dt21jb
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| posted on 14/3/10 at 04:50 PM |
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aluminium chassis
anybody made the chassis out of alulinium
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nitram38
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| posted on 14/3/10 at 04:59 PM |
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OMG! not again!
Do a search, the conclusion is it's not viable
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Ben_Copeland
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| posted on 14/3/10 at 05:00 PM |
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Please search it's been discussed so many times!
Ben
Locost Map on Google Maps
Z20LET Astra Turbo, into a Haynes
Roadster
Enter Your Details Here
http://www.facebook.com/EquinoxProducts for all your bodywork needs!
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scootz
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| posted on 14/3/10 at 05:19 PM |
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Plenty, but they tend to be a monocoque construction or ali honeycomb planks.
The thinking is that an Ali space-frame is not a viable option as the metal thickness required to prevent fatigue would negate the weight saving of
ali over steel (and cost more!).
HTH!
It's Evolution Baby!
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mangogrooveworkshop
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| posted on 14/3/10 at 08:48 PM |
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stolen from the blat chat
Aluminium Chassis
The subject of Aluminium and its Alloys is quite complex and a correctly designed chassis with the correct alloy selection and proven welds may well
be feasible but I am convinced that this concept would be beyond the scope of the average amateur in a "shed " environment. The main
problem with Aluminium is that it has relatively low fatigue strength and unlike most ferritic steels it does not have an "endurance
limit". The endurance limit is the stress at which fatigue failure will never occur. Aluminium will always fatigue even if stresses are very
low. In practice it may take millions of cycles for this to occur and it may not be of any practical significance but you would need to consider the
fatigue loading of every critical chassis joint and pick-up point to be confident. It would be practical to run an FEA analysis of the structure for
an Aircraft but the cost of analysing a Locost chassis would be too high.
There are some other points to consider:
1. Material Selection
I see many adverts that state "manufactured from aircraft alloys" -- What the bloody hell does this mean?
Some aircraft parts are made from relatively weak alloys and other parts are made from relatively strong alloys. They will all have
"aircraft specification" and a release procedure if they are to fly. Just because an alloy is used for an aircraft part doesn't mean
it will be suitable for every automotive application.
The common alloys used for aircraft components are 2000, 6000 and 7000 series alloys (Names and designations keep changing and I may be a little
out of date). Most of these alloys are quite strong and gain their properties by using a technique known as "precipitation" or
"age" hardening. In general terms none of these alloys should be welded unless the manufacturer can provide a specific data sheet with all
details of techniques and current densities etc, etc. It is possible to join them relatively easily using a good quality TIG welder but the material
properties close to the weld will be damaged and a considerable amount of strength may be lost. This wouldn't be great as the joints in a
spaceframe are used to transfer load.
The only alloys I would consider welding would be Aluminium/Magnesium/Silicon? alloys and these materials gain their strength from "solid
solution strengthening". They are weaker than the heat treatable alloys but more difficult to damage by welding.
From memory the tensile strength of these alloys is about 60% that of the heat treatable types.
2. The strength of the heat treatable alloy is similar to the mild steel that Lotus 7 and Locost chassis are made. Corrosion Resistance
We all believe that Aluminium has excellent corrosion resistance. This is due to our experience with very weak sheet materials used for body
work or anodised parts. The type of alloy used for anodising is traditionally HE9. This is easy to extrude and easy to anodise but it is very weak.
Its main use is in kitchen and bathroom fittings.
The high strength heat treatable alloys do not like to anodised. Even if you could anodise a complete chassis after fabrication it would not be
successful and you could not successfully weld after anodising. Anodising produces a relatively thick oxide layer on the surface of aluminium and it
is this oxide which seals the surface and gives Aluminium its excellent corrosion resistance.
Low Alloy sheet materials also have this oxide film. (N.B. Sapphire is an Aluminium Oxide) The crystal size of aluminium and aluminium oxide are
almost identical and this is why the oxide seals and protects so well. Iron oxide has a very different size crystal to iron and so it continually
cracks as it grows and doesn't give good protection. The alloy additions used in high strength heat treatable alloys (Copper, Zinc Magnesium
etc) prevent the formation of this oxide layer over the complete surface and if you look at the surface with a microscope it would appear to have
pinholes all over it. This produces a large anode surface and a small cathode which is great for corrosion ! None of these alloys are generally
recommended in marine or saline environments and they can rot badly in the presence of salt. ( The Harrier GR3 is a very different aircraft to the Sea
Harrier and nearly all exposed components are made of different and more expensive materials to eliminate corrosion)
3. Another problem could be stress corrosion fatigue. When a fatigue crack does start it exposes a clean new metallic surface which is very, very
active and is looking to become passive. As oxygen is stripped from any liquid present in the crack, the pH of the liquid increases and causes even
more corrosion, etc, etc. This type of fatigue is extremely dangerous and unpredictable and causes many serious accidents every year, It really needs
thinking about in a road car environment. Strength vs Stiffness vs Weight
IMHO the "stressed skin" design of the 7 Chassis is a load of Bollocks. Unless the skin is attached whilst pre-loaded it cannot
provide any stiffening until sufficient elastic deformation has take place to allow the skin to be stressed. This may have occurred on Series 2 cars
which had many tubes missing but is less significant with Caterhams and Locosts which have more tubes.
The floor panel may improve the bending stiffness but I am to convinced that the rest do too much.
By copying the steel chassis in Aluminium (to an identical design) the weight will be reduced to one third. This seems good but as a proportion
of the complete car it is quite small.
As the G and E of Aluminium is also one third of steel all of the chassis deflections and deformations will increase by a factor of three. All
Moments used in the stiffness and deformation calculations are effectively multiplied by E - The Young's Modulus of the material) I think that
this could result in dodgey or uncomfortable handling.
If the tube wall thickness are increased by three time the deflections will be the same as a steel chassis and so will be the weight. It seems
like an exercise in futility to me!
The only way to really use aluminium is to re-design the chassis to suit the different characteristics.
The Locost uses 16swg tube in places where a real 7 uses 18swg. Using Lotus sized tube will reduce weight by about 25% without affecting
integrity, a detailed study of the extra tubes that have been introduced may allow a few others to be removed and weight eliminated.
Conclusions
Unless the whole concept is studied and analysed in detail I feel that an aluminium copy of a 7 chassis could be bloody dangerous. The impact loading
on pick ups would need to be modelled and design changes made. Some math should be used instead of "it looks OK". I think this would be an
interesting but time consuming process but if you are keen I would help with material data and mathematical models. The welding control that would be
required is very advanced and quite costly and I would advise a manual TIG system with an HF supply and a "crater elimination" unit. The
correct shielding gas would depend on final alloy selection. Other thoughts:
* Very few Aluminium spaceframes exist. Even F1 cars of the sixties used the same tube as the 7 and a brazed structure.
* Aluminium wasn't common until the introduction of monocoque chassis with the Lotus 25.
Chris Flavell
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kb58
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| posted on 14/3/10 at 09:00 PM |
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I think I'll sit this one out this time.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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minitici
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| posted on 14/3/10 at 09:05 PM |
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am oot!
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Rod Ends
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 posted on 15/3/10 at 04:10 PM |
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He said "Aluminium Chassis" - Stone him!
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Benonymous
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| posted on 17/3/10 at 04:25 AM |
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Aluminium pffffft....
I'm considering making a 7 chassis entirely out of matchsticks and PVA.
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iank
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| posted on 17/3/10 at 07:44 AM |
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Foam filled Carbon fibre spaceframes have been done experimentally (by Cranfield University and Caterham).
There was a pdf here (and google says it's still there, but the link claims it isn't)
I have a copy if anyone want's it emailed (u2u me your address).
http://www.lowcgen.co.uk/presentations/thurs04.pdf
But not practical for home construction.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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RIE
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| posted on 24/3/10 at 09:04 AM |
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I know very little about cars, but have a bit of observational experience of motorbikes, where both steel and aluminium chassis are used (and
sometimes combined).
Steel trellis:
Aluminium twin spar frame (usually uses engine as stressed part of the chassis):
Aluminium perimeter frame:
The aluminium frames all use big box sections, not tubes - if it was just a simple case of welding some tubes together I'm sure some
manufacturers would have done it by now rather than following complicated manufacturing methods.
Of course, they're very different applications so there may be no comparison at all, so ignore me
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