Simple question with probably a complicated answer:
When building a spaceframe, why are you concerned with the second moment of inertia of the tubes?
I guess the spaceframe isn't really a spaceframe, but I'm curious for your answers.
I was more concerned with it holding together and looking pretty. I don't know what inertia is and when it's second moment occurs. Hell, I didn't even notice the first. I'm all of a tizzy now looking for answers to a problem I never knew I had. Don't come on here with your college ways trying to cause arguments or there will be trouble, you mark my words. If there's space for me inside it and I can see through it it's a spaceframe to me. I'm going back out now to strip this wiring loom and think about my place in the universe and the scheme of things. Thank you.
Do you mean second moment of area, which relates to the strength of the individual tubes?
Why do you say this isn't important? I suppose if you assume the spaceframe is loaded entirely in tension or compression along each tube then the
second moment of area wouldn't be important, but unfortunately the frame will have all kinds of loads pushing sideways on the bars. If the second
moment of area (essentially strength) of the bars wasn't good enough, they would just buckle and fold up, which obviously isn't very
desirable!
Hope that helps
Pete
[Edited on 14/12/04 by scoobyis2cool]
urgh, this stuff's confusing
can we not go back to joining tube A to tube B and added extras if it looks weedy?
Yeah, I agree with staple balls.
There are hundreds of book chassis'sss (sp) out there and how many report failures?
Pat...
quote:
Originally posted by Avoneer
There are hundreds of book chassis'sss (sp) out there and how many report failures?
Pat...
Well - It's like this you see;
Second moment of area (or second moment of inertia) is a measure of the ability of a tube of any cross-section (eg rectangular or circular) to resist
bending.
Some cross sectional shapes are better at resisting bending than others, an I-beam for instance is better at resisting bending (in the correct plane)
than then a rectangular tube of the same cross-sectional area.
That's why they build bridges and buildings using I-beams - the best means of resisting bending for the minimal amount of material.
Talking of bridges - I should probably chuck myself off one for being a boring twat!
Matt
thx scoobyis2cool, yes I meant second moment of AREA!
I guess I'm going to have to play around with FEA
Ehhh, your I-beam theory is not correct! When you compare a rectangular tube to an I-beam, and the vertical sides of the rectangular tube have the
same (combined) total thickness as the I-beam web, AND the horizontal sides of the rectangular tube have the same dimensions as the flanges of the
I-beam, their cross-sectional areas are the same, as well as their second moments of intertia. Hence, both are equal in terms of bending in the
vertical plan. And the tube is even much better when it comes to torsion or bending in the horizontal plane.
The reason why they use I-beams for bridges etc. is that they can't extrude steel tubing, and they can extrude I-beams (I think). (also, an
I-beam is easier to inspect, as there is no area you can't see).
A chassis is by no means an ideal spaceframe. It is practically impossible to prevent all planes to be submitted to torsion, by proper triangulation.
Think about passenger area, engine bay. That's why tubing is a smarter option; it's better for out-of-plane bending of the sides.
Mave
I think you will find that an I beam of the same strength as a box section (in a given plane only) actually weighs a little less because the design is
more efficient for a single direction load...
quote:
Originally posted by Mave
Ehhh, your I-beam theory is not correct!
get a life you lot and just make the bloody car....then trash it round the queen's raceway or is that highway!!!!
Based on all the highly interesting and thought provoking info in this thread, iv'e come up with
this cunning new chassis design. Is it a winner or what!
Al
Boll#x i cant get it to show, you'll have to look here to marvel at the cutting edge of design.
http://locostbuilders.co.uk/photos.php?action=showphoto&photo=rsj%20chassis.jpg
[Edited on 14/12/04 by Wadders]
Now that's just taking the p wadders 
Rescued attachment rsj chassis.jpg
That's wadders'sss (sp) pic by the way!
Hah! That beam is actually very flexible the way it's used. Turn it into a tube though and it would be much, much stiffer.
Make that sucker out of carbon fiber and now you're talkin'!
Four wheel steering as well by the looks of it
Don't you guys notice how similar the frame in the pic is to that of a Lotus? Is that the Elan or the Esprit?
yes, 4 wheelsteering, but none are driven
Nice soapbox, though
quote:
Originally posted by Avoneer
Now that's just taking the p wadders
Mmmmh, after careful evaluation of all the expert advice posted on this subject, opinion seems divided as to the strongest method of construction, so
iv'e decided to modify my design a bit. Check it out, and be amazed. Cunning in the extreme even if i say so myself.
http://locostbuilders.co.uk/photos.php?action=showphoto&photo=2rsj%20chassis.jpg
[Edited on 15/12/04 by Wadders]
[Edited on 15/12/04 by Wadders]
[Edited on 15/12/04 by Wadders]
Yup, which changes it into a box/tube/pipe. Even better are two parallel tubes, where the driver sits between them.
quote:
Originally posted by kb58
Even better are two parallel tubes, where the driver sits between them.
I think you've just invented the ladder chassis Kurt!
Yeah but I meant TUBES, as in monocoque aluminum ones, of a foot in diameter or more, as in old F1 chassis. The fuelcells were put inside the side pods. Staniforth reviews this chassis type.
quote:
Originally posted by kb58
Yeah but I meant TUBES, as in monocoque aluminum ones, of a foot in diameter or more, as in old F1 chassis. The fuelcells were put inside the side pods. Staniforth reviews this chassis type.
You fellas is just being silly ... a pyramid with 4 wheels would be much more practical ... 
-Sven
quote:
Originally posted by Aloupol
Yes of course it is.
For a EPN100 as instance (height 100 mm and width 55 mm) the flanges are 5.7 thick each and the web is 4.1 thick. So the moment of inertia is about 50% more than the equivalent tube.
now this is just getting silly....
what you want is a 'real' chassis - like this one.... (definition of 'real' - lots of tubes and the more complicated the
better!)
[Edited on 15/12/04 by protofj]
He, somebody did extensive google work here!
Gordon keeble, isn't it?
quote:
Originally posted by Mave
Please explain what you mean with equivalent tube. In this case, my equivalent tube would be 100x55, with 55 sides being 5.7 thick, and the 100 mm sides being 4.1/2=2.05 thick. In my book, that equates to the same moment of inertia. Or am I missing something?!?
quote:
Originally posted by ettore bugatti
He, somebody did extensive google work here!
Gordon keeble, isn't it?
quote:
Originally posted by Aloupol
Mine would have equal thichness all around, as all the things called "tube" I met so far.
[Edited on 15/12/04 by Aloupol]
the reason that construction uses I section beam is that,when a bending moment is applied,one surface is in comression and the other is in
tension.This means that between these two forces is a "null point" which is neither in compression or tension;so there's no need for
large amounts of material here.Try bending a ruler on it's flat then on it's edge.
All an I section beam is doing is holding the two forces apart and connecting them.
I section beams are useless in torsion,though:hence the need for box section.
re: gordon keeble - looks like a gilbern invader , now theres a car i could certainly live with , i aways like the angular styling - Anybody got one so i can copy it ?
quote:
Originally posted by fester
the reason that construction uses I section beam is that,when a bending moment is applied,one surface is in comression and the other is in tension.This means that between these two forces is a "null point" which is neither in compression or tension;so there's no need for large amounts of material here.Try bending a ruler on it's flat then on it's edge.
Got board at work this morning so I knocked up this Excel Spread sheet which calculates the "Second Moment of Area" for a number of
different tube sections.
It is only very basic, but if people find it useful then I may expand on it, or other people can have a go if they want - its not like its copyrighted
or anything.
I-Value.xls
The formula are from memory (ooh err) so if anyone spots a mistake, please let me know - I didn't get much time to check it.
Cheers.
Which is it doing, holding them apart or connecting them? It can't be doing both . I still don't understand this thread.
I will try to explain it without wading into the engineering math.
Lets say you have a piece of steel, its 30mm wide, 5mm thick and 1000mm long. You lay this flat on the bench so that you can clamp 500mm to the bench
and let 500mm overhang the bench. Now if you push down on the very end of the steel thats unclamped and overhanging the bench and you will bend it
very easily. I am sure you will have probably done something like this in the past.
Now repeat the experiment, but this time put the steel up on its edge and see if you can bend it - probably not, but you still have the same piece of
steel so whats changed ?
You could try this quickly using a ruler on a desk.
The " I " value (second moment of area) makes up part of a formula that helps us determine how much force can be applied for the steel to
bend. The bigger the " I " number, the more resistant it is to bending.
[Edited on 20/12/04 by silex]
So the shorter the piece the higher the value? If you look at a chassis with all the different lengths and the way they interact it must be a nightmare to work out.
Hi,Paddy;
No, when you talk about "overhang" you are really talking about "bending moment" when you talk about "second moment of
area" you are talking about resistance to bending moment of a given cross sectional area(remember the ruler analogy).....And yes,it's a
complete B*****d to get your head round.I,m glad I,m not in collage no more!
quote:
Originally posted by Wadders
Mmmmh, after careful evaluation of all the expert advice posted on this subject, opinion seems divided as to the strongest method of construction, so iv'e decided to modify my design a bit. Check it out, and be amazed. Cunning in the extreme even if i say so myself.
http://locostbuilders.co.uk/photos.php?action=showphoto&photo=2rsj%20chassis.jpg
the bending moment is the edge of a desk if your ruler is flat and the second moment is when you turn your ruler over, or ten past three or something.
I give up now.
your teachers must've loved you at school pete... an uncooperative pupil
in my foolish understanding, square tube is very similar to I-section beams. Except that the tube is 'evened out' so that it isnt as fussy
as the I section. i suspect that I-sections are only good at resisting bending in the one plane, and that is bent on their side they wouldnt be great
- certainly less resistant to crumpling than a square tube.
this thread is really scrapping the barrel, isnt it? i almost wish i wasnt about to continue it.