greglogan
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| posted on 21/2/06 at 10:37 PM |
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Steel Size
Hi Guys me again. Stupid question but I saw this on a website and it gave me some concern. This guy reckoned that 16 guage steel was nowhere near
light enough and not to consider using anything less than 3mm box. Is this correct? Seems like it would add a whole lot of weight to the overall
build. I don't think it would add much to the overall strength tho would it?
Women are meant to be loved, not understood.
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caber
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| posted on 21/2/06 at 10:42 PM |
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There are a lot of 7 clones out there built of 16guage or thinner tube I don't see too many as a heap of bits at the side of the road! If you
look through the forum you should find a few pics of crashed locosts none seem to have crumpled up !
Caber
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flak monkey
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| posted on 21/2/06 at 10:53 PM |
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Upping the thickness of tube actually makes little difference to the overall strength of the tubes in tension and compression. It makes more
difference in bending, but not as much as increasing the overall size of the tube, say from 25x25mm up to 35x35mm.
{Edit - Ignore that, its crap, see my later post}
Spaceframes can be made of light gauge tube without too much detriment to torsional rigidity. However 16g (1.6mm) wall thickness is more than adequate
for a locost. Many racing cars only use 18g and sometimes thinner in less critical areas. 16g is a lot easier to weld than the thinner stuff
though.
David
[Edited on 22/2/06 by flak monkey]
Sera
http://www.motosera.com
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Peteff
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| posted on 22/2/06 at 12:43 AM |
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nowhere near light enough
Bit of contradiction there, 3mm is going to be double the weight. Is it a Cobra website?
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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DIY Si
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| posted on 22/2/06 at 01:01 AM |
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As above, the thickness does less than you might first think. Upping the tube/box OD is what makes the most difference. And may mean you can use
thinner, lighter tube as well. On a similar vien, cymtriks did an analysis of the book chassis and made it twice as stiff, but lighter and with less
tubes!!
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britishtrident
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| posted on 22/2/06 at 08:33 AM |
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1.6 mm is the size to use --- by far the easiest size to work with easy to cut, easy to make a good weld in.
1.2 mm has been used for many spaceframes its is stiff and more than strong enough but it is more difficult to cut cleanly with a hacksaw and welding
it needs a light touch -- really best brazed or TIG welded.
2 mm or above is a lot more work to cut and getting full penetration welds starts to become a problem unless you are a skilled welder with decent
equipment, if you make a bad weld in in 1.6 mm it is relatively easy to spot in 2 mm or above serrious weld faults can be hidden below the
surface.
My own chassis is mainly 1.6mm with some 1.2 and some 2mm and 2.5 --- the thicker material being used in areas where members are subject to bending
moments.
[Edited on 22/2/06 by britishtrident]
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smart51
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| posted on 22/2/06 at 09:07 AM |
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MNR's RT+ chassis is made from various thicknesses down to 20 SWG. 16 SWG is used because it is easier to weld than the thinner stuff.
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flak monkey
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| posted on 22/2/06 at 09:23 AM |
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quote:
Originally posted by Dave Ashurst
quote:
Upping the thickness of tube actually makes little difference to the overall strength of the tubes in tension and compression.
That's interesting. What do you mean?
regards
Dave
OK, so thicker tube will obviously yeild at a higher load than a tube with a thinner wall in pure tension or compression testing as theres more
material in the cross-section.
Perhaps what I should have said to be technically correct is that it will make little difference to the strength of the chassis and even less of a
difference to the torsional rigidity. The loads through a chassis generally put nowhere near the loads on each individual tube which would be needed
to make it yeild.
Hopefully thats a bit clearer and correct this time.
David
Sera
http://www.motosera.com
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britishtrident
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| posted on 22/2/06 at 12:18 PM |
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quote:
Originally posted by Dave Ashurst
quote:
Upping the thickness of tube actually makes little difference to the overall strength of the tubes in tension and compression.
That's interesting. What do you mean?
regards
Dave
Stiffness is what is important in the chassis structure -- more precisely the torsional stiffness -- increasing the material thickness of
each member of a framed structure has little effect on stiffness.
The downside of using thinner members is the buckling load of each individual member is reduced but if the length of unsupported member (ie distance
between nodes of the frame) is kept short and loads such as spring mounts are only applied at or very close to nodes this is unimportant.
[Edited on 22/2/06 by britishtrident]
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Syd Bridge
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| posted on 22/2/06 at 02:23 PM |
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Are any of you people writing these replies educated in engineering and structures?
Have any of you seen the anecdote somewhere on this great internet thing about the Lotus that was a 'special' build by Chapman for a mate,
just for racing, made of 18 & 20gauge?
The thing broke in two under its own weight when they lifted it for a tow!!
For a given size tube and a given load......
Increased wall thickness means less stress,..means less strain,....means less deflection! This applies equally to beam, column and torsion.
If the same cross sectional area is kept, but the tube is given bigger outside dimensions and thus thinner walls, bending and column properties can be
enhanced to a point, but not tension.
Put all this in a locost chassis, and a chassis with thicker tubes will be stiffer in beam and torsion, but heavier. Thin out the tubes and the
reverse applies.
If I went by what you lot are saying, the rollcage/spaceframe I am currently charged with optimising would be 1" 20g tube!
Get real, and sign up at your local college for some engineering classes, you might learn something. And while you're there, ask someone to
teach you about Moments of Inertia, and Section Modulus, and how they are derived and applied. 
Syd.
[Edited on 22/2/06 by Syd Bridge]
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flak monkey
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| posted on 22/2/06 at 03:07 PM |
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Yes I am 'educated' in engineering. Sorry, but I am not going to take offence  Seeing as I have done it all before. Perhaps i was being
too general, BT put it a little better than myself, but heres some maths for those of you who like the numbers.
Right, heres some maths for you (based on a standard mild steel yeild point of 200MPa):
A 25x25x1.6mm tube has a second moment of area of 27461.99mm^4. Critical bending moment is 439Nm. Max load in tension is 3053kg.
25x25x1.25mm tube is 22389.32mm^4. Critical bending moment is 358Nm. Max load in tension is 2421Kg.
25x25x1mm tube is 18464mm^4. Critical bending moment is 295Nm. Max load in tension is 1957kg.
Conversely, a 32x32x1.25mm tube is 48540.16mm^4 (huge improvement even over 25x25x1.6mm even though CSA are nearly the same). Critical bending moment
is 606Nm. Max load in tension is 3135kg.
Considering most loads in a chassis are tension/compression then these figures are pretty high. Bearing in mind each figure is for only a single tube,
most loads in a spaceframe are spread between 2 to 3 tube at least.
Obviously as you reduce the wall thickness second moment of area will go down. And the stiffness of the tube will also go down. However in a
spaceframe going from say 16g to 18g tube overall will not make the chassis unsuitable for its job. Torsional rigidity will go down as you decrease
the wall thickness, but it will not have a massive impact, unless you go way too thin. If you wanted to make a much stiffer chassis without the weight
penalty, increase the tube size and reduce the CSA of the tube to close to that of the original.
The lotus chassis which broke was not really similar to the book chassis, ISTR seeing pictures of it and wondering how it held together....
David
[Edited on 22/2/06 by flak monkey]
Sera
http://www.motosera.com
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britishtrident
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| posted on 22/2/06 at 03:36 PM |
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quote:
Originally posted by flak monkey
Yes I am 'educated' in engineering. Sorry, but I am not going to take offence Seeing as I have done it all before. Perhaps i was being
too general, BT put it a little better than myself, but heres some maths for those of you who like the numbers.
Right, heres some maths for you (based on a standard mild steel yeild point of 200MPa):
A 25x25x1.6mm tube has a second moment of area of 27461.99mm^4. Critical bending moment is 439Nm. Max load in tension is 3053kg.
25x25x1.25mm tube is 22389.32mm^4. Critical bending moment is 358Nm. Max load in tension is 2421Kg.
25x25x1mm tube is 18464mm^4. Critical bending moment is 295Nm. Max load in tension is 1957kg.
Conversely, a 32x32x1.25mm tube is 48540.16mm^4 (huge improvement even over 25x25x1.6mm even though CSA are nearly the same). Critical bending moment
is 606Nm. Max load in tension is 3135kg.
Considering most loads in a chassis are tension/compression then these figures are pretty high. Bearing in mind each figure is for only a single tube,
most loads in a spaceframe are spread between 2 to 3 tube at least.
Obviously as you reduce the wall thickness second moment of area will go down. And the stiffness of the tube will also go down. However in a
spaceframe going from say 16g to 18g tube overall will not make the chassis unsuitable for its job. Torsional rigidity will go down as you decrease
the wall thickness, but it will not have a massive impact, unless you go way too thin. If you wanted to make a much stiffer chassis without the weight
penalty, increase the tube size and reduce the CSA of the tube to close to that of the original.
The lotus chassis which broke was not really similar to the book chassis, ISTR seeing pictures of it and wondering how it held together....
David
[Edited on 22/2/06 by flak monkey]
Yes Chapmans late 50s chassiss gave "sparse space frame" a new meaning there was virtually no triangulation -- hence the famous pictures
of him holding up an Eleven chassis with virtually one hand try that with a Locost -- hernia op time ;-)
It reached a peak when the the Seven series 2 was introduced the revised chassis had so few diagonals that it lozenged badly and failed at the nodes
the problem wasn't it was made of 18 guage but that Chapman had ommited structural elements -- some say to lighten the chassis others say to
reduce costs (= more profit), I personally think he was just taking the p*** and testing to see how much you could leave out still get punters to
pay big money for it.
[Edited on 22/2/06 by britishtrident]
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Syd Bridge
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| posted on 22/2/06 at 03:50 PM |
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Flakfella,
You've said the same as I put, but with numbers and flowery terms that most on here would not understand. Not that that's bad, just
gibberish to most here.
The chassis that broke had a few diagonals missing. But the basic premise of thinner tube walls holds up.
And...if anyone has had anything to do with the kit industry, they'll find it difficult to disagree with the statement I had put to me by a very
well educated and regarded engineer,' if Colin (Chapman) had built the Seven out 2x2x10g tube, every kit today would be made of it'.
'As well as that, 1x1x16g tube is easily handled and can be readily cut by hand with a hacksaw.' I won't repeat the rest of the
statement, as it may be offensive to a majority of the kit manufacturers.
The said gentleman is held in very high regard by the kit industry today. Shame he doesn't feel the same towards the manufacturers, but his
words are well founded.
Syd.
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Syd Bridge
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| posted on 22/2/06 at 03:58 PM |
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quote:
Originally posted by flak monkey
Upping the thickness of tube actually makes little difference to the overall strength of the tubes in tension and compression
David
Out of curiosity, Flakmonkey, how do you come upon that statement above? It flies in the face of everything a structural engineer relies on for his
bread and butter.
For a given outer size and load, thicker tube means less stress,...means less strain,...means less deflection. Or have I been grossly wrong for the
last 31 years since I graduated?
Syd. 
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flak monkey
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| posted on 22/2/06 at 04:07 PM |
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Umm dont know. Maybe we should put it down to not having enough sleep and too much project work at uni eh?
Yes OK I wasnt thinking straight and am man enough to admit thats a load of nutsack 
Hopefully my mathsy post above said what i should have put in the first place. Its all a trade off, yes if you use too thinner tube then the chassis
will be crap, too thick and it will be overweight, but strong. I really meant that changing the tube thickness *slightly* will not have a huge effect
on the strength of the chassis (yes you can go too far!), not the strength of the individual tubes. Hope thats clear now.
David
*Note to self - think and check before posting*
Sera
http://www.motosera.com
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Syd Bridge
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| posted on 22/2/06 at 04:15 PM |
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quote:
Originally posted by flak monkey
Umm dont know. Maybe we should put it down to not having enough sleep and too much project work at uni eh?
Too much cheap beer in the student bar last night more likely!
My eldest is finishing his Masters final year at the moment. I know what your going through.
Syd.
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Avoneer
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| posted on 22/2/06 at 04:45 PM |
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In answer to the original question without confusing this poor guy, I think most manafactures use 16 gauge and don't have any problems.
Hope that helps,
Pat...
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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greglogan
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| posted on 22/2/06 at 06:43 PM |
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Sorry guys, didn't mean to start a war. Thanks for the straight answer tho Avoneer. I think i'll try the 16guage, if I can get it as it
seem to be a little tricky to get here. Everyone wants to use 2mm. I'd prefer to go the lighter route if possible.
Thanks again everyone for your input!
Greg
Women are meant to be loved, not understood.
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Peteff
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| posted on 22/2/06 at 07:15 PM |
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If you can't get 1"x 16g see if any suppliers have 25mm x 1.5mm which is practically the same. The only difference I've seen is the
metric stuff has squarer corners
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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JoelP
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| posted on 22/2/06 at 08:18 PM |
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squarer corners is probably better anyway, less gaps when butting them up together.
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kreb
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| posted on 26/2/06 at 09:28 PM |
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I am most assuredly not an engineer, but I can say that I think most of those who want to "engineer" every last gram out of their chasis
are daft. It's one thing to do a detailed analysis and add and subtract stiffness where it benefits you most. It's quite another to say:
let's just subsititute thinner tubing because all the car's going to see is track days.
I wonder how many people would even notice a 10 kg decrease in weight? OTOH how many people would like the maximum structure around oneself when you
tag that wall? Thought so.
https://www.supercars.net/blog/wp-content/uploads/2016/04/1966_FiatAbarth_1000SP1.jpg
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greglogan
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| posted on 8/6/06 at 10:54 PM |
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I know this is digging this post up from the past (sorry!) but flakmonkey dug it out for another post and I realised I had asked the question before.
I 've done quite a bit of research since the original post. 1.5mm is the closest I can get to 1.6mm. There is however a problem It's semi
brite steel which means it's twice the price of 2mm mild steel.
My question is: will there a huge gain in weight by going for the 2mm or should I just swallow the difference and go for the 1.5mm semi brite?
Women are meant to be loved, not understood.
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t.j.
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| posted on 9/6/06 at 06:15 AM |
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quote:
Originally posted by greglogan
I know this is digging this post up from the past (sorry!) but flakmonkey dug it out for another post and I realised I had asked the question before.
I 've done quite a bit of research since the original post. 1.5mm is the closest I can get to 1.6mm. There is however a problem It's semi
brite steel which means it's twice the price of 2mm mild steel.
My question is: will there a huge gain in weight by going for the 2mm or should I just swallow the difference and go for the 1.5mm semi brite?
I noticed the same problems. I've decided to build the frame out of 2 mm. I don't think it will increase the weight that much that i will
notice on a sunny day on my local street
The welding problem as earlier talked about. I've made de gaps about 1-2 mm so the welding is better under control. Also you can grind the ends
45 degr.
So the penatr. will be deep enough.
Grtz
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flak monkey
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| posted on 9/6/06 at 11:55 AM |
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Well it does say in the post to ignore it as its crap Was probably worse for wear at that time of night too!
My other post(s) puts it right... the one with all the numbers in
Sera
http://www.motosera.com
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crbrlfrost
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| posted on 10/6/06 at 01:01 AM |
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Shouldn't the loads be in newtons? jk. Cheers
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