Pseicho
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| posted on 14/12/04 at 01:31 PM |
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Spaceframe <-> second moment of inertia
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.
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Peteff
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| posted on 14/12/04 at 02:19 PM |
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You what?
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.
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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scoobyis2cool
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| posted on 14/12/04 at 02:29 PM |
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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]
It's not that I'm lazy, it's that I just don't care...
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Staple balls
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| posted on 14/12/04 at 03:02 PM |
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urgh, this stuff's confusing
can we not go back to joining tube A to tube B and added extras if it looks weedy?
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Avoneer
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| posted on 14/12/04 at 03:08 PM |
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Yeah, I agree with staple balls.
There are hundreds of book chassis'sss (sp) out there and how many report failures?
Pat...
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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scoobyis2cool
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| posted on 14/12/04 at 03:20 PM |
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quote:
Originally posted by Avoneer
There are hundreds of book chassis'sss (sp) out there and how many report failures?
Pat...
I don't think he was really suggesting the chassis' are under-engineered. As you say, experience seems to show that they are more than
good enough for the job. I think it was more a general interest question
Pete
It's not that I'm lazy, it's that I just don't care...
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mattpilmoor
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 posted on 14/12/04 at 03:40 PM |
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Second moment of area
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
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Pseicho
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| posted on 14/12/04 at 04:14 PM |
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thx scoobyis2cool, yes I meant second moment of AREA!
I guess I'm going to have to play around with FEA
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Mave
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| posted on 14/12/04 at 04:30 PM |
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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.
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DaveFJ
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| posted on 14/12/04 at 05:02 PM |
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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...
Dave
"In Support of Help the Heroes" - Always
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Aloupol
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| posted on 14/12/04 at 05:39 PM |
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quote:
Originally posted by Mave
Ehhh, your I-beam theory is not correct!
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.
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undecided
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| posted on 14/12/04 at 06:50 PM |
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get a life you lot and just make the bloody car....then trash it round the queen's raceway or is that highway!!!!
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Wadders
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| posted on 14/12/04 at 07:08 PM |
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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]
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Avoneer
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| posted on 14/12/04 at 07:16 PM |
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Now that's just taking the p wadders 
 
Rescued attachment rsj chassis.jpg
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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Avoneer
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| posted on 14/12/04 at 07:17 PM |
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That's wadders'sss (sp) pic by the way!
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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kb58
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| posted on 14/12/04 at 07:42 PM |
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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.
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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pbura
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| posted on 14/12/04 at 08:32 PM |
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Make that sucker out of carbon fiber and now you're talkin'!
Pete
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MikeRJ
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| posted on 14/12/04 at 08:39 PM |
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Four wheel steering as well by the looks of it
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Pseicho
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| posted on 14/12/04 at 08:41 PM |
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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?
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ettore bugatti
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| posted on 14/12/04 at 09:51 PM |
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yes, 4 wheelsteering, but none are driven
Nice soapbox, though
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Aloupol
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| posted on 14/12/04 at 10:05 PM |
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quote:
Originally posted by Avoneer
Now that's just taking the p wadders
Who the hell told you that I-beam is good in torsion?
Try again, you can do it...
[Edited on 14/12/04 by Aloupol]
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Wadders
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| posted on 15/12/04 at 12:48 AM |
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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]
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kb58
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| posted on 15/12/04 at 01:47 AM |
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Yup, which changes it into a box/tube/pipe. Even better are two parallel tubes, where the driver sits between them.
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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Rorty
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| posted on 15/12/04 at 02:22 AM |
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quote:
Originally posted by kb58
Even better are two parallel tubes, where the driver sits between them.
OMG! I think you've just invented the ladder chassis Kurt!
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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kb58
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| posted on 15/12/04 at 05:36 AM |
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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.
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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