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Thread: Beam Axles - Front, Rear or both.

  1. #161
    The examples mentioned aren't attempts to approach the limit dictated by the underlying principles like motorsports and engineering competitions.

  2. #162
    Z,

    1. Old texts:

    Yes, I too seem to find many of the older texts more 'readable' and understandable. Newer texts (physics, mechanics, machine design etc.) are very flashy, lots of useless graphics, colors, etc. but totally disjointed and frustrating, to me anyway.

    2. Professor Walter Lewin:

    I will not defend him or his teaching methods. I will just say I personally learned something from his video lectures and that his showmanship helps make the basic points stick in some way for me.

    His is the perspective of a physicist not an engineer. And as you say there is a pretty big difference.

    Lastly from the course outline:

    "8.01 is a first-semester freshman physics class in Newtonian Mechanics, Fluid Mechanics, and Kinetic Gas Theory. In addition to the basic concepts of Newtonian Mechanics, Fluid Mechanics, and Kinetic Gas Theory, a variety of interesting topics are covered in this course: Binary Stars, Neutron Stars, Black Holes, Resonance Phenomena, Musical Instruments, Stellar Collapse, Supernovae, Astronomical observations from very high flying balloons (lecture 35), and you will be allowed a peek into the intriguing Quantum World."

    Quite a bit of ground to cover for a fall semester freshman course on classical mechanics. And also quite an advertising pitch for a basic freshman physics course.

    Hmm...I guess I did wind up defending him in a way despite what I said above.

    Oh, and for those interested and who did not come up with an answer for the ball and track demonstration problem, the answer can be found in lecture 29.

    3. Too many equations:

    Try a few of these videos on for size from one of MIT's basic dynamics courses:

    http://ocw.mit.edu/courses/mechanica...2011/index.htm

    Now there is an alphabet soup course designed to be 'specific' after a freshman gets past Lewin. Makes Lewin's course seem absolutely devoid of the maths.

    I fundamentally agree with your insistence for a grounding in the very bedrock of the logic and deductive reasoning used by the creators of this subject. However, I also know that we have created an engineering profession so heavily reliant on technology, computing power in particular, that there just isn't time in four or five years to even scratch the surface for a student.

    That is what makes FSAE important in my opinion. If for no other reason than there will at least be a very few engineering students who have truly attempted to build something, anything, and found out for themselves how difficult the process can be. What they build is immaterial to me, personally. Think of all the rest...

    As far as the false profits of engineering science are concerned, they have always been there and will continue to be, most likely even in vastly greater numbers with the availability of the internet to pedal their wares.

    And... Back to Beam Axles!

    Ralph

  3. #163
    Z,

    One last point on all the confusion concerning the terms moment, couple and torque. Look for torque in the Hartog index, none. In index of Statics by J.L. Meriam 'Torque, see Moment of Force'. In any physics text I have torque is defined as r X F.

    Couples in Hartog are designated as C in Meriam as M (same as moment).

    Point, physicists use torque, no moment or couple and engineers use moment, couple and torque with only one source of clear distinction I could find on Wikipedia under Torque of all places. Did you write that entry?

    No wonder students get all turned around and confused.

    Ralph
    Last edited by rwstevens59; 05-14-2014 at 07:19 PM.

  4. #164
    Quote Originally Posted by Jay Lawrence View Post
    Z,

    I was thinking about this 'everything from 1st principles' point of view and wonder if we as humans skip steps so that we may progress? I don't need to know how an engine works to put one on a cart and therefore make it 'better'. I don't need to know how radio waves work in order to control an R/C car. I don't need to know a great deal about aerodynamics to play golf. I learn about things such as these because they are of interest. If everyone had to learn all there is to know from the very basic principles upwards, we'd run out of life before we made any progress! Something about giants and their shoulders...

    Just a thought.
    hear hear..
    University of Tasmania (UTAS)

  5. #165
    Quote Originally Posted by Jay Lawrence View Post
    Z,

    I was thinking about this 'everything from 1st principles' point of view and wonder if we as humans skip steps so that we may progress? I don't need to know how an engine works to put one on a cart and therefore make it 'better'. I don't need to know how radio waves work in order to control an R/C car. I don't need to know a great deal about aerodynamics to play golf. I learn about things such as these because they are of interest. If everyone had to learn all there is to know from the very basic principles upwards, we'd run out of life before we made any progress! Something about giants and their shoulders...

    Just a thought.
    My post in response to this was not clear.

    If I may, I'd like to restate my point;


    I don't believe that Z is advocating reducing every task to first principles. As you state, that would be inefficient.

    However, there is a fundamental difference between pursuits like motorsports and the examples given in the above quote.

    Take the case of controlling an RC car. The underlying physics of signal transfer through electromagnetism are not an area where you can get much of a performance boost. Radio waves are, in this case, a commodity. If one were to devote years of study to devising a more efficient manner of sending control inputs to the car, the improvement over standard off the shelf items would be minimal.

    A similar situation applies to the golf example. No amount of study into aerodynamics will net you the same performance improvement that learning how to swing a club consistently will.


    Contrast those with the design of vehicles whose primary purpose is to operate at the very edge of their performance envelope. Motor racing vehicles spend almost there entire life at or near design limits. With the exception of maybe single use space launch vehicles, nothing else even comes close.

    This performance envelope is defined by the laws of physics. Understanding these laws has a massive impact on our ability to develop a vehicle able to test these limits.




    Cory

  6. #166
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    Yes, but like learning how to swing a golf club consistently, I can learn how to apply magic numbers and simple 'roll centre' approximations without having to go down into first principles of mechanics (or aerodynamics/projectile motion in the golf example). Maybe I will lose a tiny amount of initial performance (i.e. my more learned competitor might get to the track with perfect spring rates and no need for dampers and have a car that is immediately at its design limit) but it is likely that I would have a vehicle ready quite earlier than he/she because I have stood on some shoulders, applied a top down approach, and not spent the time to get to the very basic fundamentals. The extra time is what I use to tune the car in the real world and bridge my theory gap.

    Perhaps my examples were a bit off, and I'm not necessarily advocating to forget the classical side of things, it was just a thought.
    Jay

    UoW FSAE '07-'09

  7. #167
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    I agree more or elss to the last few posts. The problem with many óf these short cuts (e.g. roll centres...), is that it is often taught as though they ARE the first principles.

    I'm pretty resentful of the fact that the first few years of my professional life as an automotive engineer were spent unlearning stuff that I was "taught" in various books and seminars

  8. #168
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    Firstly, thanks to everyone above for giving some thought to these educationally fundamental, but not-directly-FSAE-related, issues. Here is some more general ranting. More specific responses next post...
    ~~~~~o0o~~~~~

    I do not blame Prof. Lewin for any of the teaching errors above. As noted, he does a great job of explaining the subject in an interesting way. The problem is with the modern education system in general, which nowadays seems to be,
    "Well, it's all about this humungous jumble of equations, all of which you will have to learn. You have to learn all of them individually, because they are all a bit different. But they are also all kind of the same. Err..., in the same way that these days 'all answers are equally correct', and 'everyone's opinions must be equally respected'. Yep..., that way we can always give all the kiddies a gold-star...".

    IMO the best way to teach Mechanics would be more of the Lewin/Mythbusters style of simple, real, and interesting experiments, but with much more honesty, clarity, and rigour in the explanations. This takes no more time than the current method. In fact, probably less, because most of the unnecessary equation-crunching can be dropped. So;

    HONESTY - Don't tell lies about what people did or said hundreds of years ago! How is that educational?! An honest historical account is more ethical, and also often more interesting. For instance, in Newton's Definition 1 he defines the "quantity of matter" (= "mass" nowadays) as "... its density and bulk conjointly". Note how we do it the other way around these days (ie. density = mass/volume) .

    Also, the notion that "We are cleverer than Newton because we know that General Relativity gives more accurate answers than Newton's ULG..." is nonsense, as noted before. High school teachers might mistakenly suggest this, but University Professors should not teach this sort of slanderous rubbish. Unfortunately, when they all do it, they can all get away with it. And all you students are the losers.
    ~o0o~

    CLARITY - "F = P-dot" is really much easier to understand than "F = mA". I only started to think that way well after I finished my schooling, and I highly recommend it. Lewin often mentions that rotating dynamics are very "non-intuitive". This is, IMO, due to too much "F = mA". And you can forget about ever understanding gyroscopes (the most "non-intuitive" of the lot, according to Lewin) until you move to "F=P-dot". Then it all suddenly falls into place!

    BTW, "Angular Momentum" (Lewin calls it L) was in the olden days called the "Moment of Momentum". It is simply the sum of all the "Moments" (= Cross-Products) of the Linear-Momentum-Vectors of the various particles and their Radius-Vectors, taken at a particular point. There is still a bit more explaining required here (maybe with some simple experiments), but seen geometrically it is all quite simple, and it gives "T = L-dot".

    In the preface to Bevan's ToM book (ref'd earlier) he says "As so many of the problems which arise may be solved more quickly and easily by graphical methods, particular care has been taken to draw the diagrams correctly...". I note that in Lewin's lectures the "man climbing a ladder" problem is "solved more quickly and easily" in the simple act of drawing the FBD! (Lewin has to work through quite a lot of algebra for the same end result).

    Similarly, the "time period of the hula-hoop pendulum" is solved in one, simple, do-it-in-your-head step, based on "equivalent mass systems". (Hint - What length dumb-bell has the same (2-D, planar!) mass-distribution as the hula-hoop?) And the "sliding vs rolling-ball pendulums" (ie. problem asked on other thread) is solved by simply noting the paths of motion, in side-view, of different points on the two bodies. (Hint- Which body has greater changes in its "quantity of motion"?)

    Bottom line here, geometrical (or "graphical", as Bevan calls them) methods can give greatly improved insight into problems, and much quicker solutions. Your schools NOT teaching them is your loss.
    ~o0o~

    RIGOUR - It should be constantly restressed that Newton's 3 LoMs are unprovable "Axioms", while most of the other "Laws" are deductions from these. It is a heirarchical system, starting with foundations at the bottom, and then other stuff built on top. The closer to the foundations, then the more widely applicable are the concepts. The further away from the foundations, then, typically, the more simplifying assumptions that have been used, and the LESS USEFUL is that "Law" (see eg.s below).

    For a general feeling for how this works, ask the good citizens of Pisa how things go when you get the foundations wrong. Ok, so they do make some tourist-lira out of it now, but that is mainly because people enjoy looking at huge cock-ups. And the good citizens are having to pay quite a lot to prop-up their cock-up, lest it disappear into a pile of its own rubble...

    As an example of the heirarchical approach, Kinetic Gas Theory is deduced from Newton's Axioms, typically with a few other assumptions thrown in (eg. the molecules are assumed to bounce off each other like elastic billiard balls). Likewise Bernoulli's Law is deduced from N's Axioms, again with a bunch of simplifying assumptions thrown in. Now, if you happen to forget any of these more superficial Laws, then, with practice, you can always re-deduce them from the very small set you started with (ie. N's I, II, & III). Fortunately, doing this reminds you of ALL those simplifying assumptions. Namely the times when said "Laws" DO NOT APPLY! (<- A hint here to FSAE-aero-guys that Bernoulli does NOT ALWAYS apply!).

    An even more superficial example is the "Law of Friction", which barely qualifies for that title. Worth noting that back in the 1960s, when racing tyres started to get ridiculously wide (from 4" to 6"!, then 8"!!, then !!!) there where many experts, those that, ahem, understood the "Laws of Nature", that claimed that "wider tyres won't make any difference!". The "Friction Law" is a reasonably good approximation for hardish and smoothish materials sliding on each other, over a smallish range of pressures. But not much good for soft, sticky stuff sliding on a rough surface. And certainly no good at balancing your car's handling via LLTD and TLS.
    ~o0o~

    Bottom line, it does not take a lot of time to constantly restress this idea that there are different "levels" of Laws (and some SHOUTING might help get the message across in less time ). The fundamentals are by far the most important, and should be understood the best. The more superficial stuff is less important, and if you happen to forget some of it, then no problem, because it is not that accurate anyway...

    MOST IMPORTANTLY, never forget that the superficial stuff is usually built on a whole lot of simplifying assumptions, many of which might not apply to your particular problem!

    Z

  9. #169
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    Quote Originally Posted by Jay Lawrence View Post
    I was thinking about this 'everything from 1st principles' point of view and wonder if we as humans skip steps so that we may progress?
    ...
    If everyone had to learn all there is to know from the very basic principles upwards, we'd run out of life before we made any progress!
    Jay (and others with similar comments),

    I hope I have addressed this sufficiently above. But in case not, just be aware that only a very small part of what you learn in Science/Engineering is solid bedrock. The greater part is [...thinking of metaphor...] like quicksand with a very thin, but hardish looking, crust on it. You can pitch a tent on the latter, but don't go building an office block there (or a Pisa-esque tower ).

    And racecar VD is even worse. For example, there is the "Law of Never Letting Your RCs Migrate Sideways". This is pure poppycock that has never been "deduced" from the more fundamental stuff, but has somehow entered the field as an old-wives-tale. It is relatively harmless (it just stops you having "ground level RCs", or horizontal n-lines), but it can waste you a lot of time.

    Knowing how to distinguish the important stuff from the trivial nonsense, is, well..., IMPORTANT!
    ~~~o0o~~~

    Originally posted by Ralph:
    One last point on all the confusion concerning the terms moment, couple and torque...
    Couples in Hartog are designated as C in Meriam as M (same as moment)...
    ... only one source of clear distinction I could find on Wikipedia under Torque of all places. Did you write that entry?
    Ralph,

    I don't do any Wikipedia editing. It is too much "design by committee" for me. In fact, I reckon it is accelerating the descent down the S-bend by introducing more and more poorly phrased definitions, and thus legitimising this sort of sloppiness in the eyes of the younger generation. For example (from Wiki- "Torque");
    "In US mechanical engineering, the term torque means "the resultant moment of a Couple,"[5] and (unlike in US physics), the terms torque and moment are not interchangeable. Torque is defined mathematically as the rate of change of angular momentum of an object..."
    So two different definitions right next to each other!

    The second one comes from "T = L-dot", and, strictly speaking, should read as "a Torque CAUSES, and is proportional to, a rate of change of angular momentum". But the double translation, first into algebra, and then back into English, gives the impression that the torque is an end result, or effect, with the L-dot being the cause. Anyway, the whole quote above is very misleading, because it loses the direction of the causality.

    Personally, for "a purely rotating, forceful, action" I prefer "Couple". This makes it very clear that for such a rotating action you need at least TWO matched forces (ie. "a couple of..."!). "Torque" would be ok, except that it doesn't stress this "two-ness". "Moment" is far too vague, because it is used in too many different ways (eg. 1st/2nd/3rd - Moment of - Area/Mass/Force/whatever...).

    With regard to symbols, I think "T" (often Greek Tau) first started to be used for Couple (which later became Torque) because in Descartes' "Analytic Geometry" (c. ~1600) he used A, B, C for general purpose constants, and X, Y, Z for the unknown variables. This has since become universal, so "C" doesn't really suit a variable vector. Descartes' abstraction into alphabet soup means that even with two whole alphabets (Greek and Roman), and quite a lot of different fonts these days, we still don't have enough symbols to uniquely label all the different concepts (T is also time, L is also length, etc...)

    Doing it graphically is much easier because we just draw a different sort of arrow for each concept. So, as in the earlier figures, a ring-like arrow around a normal arrow indicates rotational stuff. Add different sorts of feathers, and you can represent Force, Motion, or whatever you want...

    Z

    (PS. Don't bother googling images of "Couple Moment". You just get pictures of couples canoodleing...)
    Last edited by Z; 05-17-2014 at 06:41 AM.

  10. #170
    Tim.Wright,

    I would have to agree with you 100%.

    What frustrates me the most is I feel that I should have known better but I took the ride anyway.

    Starting to learn VD much later in the game then most students and having a fairly good but ancient (i.e. pretty rusty) understanding of classical mechanics I always had the feeling I came in at the middle of the story while studying the excepted seminal works.

    I would be skeptical many times and fall back to drawing very basic FBD's then convince myself that my gut instinct was wrong, because after all the author(s) are the practiced experts, right? I just need to read and study more, I thought.

    So, after many fits and starts, a lot of dead end alleys explored I am finally to a point of a somewhat better understanding.

    Am l resentful, no. But I am quite disappointed and concerned with many of the sources proclaiming educational content in vehicle dynamics.

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