is little existing base to draw experience from you are more on your own. In general, aluminum construction of race cars and airplanes has tended to be monocoque. For one thing since they often have had aluminum skins anyway, one of the advantages has been to combine the two.

We do have at least two aluminum monocoque builds under way, kartracer and airframefixer, I believe.

Being a good aluminum welder would be a big help. Something like 1.25" x 1/8" tubing might be fine. I don't know. Would hesitate to try building the frame much lighter then the steel one though. Just being conservative here...

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 Post subject: Re: Aluminum Frame?
PostPosted: Sat Dec 03, 2011 7:49 pm 
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To the OP, oh God, here we go again.

Since you're a ticketed TIG weldor, you know about aluminum's fatigue properties and the need for proper heat treatment. Please use the search function, as this issue has been endlessly beaten to death many times.

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 Post subject: Re: Aluminum Frame?
PostPosted: Sat Dec 03, 2011 10:55 pm 
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Wayne,

Myself and many I have known have successfully used a simple formula of twice the thickness for aluminium as steel for many components over the years ie; 0.40" steel then use 0.80" aluminium. By the book you can go thinner but good engineering has to go hand in hand with it.

Heat treating isn't required as long as you use gussets or the weld area is big enough so the loads are distributed over the softer area sufficiently.

I have the offer to do all my chassis in aluminium but the Pleb's fear factor put a stop to that, a shame because my mate is successfully making plenty for his electric theme park buses - they are specifically done for anti corrosion reasons.

At the end of the day though steel will be barely heavier than the best effort you can put into an aluminium chassis, crash and last better. I was only interested for the visible effect that large aluminium tubing would offer.

That's "al - you - min - ee - um" by the way!


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 Post subject: Re: Aluminum Frame?
PostPosted: Sun Dec 04, 2011 7:39 am 
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Thanks for the input. Just exploring options at this point, as there are two bathrooms to finish in the house first, the whole happy wife happy life thing. I have checked out the links on your posts and your cars are fantastic! Sorry about the stupid question, I was so engrossed with the content of the site I never noticed the "search" function in the top right corner, I will use that from now on first before asking anything again. :oops:

"If" I were to build with aluminum I had already decided to use 1-1/2" x 1" x 1/8" for the main frame rails and try to use as much .083" sheet as possible for a sort of semi-monocoque with as many gussets as looked to be necessary. My reasoning, besides "adding lightness", has to do with how much salt they use up here in Ontario. Not that (obviously) I would drive it in the winter but the salt never completely gets off the roads until late April or beyond. Also I think it looks cool.


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 Post subject: Re: Aluminum Frame?
PostPosted: Sun Dec 04, 2011 10:12 am 
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Always Moore!
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Welcome to the board. :)

Have a look at this post - there are a few pages of discussion here that might help: viewtopic.php?f=53&t=4778&p=48993&hilit=aluminum#p48993

Keep in mind that aluminum is about 1/3 the weight of steel but its modulus of elasticity (what defines its stiffness) is also about 1/3 that of steel so they have the same stiffness to weight ratio. If you have two columns that are loaded primarily axially and one is steel and one is aluminum then you need your cross sectional area to be three times as great for aluminum as for steel to achieve the same stiffness.

Since a stiff chassis is desirable, an aluminum space frame and a steel space frame will end up weighing about the same but the aluminum could potentially cost more, will be more difficult to weld, and will require heat treating to achieve the material strength (6061-O for instance has a yield strength of 8,000 psi versus 6061-T6 which is 40,000 psi - gusseting won't make up for this). I have heard of several FSAE teams trying aluminum frames only to have them twist and distort during heat treating since the heat also stress relieves the structure.

If you do go aluminum, I would not copy the book frame. It is already pretty soft when made from steel and would be terrible in aluminum without some major redesign work.

Aluminum and salt are still a recipe for corrosion. Just instead of turning into nasty brown flakes, aluminum turns into a white powder. You will need to worry about some sort of coating.

I don't want to discourage you but there is more to it then welding some aluminum tubes together.

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 Post subject: Re: Aluminum Frame?
PostPosted: Sun Dec 04, 2011 11:33 pm 
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I say go for it ,so it cracks?...weld it up..look at everything in the world ,everything cracks up ..wings fall off planes,boats sink..the thing is,if it ever does crack..and thats a "if" ,you could easily repair and gusset that area.If we all did stuff on someone elses presumtions then I doubt 10% of these things would exist.I have lost count of the people that have looked at my half built car and can simply not grasp that it wont snap into peices ...have you looked at a morgan? They are made from wood,but it works cos someone tried it once and got away with it...dont be put off by predictions...go for it!


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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 8:20 am 
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Wayne-O,

The nice thing about a Locost frame is that for the most part, the structure has some redundancy, and you can live with cracks. On the other hand, there are bits like suspension brackets, where the crack might not be on one ear of the bracket, but might extend through the supporting tube, leading to separation of the bracket. An aluminum frame might be neat, and certainly would be attractive, but it would demand greater care in design and construction of details than is often the norm in our community.

Two telling examples come to mind. Fabric-covered light aircraft can have wood or metal supporting structure, and those supporting metal structures are usually steel spaceframes. Why not aluminum? Andrew's analysis of relative stiffness is telling. I can also think of a case where aluminum was the right choice. The lab I used to work at conducted research at sea, and developed various large deployable oceanogrphic equipments. Many were large space frames carrying numerous sensors (a 15 ft diameter array weighing in exces of 5000 lbs. Pure weight saving (we had to design these arrays within the lifting capcity of the research ship's crane) often drove the designers to aluminum, but there were few dynamic loads when the device sat on the ocean bottom, and the operational life of the space frame was probably only in the order of a few hundred hours. Any fatigue-related weaknesses of aluminum were largely irrelevant in this circumstance.

An autmobile chassis is another story.

Having said that ... there are a number of very nice aluminum boat trailers around, but they typically employ fairly large I-beam sections, for stiffness, so that working stresses are low, and where inspection of critical weld is easy (although i bet that few owners do so).

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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 8:39 am 
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Always Moore!
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Warren Nethercote wrote:
Having said that ... there are a number of very nice aluminum boat trailers around, but they typically employ fairly large I-beam sections, for stiffness, so that working stresses are low, and where inspection of critical weld is easy (although i bet that few owners do so).


On a similar note, designs that see primarily bending loads instead of axial loads may be a good application for aluminum. Since bending stiffness is determined by how much and how far material is from the neutral axis, you can take a less stiff but less dense material like aluminum, use more of it, have a larger second moment of area, and end up with a stiffer structure. A ladder chassis may have some potential in this regard.

6061-T6 isn't much weaker than hot rolled mild steel so if you only need to achieve a strength number and having a lower stiffness isn't an issue, then the material differences may not be an issue. Aluminum airplanes are not stiff but they are strong. ;)

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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 10:49 am 
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a.moore wrote:
... Aluminum airplanes are not stiff but they are strong.

And commercial aluminum-bodied aircraft are taken out of service at the fuselage end-of-life because the engineers know how long they last before work-hardening causes cracking. Locost builders have no schedule because they don't know how to calculate it - or at least I don't.

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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 12:43 pm 
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Always Moore!
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I am failing to see what that has to do with my example of aircraft structures often being designed for strength rather than stiffness. A structure designed for strength rather than stiffness will probably have higher stresses and a shorter fatigue life. If you design for stiffness, you should already have more material than is necessary and thus a longer (or potentially infinite) fatigue life. I don't have any proof to the validity of this possible internet rumor, but if Chapman era racecars were fatiguing and cracking regularly I can guarantee they were not stiff and they must have been exceeded 50% of the yield stress on a regular basis. If they were designed for a stiffness number rather than a strength number, the fatigue probably would not have been a concern since the material added to increase stiffness would have lowered the stresses.

There isn't really a design "life" on an airframe and the location and time when fatigue cracking is either known from OEM fatigue testing or operational experience is used to establish a maintenance schedule. Manufacturers establish a DSO (design service objective) which states that the aircraft will be free from major issues for that number of flight cycles and hours but it does not dictate how long the aircraft is airworthy. After that time period supplemental inspections and modifications may be required to keep the aircraft airworthy but no where does it say that an aircraft must be retired after X cycles. Issues found after the fact are typically what shortens the life of an airplane. If it is financially economical for the operator, those inspections or modifications will be done (Google "Northwest Airlines DC9") - if not the plane is retired and replaced. The winter inspections and modifications most of us do follow a similar, less structured path.

Now there are many aircraft components made from ferrous materials that do have a hard life limit that cannot be exceeded under no circumstances. I could argue that we shouldn't be building cars from steel either because of this but in reality it has more to do with the tight margins these high strength steel parts are built to and the undesirable consequences when one fails.

It is really an apples to oranges comparison. An aluminum space frame can be done but it will just take more than reading the book and cutting tubes to get there.

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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 2:49 pm 
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Quote:
aircraft structures often being designed for strength rather than stiffness.


That's because aluminum, in this application, is naturally stiff for it's strength. I think you would find the same thing with wood in an airplane. Higher strength / higher density materials do not have this. If you tried to build the plane from steel you would be worrying about buckling everywhere.

This same issue shows up in a locost frame. An advantage for aluminum is that you could use a larger diameter tube which is longer and not worry about the buckling. I see this in the frame I am designing. There are places where a tube could be more then 50 times as long as it's diamter, which is much too slender. So more bays are needed to avoid this and it adds weight.

I don't think it's true that aluminum frame has no advantage, but an aluminum copy of a steel frame would have less advantage.

I think that fabric planes use steel frames because, in general, they were designed before good aluminum was available. WWII is when that started happening. I don't think 6061 was available until the mid 50's. For most people the idea of a plane made from aluminum monocoque was more attractive then a fabric plane. So in a way that's a more appropriate use of the material, Understanding the appropriate use is really a major key to a successful design.

Airplanes are designed with a fatigue life for pressure cycles and landings. A considerable number of major incidents are traced to these issues.

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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 8:05 pm 
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Location: Fresno, California
wvandyk wrote:
Has anybody ever made a frame using Aluminum yet? If so, what size tubing and wall thickness? Also, is there any benefit to going with a round profile as opposed to square/rectangular? I am new to the forum, just getting into the planning stages of a build with my 15 year old sons. wanted to know if it had been done yet. Thanks in advance for any answers I get and thanks to all of you that have completed builds and are willing to share your knowledge. What an awesome forum! :D

Wayne


High Wayne, Monte here..

There is a really good reference regarding the use of aluminum in chassis fabrication in Chapter 7 of the HP series books "The Race Car Chassis" (#1540 Sept 2008). As it turns out Porsche did some experimenting with Aluminum as well as magnesium...... Both are obviously light weight material but the short fatigue life of weldable Aluminum alloys rendered them unreliable..... hence short lived. It's a good read. BTW:That chapter also discusses / compares square to round tubing. It's an interesting book!
Cheers,
m

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 Post subject: Re: Aluminum Frame?
PostPosted: Mon Dec 05, 2011 10:41 pm 
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Hey wvandyk, where in Ontario are ya?

Tom...


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 Post subject: Re: Aluminum Frame?
PostPosted: Tue Dec 06, 2011 6:31 am 
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Hi Tom. I live just north of Shannonville. Whereabouts are you located?


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