In summary:
Very light, difficult to weld properly, fragile, will crack and fall apart if used on the road for even a short amount of time. Really only suitable for track use - and even that's questionable.
David
Too lazy to search but pretty sure this would have been asked before or done but has anyone made a book chasis from aluminium?
curious about cost, weight savings, tubr size/wall thickness, advantage/disadvantage(s) and which alloy?
RH do a rivetted panel monocoque but I refer to a square/round tube welded chasis.
[Edited on 6/1/06 by 02GF74]
Don't even go there --- has been dealt with in a lot of depth in the past "here be dragons"
"here be dragons"
In summary:
Very light, difficult to weld properly, fragile, will crack and fall apart if used on the road for even a short amount of time. Really only suitable
for track use - and even that's questionable.
David
quote:
Originally posted by David Jenkins
will crack and fall apart if used on the road for even a short amount of time.
well I am surprised by this bit! Ali bikes frames are the norm and I am not aware of them falling apart, admiteddly it is aluminium alloy, 
Do a search.
Aluminium fatigues like crazy and wont last long on road driving.
Its also 3 times less stiff than steel, so an ali spaceframe would have 3 times worse torsional stiffness if you copied the book design.
Most ali bike frames dont do the miles or are subject to the loads that a car chassis are. But they will crack sooner or later whereas a steel one
will not.
David
And yey companies do seem to use the stuff in structural applications like planes cars and bikes.
I'm sure that if designed properly using the apropriate alloy, that it could be done. Whether it would be worth it, not that is another
question.
Isn't it true that aluminuim fatigue is due to movement in the metal, not force on it (i.e by bending or twisting)? Isn't a space frame
supposed to be designed so that there is only tension / compression loads with no twisting or bending? It sounds like the two should go together well
enough.
quote:
Originally posted by smart51
And yey companies do seem to use the stuff in structural applications like planes cars and bikes.
quote:
Originally posted by flak monkey
Its also 3 times less stiff than steel, so an ali spaceframe would have 3 times worse torsional stiffness if you copied the book design.
Most ali bike frames dont do the miles or are subject to the loads that a car chassis are. But they will crack sooner or later whereas a steel one will not.
I had a simaler thought a while back, but not using the std design - here is the thread
http://www.locostbuilders.co.uk/viewthread.php?tid=26462
My 2 cents on aluminium book chassis.
The 2.5 to 1 weight advantage of aluminium is offset by the following factors:
1- aluminium is more maleable therefore less stiff than steel.
2- thin aluminium is difficult to weld properly (metal structure desintegration) and in any case must involve TIG welding which small shops do not
always have. Qualified aluminium welders time cost more than steel welders'.
3-As aluminium cost 4 to 5 times more than mild steel there are budget implications.
In essence: there are many ways to improve the weight/tortional resistance ratio of a chassis. One of them is to improve the steel chassis to sheet
metal "integrity".
In my view an aluminium chassis is pretty low on my list. But if I were racing I may feel differently.
Philippe.
I,m going for a hybrid as I can weld ali.
The frame is based upon the books external dimensions, however I will not be providing tubes for any of the Transimmision tunnel/footwells beyond the
bulkhead.
I will build a complete ali floor and tunnel in one piece and drop the frame over this.
The floor wll be braced with folder U lengths of ali etc. I,m hoping for a very light and rigid structure, the load bearing sections will still be
steel.
Regards Mark
If I wanted to make an ali chassis I'd start with a totally different design - something more 'monocoque', using folded sheet rather
than welded tube.
Trouble is, I don't have the engineering knowledge to design such a beast!
David
IIRC BSA built an ally frame for a Victor engine, they used the same design as the steel one. It was a disaster.
This suggests to me that an ally chassis would have to be totally redesigned.
Sorry my post was not very clear.
My frame is still steel, its just where you sit that I have chosen to delete the steel panels and construct a rigid ali replacement if that makes
sence.
This is based upon the information in the kit car analysis doc provided by gretrix where he builds the tunnel and floor in steel reducing weight and
increasing rigidity.
When Chapman replaced the spaceframe on the Lotus 24 (which was really just a rear engined version of the seven chassis) with aluminium to make the
Lotus 25/33 monocoque it ended up heavier than the Lotus 24 steel space frame.
Aluminium is a completely different material for the designer to work with --- for a start it has no identifyable yeild point, it always has a finite
fatigue life and welding it without heat treating the whole frame has a really bad effect on the mechanical properties.
quote:
Originally posted by mark chandler
I'm going for a hybrid as I can weld ali.
The frame is based upon the books external dimensions, however I will not be providing tubes for any of the Transimmision tunnel/footwells beyond the bulkhead.
I will build a complete ali floor and tunnel in one piece and drop the frame over this.
The floor wll be braced with folder U lengths of ali etc. I,m hoping for a very light and rigid structure, the load bearing sections will still be steel.
Regards Mark
One of the problems with aluminium IIRC is the effect of the heat from welding. Small componets (up to and including bike frames) can be easilly heat
treated after welding to over come this problem.
An alturnative way of using aluminium I have heard of is to use it in honecomb boards, bonded together with a resin.
Don't know the pros and cons of using aluminium in this way though.
If you've got the money to invest in chassis materials, they going for a suitable medium carbon steel would be a better bet.
[Edited on 6/1/06 by clbarclay]
aluminium unlike steel has no lower fatigue limit. i will explain this as so far everyone has got the explaination wrong in some way.
fatigue is failure caused by a cyclic load (eg vibrations) that are below the yield point (ie the load is not enough to casue failure on its own) that
over many cycles (many means in the order of a billion or more) causes cracks to form and propagate until failure occurs.
some metals (steel is one of them) have a lower limit below which any cyclic loads do not cause fatigue. so a good design involving steel trys to keep
below this limit however people dont always get it right see here on tim Hoverd's site the crack in his engine mounting looks like a classic
fatigue failure.
http://www.hoverd.org/Tim/Fury/images/broken_engine_mount_01.jpg
with an aluminium structure you dont have this limit to work with so any alu structure with vibrations in it will eventually fail. the time it takes
to fail is measure in cycles of the load so a high frequency load will cause failure faster if all esle is kept equal. the number of cycles needed to
causes failure depends on the size of the load.
in the aviation industry where fatigue is a big problem they get around it by having limited life parts that are check and replaced before they fail.
for a locost it wouldnt be worth it as to get any benefit you would have to completely redesign the chassis to use a box sructure approach like lotus
and aston martin use, otherwise you quite probably are either going to have a very flexible chassis or chassis that weighs the same and costs 5x more.
So, do the recent crop of aluminium bodied cars such as Jags and Audis have a fixed life specified for them as well ?
quote:
Originally posted by cossey
aluminium unlike steel has no lower fatigue limit. i will explain this as so far everyone has got the explaination wrong in some way.
quote:
Originally posted by cossey
aluminium unlike steel has no lower fatigue limit. i will explain this as so far everyone has got the explaination wrong in some way.
quote:
Originally posted by mark chandler
Sorry my post was not very clear.
My frame is still steel, its just where you sit that I have chosen to delete the steel panels and construct a rigid ali replacement if that makes sence.
This is based upon the information in the kit car analysis doc provided by gretrix where he builds the tunnel and floor in steel reducing weight and increasing rigidity.
A spaceframe, as in a typical kit car, relies on the triangulation of its tubes to give it both stiffness and strength. Most kit car spaceframes
don't have the right triangulation in the right places to get anywhere near there best results. Check my mods as posted on here or read them in
kitcaranalysis in the files section of locost7.info; these mods show how reconsidering the chassis a little can make very worthwhile improvements.
Aluminium can be used to make very light structures. But not spaceframes. as has been said above it's a third of the weight but also a third of
the strength in tension or compression so to get the same strength as a steel spaceframe you'd need to triple the amount of metal which gives you
the same weight as a steel chassis only with worse fatigue life and at higher cost. Pointless.
So why do some cars use aluminium "spaceframes"?
The answer is simple. They don't. All those "spaceframes" rely on the strength of the beam section in them and not the triangulation.
In fact if you look at any of these chassis you'll notice something about the structure, there's no triangulation but there are very big,
compared to locost chassis, tubes.
Large diameter tubes are, weight for weight, stiffer and less stressed than small diameter ones.
Take a 4x2 14gauge (100x50 2mm wall) tube in steel and a 5x2.5 10gauge (125x64 3mm wall) aluminium tube.
The aluminium tube will be as stiff as the steel tube but only 2/3 of the weight.
replacing a steel panel with an aluminium one 1.5x the thickness gives the same buckling resistance but with only half the weight.
These two facts are how the mass produced aluminium frames work. They're not spaceframes as we know them, in fact they're closer in concept
to ladder frames.
quote:
Originally posted by JonBowden
So, do the recent crop of aluminium bodied cars such as Jags and Audis have a fixed life specified for them as well ?
Adding these 1.6 mm ally pressed up side boxed panels to the sides of my proto middy chassis increased torsional rigidity from 2700 ft/lb/Deg to 5700
ft/lb/fl/Deg.
Cheers
Fred WB
[img][/img]
quote:
Originally posted by Fred W B
Adding these 1.6 mm ally pressed up side boxed panels to the sides of my proto middy chassis increased torsional rigidity from 2700 ft/lb/Deg to 5700 ft/lb/fl/Deg.
Cheers
Fred WB
Many production cars including my own R75 use alloy lower wishbones other production cars like the Mondeo use wishbones made of steel pressings --- the Mondeo wishbone is lighter it weighs next to nothing.
quote:
Originally posted by britishtrident
Many production cars including my own R75 use alloy lower wishbones other production cars like the Mondeo use wishbones made of steel pressings --- the Mondeo wishbone is lighter it weighs next to nothing.
When aluminium is manufactured it is cast into large billets.
As it cools and solidifies it forms into crystals that are uniform in shape and size.
Diagram A below is a simplified version of what might be seen under magnification.
In this state it has a tensile strength of 4 to 6 tons/sq inch. (steel is 25 to 30)
Aluminium is however extremely malleable and can be cold formed into usable shapes,
rolled into sheet, drawn through dies, or drawn over mandrels to form hollow sections.
This stretches and compresses the crystal structure as in diagram B.
This has the effect of greatly increasing the tensile strength, depending on the amount. (Work hardening).
Unfortunately, when we then weld this structure, we locally anneal it and in the weld itself
reform the crystals as in diagram A, and consequently back to it's lower tensile strength.
Diagram C shows a welded fabrication similar to areas of a chassis.
The main structure would be strong, but the red areas would be weak.
Under load, either constant, but more so alternating, these areas would be prone to failure.
So it's glue, bond, rivet or bolt.
Welding this type of structure is a no no.
Paul G
Rescued attachment Al-structure-merge-s.jpg
Nicely put 907
quote:
Originally posted by britishtrident
Nicely put 907
quote:
Originally posted by cymtriks
Aluminium can be used to make very light structures. But not spaceframes. as has been said above it's a third of the weight but also a third of the strength in tension or compression so to get the same strength as a steel spaceframe you'd need to triple the amount of metal which gives you the same weight as a steel chassis only with worse fatigue life and at higher cost. Pointless.
Most of the alloys that approach steel in strength do so through solution heat treatment and precipitation hardening. When you weld these alloys they
effective temper and return to the strength of normal ali (within a bit). To get the strength back you need to heat treat the whole chassis once it
has been welded, not easy!
As has been said, ali is ok for some structures, but not for spaceframes.
David