View Full Version : Aluminum space frame chassis

01-08-2011, 02:49 PM
I am new to this site and what a mine of information!

Tried to find something about space frame aluminum chassis but nothing really. The reason being is that I wondered if it is possible to build an all aluminum Cobra. I think Ferrari build an aluminum space frame chassis called the 'bird cage' a few decades ago. I've seen a Cobra steel space frame chassis and figured an aluminum one might be possible to manufacture if properly designed for stress and stiffness and the right square tubing could be found. Squareness enhances strength and stiffness, right? It cant be of large dimensions otherwise the body will sit too high of the ground and I want to be as low as possible seeing that I am a tall bloke. And also the space underneath the door don't allow for large tubing as oppose to what Lotus uses in their designs.

The forces that would normally be at work in the chassis of a family car (where a degree of flexibility is wanted to negotiate most road conditions) differ in that engine torque, cornering etc, are the factors that affect the necessary stiffness of a sport car chassis.

Which leads me to the question if it is at all possible to make a small diameter square tubing aluminum space frame chassis that would be able to house the power and torque of a big block engine and provide a safe platform from which to steer the car out of tight corners at high speed. Or is it a question only engineers would be able to answer?

If someone can help or guide me in the right direction it would be greatly appreciated.



James(Western Canada)
01-08-2011, 03:52 PM
Kirkham did a billet aluminum chassis....but you need bottomless pockets for this type of stuff.

Porshe did an aluminum chassis for one model of their race cars (917?)....they had a fitting on the chassis which they used to add air pressure to the chassis, and a guage to monitor air pressure, a pressure loss indicated a fracture:eek:
But I personally don't think an aluminum chassis on a street vehicle which sees regular use is going to have a very long life span, and is really not safe or practical. (and yes, there can be exceptions to this rule but none of them will be inexpensive!)
A well thought out chassis constructed from 4130 can be built that will be relatively light, and have an infinitely longer fatigue life than an aluminum one..........

If you are talking about an application for use other than on public roads, then there are many choices that would likely be as light as aluminum, and still be less $$ and have longer fatigue lives...

Just my $0.02CDN;)


anders nørgaard
01-08-2011, 04:15 PM
Welcome to MM Kabous :)

Audi R8 and 2010 Jaguar XJ are all aluminum.

01-09-2011, 09:32 AM
The "birdcage" was a steel tube frame chassis made by Maseratti in the 60's The frame had so many small tubes that they said that a bird couldn't get out. There have been very few tube frames made with Aluminum that were succesful since weldable alloys tend to not have the stiffness needed, but it can be done.


01-09-2011, 12:37 PM
If you use something like 6061 you will need to heat treat the bird cage after welding it together. The strength at the welds is nearly condition zero, almost fully annealed. I was at a "Mini Baja" competition a few years ago where the Air Force Academy entered a beautiful aluminum framed car. It fell apart on the first lap. Now, aluminum sheet 2024 or 7075 T6 monocoque chassis glued and rivited together would be interesting.

01-09-2011, 01:31 PM
Personal opinion only...

If you build one you will likely find out that it wasn't worth the effort, what weight you might technically save will be mostly eaten up in the extra bracing your going to need to keep it together.

If you do complete it your going to have to be real careful to continually inspect it for cracking.

Other than the "coolness factor" of the aluminum, steel has much better overall properties for frame construction.

Motorcycles get away with aluminum frames by using really complicated castings and internal bracing and have many years to refine it to an art.

You can do it but the view won't be worth the climb.....more than likely

Just my opinion not to be confused in any way with a true and actual fact.


01-09-2011, 04:12 PM
4130 is not a flexable steel for long term use on the street, although many have used it for street cars. Aluninium you will have to really watch the alloy. MAny that are good for this type use are very hard to bend. Jack Roush (I think it was him) made a funny car frame with aluminium tubing and it takes a great amount of pressure to bend and of course you can only TIG weld this material. A normal tube bender can't do it. It also does not flex as good as steel. Most people end up using mild steel (lowest price) or DOM steel (Higher price). I do remember one T type street roadster from C channel aluminium and then used angle pieces at the cornors and bolted it together. It was built by one of the writers for Hot Rod Magazine but never did hear how it held up. Good Luck. Fred26T

01-09-2011, 05:34 PM
Thanks for the info.

Had a glimpse at the Kirkham alu chassis which is impressive in its exploded and finished form. It looks like they cracked the stress problems and the limitations of aluminum with their design. Have not read the book which can be downloaded for free but will get to it.

My aim was to make a chassis as light as possible without compromising on strength and durability too much. However, the truth seems to be, apart from the prohibitive costs of billited aluminum, there will always remain a question about the durability of it with so many factors involved. Also, to strengthen the tubes mean weight gain which defeats the object.

Ok, so the ‘birdcage’ was of steel and made by Maserati. Thanks Bruce. I remember the myriad of struts in the picture a few years ago.

I think our grades of steel in South Africa differs in classification from the States. If it is possible, can someone post the metal make-up of 4130 so I can find the closest match possible (or what other suitable alloys are recommended?)
Well, that then settles the aluminum space frame question but I have another one.
The torque from the big block engine creates a lot of stress on the chassis, especially in the floor pan area, I would imagine (which is the flattest part in a space frame).

Some designs concentrate on strengthening the tunnel as the door sills prohibits a better configuration. Lotus has quite high door sills which allows them very large diameter aluminum rectangular tubing (which they glue - not weld! -to the rest of the bonded space frame) to solve that problem. What I don’t like about the stronger tunnel is that it is too bulky and sits very square and high between the two seats which looks decidedly uncool.

I wondered if it isn't possible to have a strong tube running as high as possible through the doors with a heavy duty and tight fitting door lock which would act as part of the space frame once the doors are closed. That way the frame should be stronger because the floor pan area is now supported from much higher. Do anyone know what kind of fast closing lock/fitting could be used? Must be able to eliminate sideways and forward/backwards motion to be effective.


01-09-2011, 08:36 PM
I found this by googling 4130:

SAE 4130 Chemical composition: C=0.30%, Mn=0.5%, Mo=0.20%, Cr=1.0%

What do you expect to gain by using 4130 instead of plain mild steel tube?

The stiffness will be the same with both. The 4130 will be stronger, but you shouldn't design so close tot he breaking point anyway.


01-09-2011, 08:37 PM
Herb Adams has an optional Aluminum Backbone chassis for his Cobra replica:


01-30-2011, 09:43 PM
From the very little I know, I'd stay away from it. But here are some thoughts.

Boyd Coddington did that all aluminum PU. I believe it had an aluminum chassis as well. Then there was the Alumacoupe also.

Aluminum chassis that I do know about are in Featherlite stock and horse trailers. Great trailers, except when they get fatigued. Aluminum work hardens, then you get failures. Well meaning people weld them back together, and can end up complicating the issues.


Dick Bear
02-05-2011, 11:24 AM
Some may remember the verbal wars that took place several years ago on metalmeet between myself and one other builder of custom design/built cars. At that time I was convinced that such a frame was both possible and desireable but after further investigation I have concluded that making a space-age frame of aluminum is both unwise and of little value in weight reduction because of the fatigue factors associated with welded aluminum tube, round or square.

If an aluminum frame structure is the only system that is of interest to you then I would suggest that you explore the historic development of an aluminum monocoque structure rather than simply replacing steel with aluminum tubing to create a space-age frame design.

An aluminum monocoque design doesn't come to life without some risks as well but the failure factors are less in a well constructed monocoque than in a space-age aluminum design. This is due to the fact that when fabricating with aluminum you are dealing with extruded materials that have an inherent brittle characteristic to them. It is the brittleness of the material (tube) and the similar characteristic at the weld-point that are the limiting actors when dealing with aluminum. To over come this, added material is required and that additional material results in a minimal amount of weight reduction for an aluminum structure of tubes over that of steel.

Therefore, I'd have to question anyones decision to build the primary structure for a dependable vehicle using aluminum unless you have both the facilities and the budget to perform critical testing of individual components within the structure and are willing to undertake the remaking of components based on those test results. If however, you have the time, budget and testing facilities available I'd say go it but make it a monocoque structure rather than a space-age frame.

Good luck in whatever you decide to do!

Dick Bear

02-26-2011, 08:01 PM
As I am understanding you are looking to build a better frame for a Cobra. Personally I would go and build me a new car from the ground up before I would try and improve on the frame Ford did on the 427 style cars. Simple Hella strong and light. I used to work for a couple of kit car companys making Cobra's and I know a number of people have tried most have failed. but the group that has had the most success has been the Kirkam folks (in my opinion).8-)

MG David
02-28-2011, 06:02 AM
If you look at the aluminium cars made by Lotus, Jaguar and Audi they are all bonded sheet structures. So no heat affected zones around the welds. Also Aluminium scores over steel in stiffness versus weight rather than tinsile strength versus weight. That advantage is maximised in the formed sheet structures.

Rick (madera)
02-28-2011, 07:49 AM
My brother bought a 3 car ramp trailer that was all aluminum. It had al. diamond plate on the sides.He looked at it in the evening and bought it. brought home with him and called me the next day. The welds on frame under it were all broken .:eek: It took me 2 days to reweld the trailer.
So I think I would stay away from the the aluminum frame:rolleyes:

02-28-2011, 12:38 PM
If you look at the aluminium cars made by Lotus, Jaguar and Audi they are all bonded sheet structures. So no heat affected zones around the welds. Also Aluminium scores over steel in stiffness versus weight rather than tinsile strength versus weight. That advantage is maximised in the formed sheet structures.

Actually, while Lotus does make a bonded chassis, Audi, Jaguar, Aston Martin, Chevys' Corvette, and Fords' GT all use welded aluminum chassis construction.

Watch this from about the 5:00 mark,


and here is an article about the Ford GT chassis,


While I understand the limitations of welded aluminum trailers, there is one major difference...they are poorly designed. If you take the same design you use for steel and apply it to aluminum you will run into problems every time because steel and aluminum behave differently. Aluminum does not like to be flexed, it doesn't "spring" like steel does, but if you design around this by triangulating the joints so they don't bend, aluminum can make a very strong structure.