Wilful ignorance

Posted on February 23rd, 2009 in AutoSpeed,Driving Emotion,Opinion by Julian Edgar

As I have said in the past, AutoSpeed’s internal data generates a daily referrers’ list, where I can see discussion groups and other web pages from which readers are coming.

I read the referrers’ list (and then follow many links) at least a couple of times a day. It tells me, indirectly, which are our most popular articles for that day, and even more importantly, it also shows me how well each article is understood.

Huh – ‘how well it is understood’?

Well, the sad truth is that many AutoSpeed articles are completely misunderstood by readers. But again, that’s useful to me – if my writing hasn’t been clear enough, or the photos of sufficient quality, then I get to see the outcome expressed loud and clear.

But there’s nothing I can do about wilful ignorance.

Recently we ran an article (highlighted also in this blog) on modelling space-frame structures. By building a scaled-down space-frame out of copper wire that is soldered together, you can very quickly model the stresses that occur when the space-frame is subjected to various forces.

The article has been popular and has high reader ratings – that’s rewarding to me when I know that the technique works extremely well, and that further, it is a better technique than other ways I’ve seen in the past for modelling space-frames.

For example, using balsa wood and glue is one approach, but the copper wire technique is better because:

1) it is easily joined quickly and with strength by soldering (far easier than glue, especially when you want to make quick changes and then immediately model the results);

2) the failure mode is typically compression failure by buckling (as occurs in full size, tubular space frames);

3) the shape of failure (ie the sharpness of the bend in the wire) clearly shows the degree of stress concentration, and location of that stress, in the member.

They are all major advantages.

A further advantage is that if a wire fails by bending, it can be quickly straightened and other changes to the model then made. Of course that wire will now be weaker than it was, but in many cases that doesn’t matter because broad-brush changes are being modelled. As I stated in the article, the final iteration should always be tested with a newly built model.

So what was that about wilful ignorance?

A person, who owns and edits a motorsport / homebuilder magazine published in Australia, wrote on a forum about the space-frame modelling article:

This article is perhaps the perfect example of why Autospeed gives me the irits. Virtually the same information WRT models has been in many books for close to 30 years (with one significant change). However wire is not a good medium for such testing, balsa wood and glue is better. However Julian Edgar has obviously not understood the fundamentals of a basic beam let alone a spaceframe. The elements in any spaceframe should be in tension and not compression. He should read Costin and Phipps book on the subject…after all it has been around for close to 50 years now and the engineering pre-dates this to the 1930’s!

This is all simply rubbish.

The statement “The elements in any space-frame should be in tension and not compression” is simply farcical, as even a moment’s thought will reveal. Unless the car is hanging statically from a crane, the loads will always be made up in a space-frame by both compression and extension.

I have the book that the poster mentioned. Let me quote from it (page 22):

“Stiffness must not in any circumstances be imparted by putting bending loads into members at joints. In fact, joints should be loaded only in tension or compression.”

No-where in the chapter (or for that matter the whole book) is a statement that supports the idea that a space-frame should have members only in tension – it is of course impossible.

I suggest that the reason that articles like this give this poster the ‘irits’ is that it’s exactly the sort of breakthrough, down to earth, easily accomplished technique that readers of his magazine would love…

13 Responses to 'Wilful ignorance'

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  1. Darren Roles said,

    on February 23rd, 2009 at 12:22 pm

    I think the Poster should refer to what Costin and Phips (along with everyone else on the planet…) based their work on, namely Isaac Newton. ‘For every action there is an equal and opposite re-action’ (3rd law) and his first law talks about objects at rest stay that way unless acted on by an outside force.
    From the stuff I did at TAFE/Uni on pin jointed structures I thought it was impossible to have a triangulated structure all in tension. I thought the idea of triangulation was to balance out forces to achieve static resolution of forces? Otherwise it’s known as a Couple which induces torsional loads? At least I think that’s what all those FBD’s and Vector resolutions were about.

  2. Tom said,

    on February 23rd, 2009 at 1:57 pm

    Sounds like the poster was just trying to sound smart by invoking age-old engineering principles that you, apparently, only just now published like you’ve been hiding under a rock.

    Never mind the constant revisions that engineers re-publish over the years which are all based on age-old physics principles.

    I can see how you get annoyed, Julian, I would too. But then the Internet is inhabited by many stupid people.

  3. Matt King said,

    on February 24th, 2009 at 8:29 am

    Of course it’s impossible to have a space frame with every member in tension. That’s pretty self evident, so it’s embarrasing that you harp on about it.

    I believe the point that the poster was trying to make is that: where there is an option to put a brace in compression or tension (eg when triangulating a quadrilateral), then the best solution is to put it in tension. That way the brace can’t buckle.

  4. Julian Edgar said,

    on February 24th, 2009 at 8:38 am

    That poster was criticising the article, and criticising me personally for my alleged lack of knowledge of the subject area, and in the same paragraph quite clearly expressing a view that showed he did not understand even a basic idea about space frames. I don’t think it is ‘harping on about it’ to point out the absurdity of his stated position.

    Whatever you may believe he was trying to say, I go on what he did say.

  5. Matt Lennon said,

    on February 24th, 2009 at 9:00 am

    Your article was an introduction to the interaction of forces and moments…at best. Do not allude to it being anything further, and certainly do not feed the public nonsense like “next best thing to FEA”. Whats next, suspension design with spring loaded pens?

  6. Julian Edgar said,

    on February 24th, 2009 at 9:12 am

    Yes Matt, I saw your post on another forum and actually thought “introduction to the interaction of forces and moments” was an excellent summary about the testing technique, perhaps with the added point: “and then see what happens when these forces and moments cause failure of the elements”.

    Spring loaded pens? Sure, any spring can be tested in a way that shows important implications for a specific suspension design.

    Next best thing to FEA? I stand by that, and put this to you: show me a BETTER way that someone without access to full-scale engineering mathematics, and the ability to use (and acess to) an FEA program, can better design space frame structures?

  7. Matt Lennon said,

    on February 24th, 2009 at 9:49 am

    I put this back to you. If someone is going to the effort, and are obviously intelligent enough to build a spaceframe, then they are intelligent enough to understand the BASIC engineering principles of moments and forces, and thus apply some form of quantifiable load. When it comes to engineering you shouldnt leave “gaps” for joe bloggs to fill, for example “This member didnt deform at all so ill just throw some EA at it. Its not expected to be able to do an FEA analysis on paper, but a basic frame load calculation shouldnt be out of the reach of the average builder, and if it is, dont use it in public (whatever you build).

  8. Julian Edgar said,

    on February 24th, 2009 at 10:29 am

    Matt, I don’t really understand what you have written. Do you mean that anyone who builds a space frame should be able to mathematically analyse it?

    I am quite happy to admit that doesn’t hold for me, and from what I have gathered over ten years of feedback from AutoSpeed readers, certainly doesn’t hold true for most readers.

    I think the copper wire modelling of a spaceframe during its development and design is a brilliantly practical, simple and effective technique. I might add that I struggled for months with other approaches, including mathematical analysis.

    I will be using the copper wire approach tomorrow to model and then build a rear trailing arm spaceframe for Chalky attempt #3(!), and I am certain I will achieve a better outcome in terms of strength to weight than any other technique of which I am capable. (Incidentally, that will involve a suspension arm that weighs ~1kg and can support dynamic loads of 100 times that.)

    If you can do the maths and the FEA (and have access to the latter) – great! If I could, I too proabably wouldn’t bother with copper wire models.

  9. Matt Lennon said,

    on February 24th, 2009 at 10:47 am

    Julian, one last parting question. How exactly do you summarise from your copper model what size member to use (for example your suspension arm)? How do you know the magnitude of the moment acting on the member? Your cooking with a recipe that has all the ingredients, it just doesnt have any quantities to the ingredients.

  10. Netduke said,

    on February 24th, 2009 at 11:08 am

    Without getting an engineering degree or getting $10k FEA software, or having a third party run a FEA simulation on my go kart design, the copper wire method is so far the easiest+fastest way I have seen.

  11. Julian Edgar said,

    on February 24th, 2009 at 11:13 am

    Matt – absolutely right. As I have said elsewhere (I have another blog column on this topic), by using the copper wire technique you cannot assess the magnitude of the forces the structure will withstand.

    However, you can assess the ‘pecking order’ of the required tube strengths within the structure – eg ‘this’ tube needs to be twice as strong in bending (or even just ‘a lot stronger’) than ‘this’ tube, etc.

    You can also optimise the structure to provide the greatest strength possible for a designated weight.

    You can also optimise the structure to provide strength to cater for the right ratio of forces eg in my trailing arm of tomorrow, I know that the bending loads it will need to withstand are about twice as high as the torsional loads, and I can model this by using a spring balance to apply the two differing loads to the copper wire model.

    It’s also better than a traditional ‘pin jointed’ engineering analysis because the soldered joins allow bending forces to be applied, if the spaceframe is so designed.

    You can also spot major design errors in the spaceframe design – and so on.

    WRT to the suspension arm:

    1. I will build the copper wire model and then test it, revise it, etc,

    2. I will then will use the smallest tubes I think I can get away with (based on looking at other bikes and trikes), with the tubes arranged in the pattern and in the right order of strength as revealed by the modelling.

    3. Then I will test the final unit under real loads and measure deflections.

  12. Sam Q said,

    on December 17th, 2009 at 2:30 am

    Matt, a rare minority know how to work out stresses on a member but many have made light, strong and rigid structures using nothing more than the “whatever looks right” approach. This is a simple method and it’s not claimed to be more than that. I personally think it’s a great tool for it’s purpose and gives clarification for a proposed design. You know it is quite possible to make something decent without knowing how much stress in exact figures is going into it.

    @Julian: surely people on that forum also saw that guy who posted as a complete lunatic right?

  13. Julian Edgar said,

    on December 20th, 2009 at 6:46 am

    Sam: they actually supported him to the hilt….