Philippe
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| posted on 10/1/06 at 11:47 AM |
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Chassis construction: Alu vs steel skin?
I have been critical (on various forums) of the way the Book Locost chassis and the Lotus chassis are assembled. The riveting of aluminum panels,
floor. bulkhead to steel tubes does not make good use of the stiffening property of sheet metal and also potentially limits the life span of the
chassis.
In my build I am considering replacing all sheet aluminium panels, floor, bulkheads with light gauge sheet steel.
My plan is to mig-weld the panels to the tubes in a sequential manner so that tube distortion will not take place. Before welding the panels to the
chassis I will roll bidirectional stiffening beads to the said panels, floor and bulkheads in order to enhance their rigidity.
My questions are these.
1) Does any one of you who work with CAD files have the total aluminium area of the Book Locost handy... and would you mind telling me what it is so
that I could calculate the amount of additional dead weight the above strategy will cause and make a final decision?
2)What do you think of my suggestion to use steel?
Philippe.
_________________
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flak monkey
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| posted on 10/1/06 at 11:58 AM |
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Loads of people use steel floors instead of ali ones.
When i did some calculations a 2mm ali floor was around 5-6kg lighter than a 1.2mm steel one.
Ali panels weigh only 1/3rd as much as steel ones. So even if you halve the thickness for steel panels an ali one will still be lighter.
I am not sure if its been suggested to you to use strutcural adhesives (3M VHB tape is good stuff) to bond the ali panels to the chassis, along with a
few rivets, you will end up with a similar stiffening effect to that of welding in steel panels.
The use of structural rivets to hold the floor and other panels on is well known, and if its good enough for the aerospace industry, it's sure
as hell good enough for a Locost. The next best thing to use are closed end rivets.
The other problem is distortion in welding thin sheet steel. Try it, and you will see. However careful you are, you will always get distortion with
thin sheet, its an inherent parts of the welding process (i.e. loads of heat!)
In short, for the amount of miles your average locost will do, theres no need to worry about failure of riveted ali panels. (To my knowledge there
havent been any)
You can use steel but it will be significantly heavier.
All IMO, but there we go.
Sera
http://www.motosera.com
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Volvorsport
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| posted on 10/1/06 at 11:58 AM |
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my chassis uses grp/kevlar to replace the aluminium , no distortion , good strength to weight ratio, and can be done at home . I also made the tunnel
completely in kevlar , so ive saved on tube also .
have you looked at an AC chassis ? complete sheet metal with tube subfames .
you still have to skin the outer layers , to make em look nice .
www.dbsmotorsport.co.uk
getting dirty under a bus
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Bluemoon
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| posted on 10/1/06 at 12:03 PM |
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I what why does the Ali pannels limit the life if the chassis? If properly constructed it should not (i.e sealed, and carfull choice of rivet to aviod
galvanic corrosion). The whole point of a space frame is that it takes the loads via compression and tension in the tubes... The pannels are not load
bearing it should not fatigue...
[Edited on 10/1/06 by Bluemoon]
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Philippe
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| posted on 10/1/06 at 12:04 PM |
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quote:
Originally posted by flak monkey
Loads of people use steel floors instead of ali ones.
When i did some calculations a 2mm ali floor was around 5-6kg lighter than a 1.2mm steel one.
Ali panels weigh only 1/3rd as much as steel ones. So even if you halve the thickness for steel panels an ali one will still be lighter.
I am not sure if its been suggested to you to use strutcural adhesives (3M VHB tape is good stuff) to bond the ali panels to the chassis, along with a
few rivets, you will end up with a similar stiffening effect to that of welding in steel panels.
The use of structural rivets to hold the floor and other panels on is well known, and if its good enough for the aerospace industry, it's sure
as hell good enough for a Locost. The next best thing to use are closed end rivets.
The other problem is distortion in welding thin sheet steel. Try it, and you will see. However careful you are, you will always get distortion with
thin sheet, its an inherent parts of the welding process (i.e. loads of heat!)
In short, for the amount of miles your average locost will do, theres no need to worry about failure of riveted ali panels. (To my knowledge there
havent been any)
You can use steel but it will be significantly heavier.
All IMO, but there we go.
R-Thanks,
I am aware of the riveting techniques in airplane construction and You may be right.
However what I would like before I make a decision is to document the actual weight saving gained by using riveted aluminium.
Philippe.
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oliwb
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| posted on 10/1/06 at 12:05 PM |
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Wouldn't bother with the steel to be honest it sounds like a bit of over-engineering to me! I'm not saying that you wouldn't notice
any difference but function over form and all that - Ali is lighter, and as mentioned I've not heard of any failures yet! I'm going with
the if its good enough for everyone else, its good enough for me plan! Obviously my opinion but hey in this world that's all you got right?!
Oli.
If your not living life on the edge you're taking up too much room!
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Philippe
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| posted on 10/1/06 at 12:08 PM |
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quote:
Originally posted by Volvorsport
my chassis uses grp/kevlar to replace the aluminium , no distortion , good strength to weight ratio, and can be done at home . I also made the tunnel
completely in kevlar , so ive saved on tube also .
have you looked at an AC chassis ? complete sheet metal with tube subfames .
you still have to skin the outer layers , to make em look nice .
R-Thanks. On the Book Locost, have you made your own comparative analysis between: alu, GRP, and steel, in terms of weight?
Philippe.
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Volvorsport
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| posted on 10/1/06 at 12:16 PM |
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yeah , the less resin you use , the lighter it will be , but you have to try and keep with the recomended resin/glass ratio - which is 1:1 for most
cloths and weaves .
altho ive not weighed it before and after - ive used 4 -5kgs of mat so about 5 kgs of resin , so about 10 kgs altogether , but i have no central top
tubes and no diagaonals in the footwell/tub area .
Also those 10 kg comprise the full enclosed tub - so the panel on the top of the footwell is closed off , and of course the trans tunnel is done
aswell .
www.dbsmotorsport.co.uk
getting dirty under a bus
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cossey
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| posted on 10/1/06 at 12:30 PM |
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quote:
Originally posted by Bluemoon
I what why does the Ali pannels limit the life if the chassis? If properly constructed it should not (i.e sealed, and carfull choice of rivet to aviod
galvanic corrosion). The whole point of a space frame is that it takes the loads via compression and tension in the tubes... The pannels are not load
bearing it should not fatigue...
[Edited on 10/1/06 by Bluemoon]
the pannels in a seven are stressed and have a considerable effect on the stiffness of the chassis.
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smart51
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| posted on 10/1/06 at 12:31 PM |
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If you use a steel panel of 1/3 the thickness of the ali you replace then you will have the same weight. You will have to make the steel a bit
thinner to counter the weight of the paint or powder coat that you will have to use to stop the steel rusting. If you think that this painted thin
steel is better then go for it. Ali is not the answer to the world's problems but it does have its uses.
My ali floor is bonded on to the chassis with PU adhesive and is also riveted at frequent intervals. The adhesive and rivets together will make the
ali add to the strength of the steel tubes.
There isn't a problem with ali or with steel. Use what you think best.
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Volvorsport
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| posted on 10/1/06 at 12:41 PM |
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some pics to clarify my chassis - its a 442 ish size so it will be heavier
http://www.dbsmotorsport.co.uk/Html/index3.html
[Edited on 10/1/06 by Volvorsport]
www.dbsmotorsport.co.uk
getting dirty under a bus
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NS Dev
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| posted on 10/1/06 at 01:15 PM |
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The problem will be that using 0.5mm steel (as opposed to 1.2mm ally) will mean that the steel will wrinkle when welded, almost impossible to stop
this happening with very thin section sheet steel.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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kb58
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| posted on 10/1/06 at 03:08 PM |
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About the riveted aluminum-paneled car not lasting as long, that may be be. But the *real* question is, will the aluminum-paneled car last longer
then you wish to own it... If so the whole issue goes away.
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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Prez
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| posted on 10/1/06 at 03:23 PM |
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I cant see anyway of getting a weld of high enough quality to bring any benefits over rivetting and / or bonding, when joining very thin gauge steel
plate to box section or tubes with considerably greater wall thickness. IMHO I think you would actually make the structure considearbly weaker for the
same given weight.
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britishtrident
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| posted on 10/1/06 at 03:29 PM |
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Just not issue ----
Early Lotus car were constructed almost exactly the same way as the Locost and there are still a high % number of survivors around that are going on
for 50+ years old. I might add Lotus used tube 1.2mm thick.
Re-skinning a Seven style car is no big deal, the alloy gets minor damage with everyday wear and tear.
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britishtrident
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| posted on 10/1/06 at 03:36 PM |
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quote:
Originally posted by NS Dev
The problem will be that using 0.5mm steel (as opposed to 1.2mm ally) will mean that the steel will wrinkle when welded, almost impossible to stop
this happening with very thin section sheet steel.
Yes ;-) it buckles or to put it anotherway because of the greater thickness an aluminium panel is stiffer in bending than a comparable steel panel
-- which is why aluminium and magnesium alloys are such excellent materials for making road wheels.
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quattromike
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| posted on 10/1/06 at 04:06 PM |
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Plus you would have to have the welding current turned down so low for welding the thin mild steel sheet with mig that it would not fuse to the
thicker box section tubes.
Every tried laser welding?
Mike
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02GF74
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| posted on 10/1/06 at 04:56 PM |
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is it not possible to spot weld the thin sheet to the square tubes or the difference in thcknesses too great?
using steel has anther disadvantage - rust! so you need rust treatment and paint = more weight!
the question is how much rigidity you gain by a more solid join of sheet to the frame, if you knew that, then maybe you could use lighter tube so shed
some weight there.
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britishtrident
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| posted on 10/1/06 at 05:15 PM |
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quote:
Originally posted by cossey
the pannels in a seven are stressed and have a considerable effect on the stiffness of the chassis.
In a real world as per book build apart from the floor panel they don't do anything but keep the wind out, to get any structural gain you need
to use a decent thickness of panel and lots of carefully spaced rivets and careful use of bonding.
Volvorsports locost (hi-carb !) chassis is impressive because he understands the material he works, nearly everyone else pop-rivets oa thin sheet of
unknown grade aluminium alloy sheet with any old B&Q rivets.
Most alumium alloy sheet sold to Locost builders is the same stuff used to make Luton van bodies -- it has the structural properties of tin foil,
van builders use it because it is easy to work. The stuff aircraft and old F1 car monocoques were made was a bit different.
[Edited on 10/1/06 by britishtrident]
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britishtrident
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| posted on 10/1/06 at 05:28 PM |
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quote:
Originally posted by 02GF74
is it not possible to spot weld the thin sheet to the square tubes or the difference in thcknesses too great?
using steel has anther disadvantage - rust! so you need rust treatment and paint = more weight!
the question is how much rigidity you gain by a more solid join of sheet to the frame, if you knew that, then maybe you could use lighter tube so shed
some weight there.
Lotus or at least Arch Motors used 1.2 mm round tube for the spaceframes. 1.6 RHS is easier work with.
Early Lotus space frames were a lot sparser than the Locost --- somewhere on the net is a picture of chunky Chapman holding up an Eleven chassis--
just try that with book chassis.
With the Seven Series 2 Chapman tried to take even more weight out cutting out nearly all of the diagonals -- the result was a lot of broken chassis
(mainly fatigued brazed joints), with the late Series 2 and Series 3 the missing members were re-instated. Locost are about twice the weight of a
Lotus Seven S2 chassis.
Chasing grams on the chassis isn't worth it, the chassis dosen't way a lot compared to the bits bolted on to it.
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Philippe
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| posted on 10/1/06 at 06:25 PM |
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quote:
Originally posted by Prez
I cant see anyway of getting a weld of high enough quality to bring any benefits over rivetting and / or bonding, when joining very thin gauge steel
plate to box section or tubes with considerably greater wall thickness. IMHO I think you would actually make the structure considearbly weaker for the
same given weight.
R-Could you elaborate? I would join the plates to the tubes in a common mig or tig welded spot by spot fashion, the way car bodies are built in the
industry. There is nothing weak about that?
Wheter is is better than riveted+chemical bonding, I don't know. This is what I want to find out.
Philippe.
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Philippe
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| posted on 10/1/06 at 06:31 PM |
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quote:
Originally posted by britishtrident
quote:
Originally posted by NS Dev
The problem will be that using 0.5mm steel (as opposed to 1.2mm ally) will mean that the steel will wrinkle when welded, almost impossible to stop
this happening with very thin section sheet steel.
R-I was refering to sequential spot welding. This is the way the industry stitches car bodied together. Punch holes all around the perimeter of your
panels, turn the amp at the lowest on your mig welder, weld one second hole at the time, etc.... No distorsion, no buckling, perfect.
Philippe.
Yes ;-) it buckles or to put it anotherway because of the greater thickness an aluminium panel is stiffer in bending than a comparable steel panel
-- which is why aluminium and magnesium alloys are such excellent materials for making road wheels.
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Volvorsport
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| posted on 10/1/06 at 06:32 PM |
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impressive eh !! wooo .
it could be a lot better - that entails makig another chassis to make the GRP/kevlar bit even better/easier to do , it was quite fiddly but rewarding
when i stood in the tub and jumped up and down !!! .
by my rule of thumb , if i can remove most of the tube work within the tub apart from the perimeter frame, and using a rear gearbox UN1 style , i can
get one below 500kgs , which aint bad for the volvo turbo lump up front .
www.dbsmotorsport.co.uk
getting dirty under a bus
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Philippe
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| posted on 10/1/06 at 09:17 PM |
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quote:
Originally posted by Bluemoon
The whole point of a space frame is that it takes the loads via compression and tension in the tubes... The pannels are not load bearing it should
not fatigue...
[Edited on 10/1/06 by Bluemoon]
R- The panels are not load bearing but they should. Then you would have the perfect combination: a light chassis assisted by a monocoque structure.
Philippe.
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quattromike
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| posted on 10/1/06 at 10:27 PM |
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quote:
Originally posted by Philippe
R-Could you elaborate? I would join the plates to the tubes in a common mig or tig welded spot by spot fashion, the way car bodies are built in the
industry. There is nothing weak about that?
Philippe.
I think most of the structual joints of modern day cars are seam welded, which is a bit like spot welding but instead of a contact electrode on ether
side of the surfaces being welded together there are little contact wheels which run along the seam pulling the arc between them IIRC.
Did this kinda stuff at the big boys school but that was a while ago 
Mike
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