Caber
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?
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
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]
Bit of contradiction there, 3mm is going to be double the weight. Is it a Cobra website?
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!!
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]
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.
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
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
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]
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]
quote:
Originally posted by flak monkey
Yes I am 'educated' in engineering. Sorry, but I am not going to take offence
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]
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.
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
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*
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?
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...
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
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
squarer corners is probably better anyway, less gaps when butting them up together.
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.
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?
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?
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
Shouldn't the loads be in newtons? jk. Cheers
Yep, but i converted them to Kg for the purposes of this. By all means carry out the calcs for yourself. It doesnt matter anyway, just multiply
thenumbers by 9.81 if you want them in N...
People identify more easily with N than Kg, well most probably do anyway, hence i just converted the loads, sorry if this upsets the
engineer/physicist in you
How thin could you go on tubing if you use 4130 cr-mo,for chassis.I not not locost but it would be pretty light
so a chassis built from 35mm 16ga steel tube would be plenty strong and stiff enough for street/limited track use?
I've been thinking 25mm may work but looks a little small (I ususally way overbuild things)
so what is the final results of all the debate between everyone with more education than me?
which size and thickness of tube is the best balance of safety and weight.
[Edited on 11/6/06 by violentblue]
In terms of strength, you may be able to get away with slightly thinner 4130, but then you have to start looking at the lengths of the spans and oil canning (flexing and collapsing of the wall). On the other hand, the modulus of 4130 is really no better than mild steel, so your stiffness (what we all basically strive for) will go to pot. Actually, with the exception of extreme loads, I don't understand peoples fixation on 4130 as a chassis material. There's the welding issues, the heat treating issues, the cost, etc, with no appreciable increase in stiffness. For a rollbar and its mounting yes, but thats because by themselves they can be heat treated post weld. Oh well, perhaps I'm wrong, and I'm rambling. Cheers!
25mm x 16G.
That's what 99.9% of us use without any problems.
Why over-complicate things and pay more?
Pat...