smart51
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| posted on 18/12/09 at 12:38 PM |
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very thin walled tube in a chassis
Just thinking out loud again. I read somewhere that a rule of thumb limit for tubes in a space frame was a wall thickness a 50th of the overall
diameter. 24 gauge steel is 0.511 mm thick so would be suitable for 25mm OD tubes. These have an area of 39.3 mm2 so with a tensile strength of mild
steel would yield at 1600kg. plenty for a kit car chassis. Now assuming you could successfully weld steel that thin and assuming your space frame
was fully triangulated in 3 dimensions, would such light weight material be OK for use in a chassis (in areas where a direct load was not applied,
such as suspension pick up points)?
[Edited on 18-12-2009 by smart51]
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02GF74
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| posted on 18/12/09 at 12:48 PM |
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quote:
Originally posted by smart51
24 gauge steel is 0.511 mm thick so would be suitable for 25mm OD tubes.
that is gotta be like bicycle frame steel tubing. 
dunno the answer but I'd you'd need lots of triangulation without any long unsupported runs.
you best be would be to run a 3-d simulation on it - I suspect that our chassis, excpet for that one Westfied one whcih faell apart, are gossly over
engineered.
anyways, I'll stop and let the knowlegeable peeps to have their say.
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MakeEverything
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| posted on 18/12/09 at 01:07 PM |
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The additional amount of triangulation to obtain strength would probably weigh in at just as much as usingnormal stuff....
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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turbodisplay
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| posted on 18/12/09 at 01:07 PM |
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The problem is that a pure tensile load doesn`t exist except for a really good design.
This good design will have no passenger space.
So it will take bending loads as well, hence be very weak in this aspect.
Darren
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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Talon Motorsport
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| posted on 18/12/09 at 01:24 PM |
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I've got a broom handle made of 24 gauge steel the we use round the work shop and it's bent from use, does that tell you any thing?
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MikeR
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| posted on 18/12/09 at 01:29 PM |
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you've got fat hands????? 
(my wooden ones are bowed from over enthusiastic use)
[Edited on 18/12/09 by MikeR]
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russbost
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| posted on 18/12/09 at 01:41 PM |
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Mike R - You have wooden hands!!!!????
Even 20 guage tube has a habit of cracking & breaking away fron welded points, for road use I wouldn't be inclined to go much thinner than
the 16g we all normally use. Thinner might be ok for racing where it's going to be regularly inspected & not used daily & where you
expect to carry out maintenance & repairs fairly regularly.
Something which you can leave alone & forget as with a daily driver needs to be more durable.
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furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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smart51
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| posted on 18/12/09 at 02:02 PM |
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quote:
Originally posted by Talon Motorsport
I've got a broom handle made of 24 gauge steel the we use round the work shop and it's bent from use, does that tell you any thing?
That it isn't triangulated? That it isn't used purely in tension or compression? Or is it just that thin steel takes dents easily?
I'm just being provocative here. I've been reading cymtrics essay on the locost chassis and also about thin walled tubes. My points are
that 16g tube is massively strong for the loads of a spaceframe but is readily available and easy to weld. The locost chassis is under triangulated
and is easy to improve by a huge amount. Thinner walled tube is more than strong enough and with sufficient triangulation could be even stiffer than
cymtyics mods and lighter. Being an engineer but not a mechanical engineer, I don't know how thin you could go and it still stand up to road
use but from what I've read, 24 gauge tube with plenty of triangulation fits within what seem to be usual guidelines.
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t.j.
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| posted on 18/12/09 at 02:25 PM |
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Lower then 1.2mm is not a good idea.
If you want to go to the limit, you must think of other welding methodes.
And laser-fit cutting.
Around the wishbone brackets, engine mounts and safety belts 1.2 mm will be hard to use.
So, I will stick with the 2.0mm for mine.
Please feel free to correct my bad English, i'm still learning. Your Dutch is awfull! :-)
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procomp
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| posted on 18/12/09 at 03:00 PM |
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Hi
5 or so years ago it was perfectly good to do a 20G race chassis with the usual extra triangulations Etc.
But in this day and age the quality of metal is bad is getting beyond a joke. You would not entertain trying a 20G chassis with today's quality.
and dont forget that all Well un certificated tubing anyhow is all down on Min tolerance. So even a 16G chassis is actualy 17.2G
Cheers Matt
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kennyrayandersen
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| posted on 18/12/09 at 03:42 PM |
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1) the right size tube is the right size tube
2) Some tubes are in tension, and some are not
3) there is more than one way to fail a tube
4) Even if a tube is OK for static strength, it might not be for durability (fatigue).
5) sometimes even if you make a model, or do a fancy calculation, the size is still wrong; time will tell.
Uncle Kenny, the Engineer
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kb58
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| posted on 18/12/09 at 04:22 PM |
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quote:
Originally posted by smart51
... Being an engineer but not a mechanical engineer, I don't know how thin you could go and it still stand up to road use but from what
I've read, 24 gauge tube with plenty of triangulation fits within what seem to be usual guidelines.
The problem is that no one hands us numbers on what a "pothole" is. Size, shape, speed at which it's taken, tire section, air
pressure, shock settings, all these alter how much force is fed into the chassis. Without good data, an appropriately strong chassis cannot be
designed. The only way round this is crash testing (nope) or making everything heavy duty to cover the unknown.
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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alistairolsen
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| posted on 18/12/09 at 06:17 PM |
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quote:
Originally posted by kennyrayandersen
1) the right size tube is the right size tube
2) Some tubes are in tension, and some are not
3) there is more than one way to fail a tube
4) Even if a tube is OK for static strength, it might not be for durability (fatigue).
5) sometimes even if you make a model, or do a fancy calculation, the size is still wrong; time will tell.
Uncle Kenny, the Engineer
Indeed, buckling failure in the top members in the chassis would be a significant risk, if not fatigue in the heat affected zone surrounding the welds
on the suspension mounts etc.
My Build Thread
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Dusty
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| posted on 19/12/09 at 01:29 AM |
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Iv'e always wondered how much strength you loose with all the rivet holes for panels in a 16G chassis. I bet the same holes in a 24G chassis
would be a problem.
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kennyrayandersen
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| posted on 19/12/09 at 04:08 PM |
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it's a bit of nonsense as KB58 pointed out. There ont enough data in the question to even estimate a reasonable solution. Now, if you were to
say book chassis tube XXX with this engine and that whatnot and you could proa\bably use 3g's for over-the-road vehicles (this is what the big
rigs use anyway (I read a book once...). Off road vehicles use a g-factor of 5. Then you could start summing the forces and moments, or build an FE
model. Use that to get the loads. Then using those loads you could do a buckling and crippling check as well as a net section check (section less
all of the rivet holes) and this would be for ultimate loading. If you sized it for these factors, and used steel for the material dejour, you could
take a stab at what gages you could use. I rarely see all that done though...
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smart51
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| posted on 19/12/09 at 05:54 PM |
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OK, I think I need to point out that I'm not planning to make a car from 24 gauge tube let a lone a locost, I'm just trying to start a bit
of informed debate. Clearly, dent resistance would be a problem, particularly for tubes under compression load. Drilling holes for rivets would be a
bad idea too. Cymtrics recommends welding in sheet steel panels in places, rather than diagonal tubes and as a 0.7mm steel floor is the same weight
as 2.0mm aluminium and adds more stiffness, welding would be the way to go.
I reckon 18 gauge is plenty strong enough for a locost with 20 gauge in places. The car I had in mind would weigh about 350kg and have 20 BHP and I
suspect that 20 gauge all round (in certified material) would be plenty.
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iank
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| posted on 19/12/09 at 06:23 PM |
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quote:
Originally posted by turbodisplay
The problem is that a pure tensile load doesn`t exist except for a really good design.
This good design will have no passenger space.
So it will take bending loads as well, hence be very weak in this aspect.
Darren
No worries, it won't have an engine compartment either  so it won't suffer too many loads that can damage it 
If you got aircraft spec certified tube, tig welded it and had the whole thing heat treated by someone with a very big oven, it might just be possible
to produce something workable. But at the end of the day how much weight will you have saved? 10kg, 20kg?
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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sebastiaan
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| posted on 20/12/09 at 04:25 PM |
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Donkervoort used to use 1.5mm tubing. Laser cut and brazed together though. Don't know what they are up to now, it's been a good 5 years
since i've been to the factory...
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