02GF74
|
| posted on 6/1/06 at 09:44 AM |
|
|
aluminium book chasis
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]
|
|
|
|
|
britishtrident
|
| posted on 6/1/06 at 09:57 AM |
|
|
Don't even go there --- has been dealt with in a lot of depth in the past "here be dragons"
|
|
|
David Jenkins
|
| posted on 6/1/06 at 10:09 AM |
|
|
"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
|
|
|
02GF74
|
| posted on 6/1/06 at 10:59 AM |
|
|
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,
Will do the search as I am all curious now. I guess Ti chasis is out of the question? 
|
|
|
flak monkey
|
| posted on 6/1/06 at 11:05 AM |
|
|
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
Sera
http://www.motosera.com
|
|
|
smart51
|
| posted on 6/1/06 at 11:09 AM |
|
|
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.
|
|
|
David Jenkins
|
| posted on 6/1/06 at 11:11 AM |
|
|
quote:
Originally posted by smart51
And yey companies do seem to use the stuff in structural applications like planes cars and bikes.
Not in the way the Locost chassis is designed...
...oh, and aluminium will work-harden all by itself, if left alone for long enough.
David
[Edited on 6/1/06 by David Jenkins]
|
|
|
02GF74
|
| posted on 6/1/06 at 11:51 AM |
|
|
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.
Those are generalisation that I don't think that is correct - all these arguments/comments have been already made - I started to read through
them.
... but just to coment ... arrghgghhh!!! got drawn into it!!
So ali is 3 times less stiff than steel; ok; so use 3 times the material - i.e. you design the structure for the appliaction. (NB ali weighs 1/3 of
steel so no weight saving if you did that).
Again with loads on mountin bikes; look at the size and thckness of bike tubes; they probably get as much stress, maybe more if you do big drop offs,
relative to their size than a car chassis. Again you design structure for the application.
Now I will not post to anymore as all the sparring on the merits or not have been stated already.
|
|
|
u401768
|
| posted on 6/1/06 at 12:11 PM |
|
|
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
|
|
|
Philippe
|
| posted on 6/1/06 at 12:36 PM |
|
|
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.
|
|
|
mark chandler
|
| posted on 6/1/06 at 12:48 PM |
|
|
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
|
|
|
David Jenkins
|
| posted on 6/1/06 at 12:58 PM |
|
|
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
|
|
|
trikerneil
|
| posted on 6/1/06 at 01:07 PM |
|
|
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.
|
|
|
mark chandler
|
| posted on 6/1/06 at 01:14 PM |
|
|
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.
|
|
|
britishtrident
|
| posted on 6/1/06 at 01:15 PM |
|
|
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.
|
|
|
02GF74
|
| posted on 6/1/06 at 01:34 PM |
|
|
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
I was thinking on these lines on my walk into town. Steel for load bearing parts: engine, gearbox and suspension and ali to hang of bodywork, not
that I plan to build one. Ofcourse sacrificing strrengh around the occupant shell should be avoided.
The addition of lugs and bolts to join steel and ali would soon add up in wieght 
|
|
|
clbarclay
|
| posted on 6/1/06 at 03:09 PM |
|
|
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]
|
|
|
cossey
|
| posted on 6/1/06 at 03:12 PM |
|
|
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.
|
|
|
JonBowden
|
| posted on 6/1/06 at 04:42 PM |
|
|
So, do the recent crop of aluminium bodied cars such as Jags and Audis have a fixed life specified for them as well ?
Jon
|
|
|
britishtrident
|
| posted on 6/1/06 at 04:47 PM |
|
|
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.
!
|
|
|
flak monkey
|
| posted on 6/1/06 at 04:56 PM |
|
|
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.
In past times I have explained it describing fatigue limits etc. But this time I couldnt be bothered and was about to get kicked out of the IT room
A search on this subject should bring up all you need to know on it.
You can make ali chassis lighter than a steel one. But its no good making it as a spaceframe...
Ali monocoques work very well (Jaguars new cars for example) and have a long life, as loads throughout are not as high as thos transmitted through a
spaceframe.
Likewise ali ladder frames, as featured on the Elise and others work well and again have a much longer life than an ali spaceframe. Ladder frames work
on the principal that the larger you make the diameter of the tube, the stiffer it becomes. Ever wondered why the sills are so wide on an elise? Thats
one of the chassis beams!
A chassis can be made from any material you like, but it has to be designed to make the most of the properties of the material it is being made from.
You cannot simply copy a design produced for a steel chassis, and expect it to work as efficiently in aluminium.
I am currently working on a chassis design using sandwich panels. The rules that must be followed are different to those in a spaceframe!
Its worth noting however that any structure subjected to varying or cyclic loads, and made from aluminium will eventually fail through fatigue. If the
loads are small and not concetrated then failure will take a long time, and it may be decided that by the time that failure occurs some other part of
the product will have failed.
David
[Edited on 6/1/06 by flak monkey]
Sera
http://www.motosera.com
|
|
|
britishtrident
|
| posted on 6/1/06 at 04:59 PM |
|
|
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.
If you put a light alloy tunnel in it won't take any significant load and contribute practically zero to the torsional stiffness --- the
reason being the much lower Youngs modulus of aluminium alloys.
The tunnel structure was a latter add-on the original Lotus Seven structure didn't have it but it is required with the massive increases
engine power since the days when Lotus Sevens were fitted with Ford Pop sidevavle engines.
The tunnel contributes a lot to the stiffness of the rear cockpit bulkhead.
[Edited on 6/1/06 by britishtrident]
|
|
|
cymtriks
|
| posted on 6/1/06 at 07:36 PM |
|
|
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.
|
|
|
I love speed :-P
|
| posted on 6/1/06 at 08:05 PM |
|
|
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 ?
A while ago i was talking to someone who designs rear sub frames for a luxury car manufacture, he was showing me one and said that it has to last for
300,000km and i asked about the fact its made from alu and he replied that its not ideal and the car is only expected to last 300,000km so it will be
ok, he then added that he wouldnt buy one which has done more than 300.000km
Don't Steal
The Government doesn’t like the competition
|
|
|
Fred W B
|
| posted on 7/1/06 at 10:11 AM |
|
|
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]
|
|
|
