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4 Apr 2009, 15:41 (Ref:2433335) | #1 | |
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Aluminium Monocoque - Tub design
The sheetmetal moncoque flew over from aviation, it was first adapted in to the Lotus 25 formula 1 car. It is still used today in lower prototype and formulae classes.
However, it is quite hard to find information on the particular subject when you are trying to design one. What is the benefit over a sheetmetal covered tubular spaceframe for example? I hope we can discuss the advantages, disadvantages and design principles of this very interesting subject. The Lotus 25 monocoque - drawing The Lotus 25 monocoque The famous Porsche 962 moncoque |
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8 Apr 2009, 14:09 (Ref:2436723) | #2 | |
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I've asked some questions like this before, and I think some of the answers are applicable here:
Aluminum is not stiff, so to take full advantage of it's light weight requires using large structures, like the side pods on the 962, to use geometry to overcome the flexibility of the material. The geometric benefits only hold as long as there is no local deformation to the material (dents), which, with a flexible material used in large structures, is quite likely. Safety wise that means it can all go wrong in a hurry: That 962, before they started using honeycomb, would fold like a lawn chair if it hit a dog at 27 kph. It is boocoo expensive compared to a steel-tube space frame, and expensive to repair if something goes wrong. It work hardens where steel wouldn't. Steel can flex to a a certain point an infinite number of times, while aluminum will eventually fail given any degree of flex. I hate to sound discouraging, and have tried to find pros to outweight cons, but it seems like to be fast and safe would require a great deal of money and skill. That said, if you have or have access to those, go for it! |
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8 Apr 2009, 17:06 (Ref:2436850) | #3 | ||
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just out of intrest isnt the Elise and the new Evora a Aluminium Monocoque chassis ? and they are pretty stiff? I wonder how these compare to the 962 chassis ?
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15 Apr 2009, 03:18 (Ref:2440984) | #4 | ||
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Quote:
The Elise and Evora are not monocoque design. They use aluminum extrusions bonded and fastened together to make a spaceframe. They are not stressed-skin design like the 962 and others shown here. The loadings and layout are very different for the Elise. Cheers |
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9 Apr 2009, 02:55 (Ref:2437092) | #5 | |
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Fair point Wooshy. I may have spoken too soon and in too narrow a fashion; I was thinking in regard to a DIY job.
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9 Apr 2009, 16:50 (Ref:2437546) | #6 | |
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Thanks for the replies.
The elastic modulus of aluminium is more or less a third of steel, however so is the density. When you divide the elastic modulus by the density you will get the specific modulus [Nmm^3/kgmm^2.] Due to the difference of a factor three the specific modulus is the same for steel and aluminium. It does mean that the actual steel sheet should be a third in thickness. Possibly the biggest advantage of aluminium is its corossion resistance. As mentioned earlier by chapmanite deformationes due to large unsuported sheets should be avoided. The deformation can be reduced by the well-known circle structures, altough not fully. The best would be to add some sort of fill-material, which doesn't nessecairely have to be a honeycomb structurized aluminium. Suport structures could be added aswell. The Elise has an aluminium chassis as well. It features two large side beams, these are extruded and slightly bend. A nice feature is the use of glue to save weight. In terms of a DIY job I would recommend a tubular spaceframe. I think an aluminium tub would require quite a bit more research to make it work. Let the discussion continue. |
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21 Apr 2009, 12:44 (Ref:2446288) | #7 | |
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Could you elaborate on the circle structures just-right? Are you referring to something like the holes cut in the tub of a Miura?
The similarity of specific modulus isn't a deal-breaker, if I understand the mechanics of it: If you have a sheet of 1mm aluminum and of 1mm steel, the steel will be three times heavier and three times stiffer. A 2mm piece of aluminum will two thirds the weight of the 1mm steel, but also 1/3 more stiff. Regarding corrosion, the higher strength alloys are not particularly corrosion resistant. |
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21 Apr 2009, 14:55 (Ref:2446363) | #8 | ||
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Quote:
Thanks to all for your contributions, highly appreciated. The holes I'm referring to are more or less like the above. They reduce the dents that occur when using unsupported sheets, they also add rigidity. However an internally supporting structure like honeycomb or something similar is way better. The point regarding the specific modulus isn't that refreshing. I am just pointing out that a sheet of 3mm aluminium has the same weight and stiffness of an 1 mm steel sheet. However when looking at availability of small sizes I would favour aluminium over steel. Not to mention the procentual impact of corossion on the effective surface. Both aluminium and stainless steel can form a thin film of corrosion products also known as passivation. High strength steel and aluminium is a whole new chapter and not all that applicapable to sheet moncoques other than the bulkhead. High strength alloys or super alloys are more used for machined parts like uprights and sort. I would reckon that using clever box sections and profiles is way more effective than using costly materials. Does anybody have pictures of the various box profiles and connections used within these monocoques? We could maybe start something about design strategies? |
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19 Apr 2009, 00:19 (Ref:2444166) | #9 | |
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i have some pictures from when i used to work at a company specialising in fabricating LOLA sports cars..
the T280`s 240`s and a few others were based upon a aluminium monocique.. i just need to find a scanner though as these were taken in a pre-digital age.. i do rememeber the suspension pick ups were all fabricated from sheet steel and bonded/rivited to the ally tub... |
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22 Apr 2009, 00:59 (Ref:2446696) | #10 | |
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I am under the understanding that stiffness goes up with the square of thickness, so that a 3mm sheet of ally would be nine times stiffer than a 1mm sheet, and three times stiffer than 1mm steel at the same weight. Is this not true for sheet? (Oh boy would I have to back and do some new math!)
What are those pieces in your picture a part of? They look like they could be stiffeners in the side pods of that 25 above. |
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22 Apr 2009, 18:21 (Ref:2447205) | #11 | |
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What you are referring to is the polar moment of inertia I believe. It is the resistance against deformation due to torsion. The formula for a closed profile is:
J = (G * 4 * A_0^2) / sum (b_d/t_d) G = the shear modulus A_0 = the closed in surface area b = distance of the centre of the element to the centre of gravity t = the thickness of the element Let's apply this on a rectangle profile of which the outer dimensions are as following: Height: 200 mm Width: 50 mm Thickness: aluminium 3mm, steel 1 mm A_0_al = (200-2*3)*(50-2*3) = 8536 mm^2 A_0_fe = (200-2*1)*(50-2*1) = 9504 mm^2 sum (b_al/t_al) = (197/3+47/3+197/3+47/3) = 162.6667 sum (b_al/t_al) = (199/1+49/1+199+49) = 496 J_al = G_al * 179.172*10^4 mm^4 J_fe = G_fe * 72.844 *10^4 mm^4 J_al = 25.5 * 10^3 * 179.172 * 10^4 = 4.5688 * 10^10 mm^4 J_fe = 79.3 * 10^3 * 72.844 * 10^4 = 5.7765 * 10^10 mm^4 m = rho * A * l m_al = 2702 * 10^-6 * (A - A_0) = 2702 * 10^-6 * (200*50 - 194*44) = 3.956 * l gram m_fe = 7850 * 10^-6 * (A - A_0) = 7850 * 10^-6 * (200*50 - 198*48) = 3.894 * l gram As we can see in this small calculation steel is actually better. The chosen profile is lighter and has more stiffness. The stiffness I referred to earlier in this topic is of a different nature. It is based on stiffness defined by Hooke's law. delta L = F*L/E*A delta L = length change F = force L = length E = young's modulus A = surface area Force and length are the same for both steel and aluminium. The variables in the denominator are not the same for the materials. For example if E is a third then, A should three times larger to have the same outcome. You can see that a specific profile is not important when the loading is normal to the surface, it is based on surface area. If the surface area is three times larger then the volume of the beam is three times larger. The density is about a third which results in equal mass. Same stiffness (when using normal to plane - loading) Same weight Different Volume Hope this helps. |
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