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  1. #1
    Senior Member trmcgeehan's Avatar
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    Static vs Rotating Pounds

    David Lamb, in his 1995 trans-America book "Over the Hills," talks about removing several water bottles to reduce weight. He claims "According to physics, every static pound is the equivalent of two rotating pounds." What does this mean? Does this have anything to do with unsprung weight?

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    As an engineer, I can confidently say that he doesn't have a clue as to what he's talking about. I can't even imagine how he could come up with such an opinion even based on personal observation. Would any other engineers care to agree?
    Je vais à vélo, donc je suis!

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    Member SpotmaticF's Avatar
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    So, if D.Lamb mounted his water bottles on his wheels they would weigh half as much.

    Ok consider this:

    Move 2 pounds from your frame and add it to your wheels. Then try and climb a hill, or accellerate from a stop. You will feel as though lots of weight has been added to your bike.

    Please don't ask me to get out my textbooks to give proof.

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    serial mender
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    I am an historian not an engineer, but I have always understood that rotating weight (wheels, cranks, pedals) costs more energy because of the fact that you have to overcome both its rotational inertia, as well as its static inertia. I think that is why they say rotating weight counts twice. But, I would imagine that you have to account for how far out from the axle the weight is placed--which would make any simple 1 + 1 = 2 statement only coincidentally correct.

    Now, as I understand it, this only affects acceleration, not constant speed climbing. Once the wheel is moving, it will want to keep moving (Newton's law), and that doesn't change just because gravity is making life hard on you up that hill. So, SpotmaticF's test will be true for acceleration, but false for climbing (once a constant speed is reached).

    For the same reason, on flat ground your weight makes no difference whatsoever in the energy you expend once you have reached a constant speed.

    By the way, I bought new pedals this year that were about 1/2 the weight of my old ones. Although it was a difference of less than 200 grams (far less than carrying an extra water bottle), I really noticed the change--but only while accelerating.

    The above is based on my meager memory of Physics 110. Any confirmations (or rebuttals based on physics) would be much appreciated.

    Cheers,
    Jamie

    P.S. As an historian, I can confirm that this has always been true in the past--provided it is still true today!

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    "According to physics, every static pound is the equivalent of two rotating pounds."
    Well, rotating mass (not the same as weight, BTW) does count both towards the total mass and also will require extra torque to spin, but that amount of torque is dependant on how far the mass is located from the polar moment of inertia, and the angular acceleration. So, in a non-scienticic way, perhaps the mass is counted "twice", but this in no way is the same as saying that Mstatic=2*Mrotating, which is essentially what this uninformed author is saying.

    Perhaps what he meant was that Mrotating=2*Mstatic, which, although a fallacy in most circumstances, might at least make sense in very, very specialized circumstances. In other words, perhaps what he meant to say was "every rotating poung counts greater than a static pound", which is diametrically opposed to what you quoted.

    I am an historian not an engineer, but I have always understood that rotating weight (wheels, cranks, pedals) costs more energy because of the fact that you have to overcome both its rotational inertia, as well as its static inertia. I think that is why they say rotating weight counts twice. But, I would imagine that you have to account for how far out from the axle the weight is placed--which would make any simple 1 + 1 = 2 statement only coincidentally correct

    Jmlee: You missed the point that I was making. His facts were backwards. Did you actually pass your physics class? Do you have a degree in either physics or engineering? Stick to history-your lack of knowlege of physics is frightening-not to mention likely embarassing.

    Move 2 pounds from your frame and add it to your wheels. Then try and climb a hill, or accellerate from a stop. You will feel as though lots of weight has been added to your bike.
    spotmatic: Go back and read the original post. Now read your rebuttal. Notice your error? He was saying that static mass counted at TWICE what rotating did. Did you also flunk physics 101? I certainly hope you aren't an engineer!
    Last edited by D*Alex; 11-15-02 at 01:08 PM.
    Je vais à vélo, donc je suis!

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    What static weight? Most of us ride our bikes so everthing is moving (either translational only or both rotational and translational.)

    If he has static weight, then he's either a relly good trackstander or he sits still while the wheels and earth rotate under him.

    Physics is great. We can say all kinds of wierd stuff. Like the contact point of the tire really isn't moving (instaneously anyway) as is moving around the circle at the same speed as the bike is moving, but in the opposite direction. Conversly, the top of the tire is moving twice as fast as you are! I guess I just contradicted myself as I said earlier that everything on the bike is moving, but oh well.

    At this point I forgot who asked, but yes the distance of the mass from the point of rotation is important when calculating angular momentum and therefore the force required to accelerate something along a curved path.

    Please correct me if I'm wrong as I'm a student with a test coming up soon.

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    Member SpotmaticF's Avatar
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    Alex:

    I did realize that the author had his logic backwards.



    My first statement contained a measure of sarcasm which was not quite as obvious as I had thought.
    So, if D.Lamb mounted his water bottles on his wheels they would weigh half as much.
    By this perverse science we should consider moving all manner of ancillary equipment to our wheels. Maybe my tool kit could be strapped to my spokes to reduce its weight by half.

    This is ludicrous, of course, though in over simplified terms, the opposite could be true as you pointed out.

    Perhaps what he meant was that Mrotating=2*Mstatic, which, although a fallacy in most circumstances, might at least make sense in very, very specialized circumstances. In other words, perhaps what he meant to say was "every rotating poung counts greater than a static pound",
    Boy this is fun.

  8. #8
    serial mender
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    D*Alex,

    In your first post you made no other point than to say that the guy was clueless and that you were confident about that.

    I, too, recognized the mistake in the quotation. But, I decided that I would try to answer what I assumed was the poster's actual question: what is the relationship between mass of rotating parts and total mass? I forwarded my understanding of the matter, appending several qualifications about my lack of expertise.

    As I read your second post, I understand it as confirming the essence of what I said, including my point about the need to account for the distance of the mass from the center of the rotating object.

    In short, based on the information you provide, I see nothing grossly wrong in my post, even if my language betrays a layman's understanding of the matter. Instead of insults, I would appreciate a demonstration of what is actually wrong in what I was trying to convey.

    Furthermore, would you please enlighten us to the "very, very special circumstances" under which this is true, and confirm whether they apply to a bicycle.

    Cheers,
    Jamie

    P.S. I, too, could make fools of most of the readers of this forum in several areas of historical, cultural, and linguistic knowledge. When a question that falls in my area of expertise pops up, I don't respond with, "hah, you don't know the importance of 1789 or how to spell peloton. Did you flunk both history and French?" Just as I have always done for my students, I accommodate myself to my readers and share with them what I know--which, of course, you started to do in your second post.

  9. #9
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    Ok, for those of us who dozed off in physics.

    Is the reason rotational mass expends more energy on something like a bike, because the additional effort to get the wheels, cranks or peddles moving, must also be maintained, as well the energy required to maintain the bike at a constant speed?

    (Mrs. Engle, I hope you cringe at that run on scentence!! Plus the punctuation and spelling!!!)

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    The real beast is air friction once you've gotten to the speed you want to maintain. (unless you ride a Huffy) All other forces are neglible by comparison. As far as the rest, I forgot the equations but for an example let us say you have a choice between 2 20lb bikes on the first the frame weighs 10 and each wheel weighs 5. On the second the frame weighs 15 and each wheel weighs 2.5. Now both bikes weigh the same but you'll be better off with the heavier frame and the lighter wheels. (Unless it's a Huffy)

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    JDP
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    Originally posted by jmlee
    For the same reason, on flat ground your weight makes no difference whatsoever in the energy you expend once you have reached a constant speed.
    In a perfect world, yes, but we have friction to overcome as well.

  12. #12
    The clock's run out kewlrunningz's Avatar
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    I remember someone saying on the forums that 4 lb on the bike was equal to 1 lb on the wheels, can't confirm or disprove this. I will ask my "fizziks" teacher monday about this and report back with a large formula with theta, pi, and a bag of other greek symbols so that we all may feel less of our selves :confused: . Anyways, taking weight off of anything rotational will improve your performance more than an equal amound removed from the non-moving parts of the bike. That said, what good would a formula do us?
    Hello moto

  13. #13
    Senior Member knifun's Avatar
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    Originally posted by D*Alex
    [B]
    Well, rotating mass (not the same as weight, BTW) does count both towards the total mass and also will require extra torque to spin
    D*Alex, but what happens as you approach the speed of light? Does a 17 pound bike even matter as one rides at the speed of light, or maybe even faster?
    SpongeBob

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    i think we can all agree that rotational mass is what we feel more when we ride as opposed to static mass. i spent the dollars buying a nicer set of wheels and the ride felt much lighter than with the cheap oem wheels.

  15. #15
    Plays well with others. greg360's Avatar
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    jmlee, I think you've got it right and said pretty much what had to be said.

    Another good perspective,
    Originally posted by tFUnK
    i think we can all agree that rotational mass is what we feel more when we ride as opposed to static mass. i spent the dollars buying a nicer set of wheels and the ride felt much lighter than with the cheap oem wheels.
    This is why I don't obsess over the weight of the frame too much (does anyone ever say anything about the weight of the rider?).
    I ditched some heavy (sigh, high mass) OEM wheels & sprang for a nice pair of Bontrager Racelights for my hybrid, with cool Conti GP 3K tires and lightweight tubes. That was all it took to take my ride from milquetoast to megafun!
    "We want to make good time, but for us now this is measured with emphasis on 'good' rather than 'time' and when you make that shift in emphasis the whole approach changes."
    Robert Pirsig, Zen and the Art of Motorcycle Maintenance

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    It is nearly 40 yrs since I got my engineering degree, and I cant remember the formulae. However I once did the calculations and I think they told me that 1 oz on the rim of a 27" wheel was equivalent to 1.3 oz on the frame when accelerating. The joy of riding light rims is all in the mind and these formulae dont apply there.

  17. #17
    aka old dog greywolf's Avatar
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    i dont know how you jokers sleep @ night with all that stuff buzzing round in your heads??? i dont think cycling is the exact science some think it is.
    :D
    dont worry be happy ????

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    Beer

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    I once ran a set of very heavy (but aero) Aerospoke wheels. They would never do for sprinting or climbing, but on the flats they would require ridiculously minimal effort to keep at speed. Perhaps angular momentum working in my favor?

  20. #20
    Hucker Extraordinare BigHit-Maniac's Avatar
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    Isn't it true that when something is rotating... that it's weight is 3 times as much as it was standing still ? ...


    (excuse my ignorance).

    I do however know that when something weighs more it is:
    • harder to get going (accelerate)
    • slower to stop
    • harder to push against when trying to maintain a speed
    • harder to pull wheelies, endo's, etc


    The wheel idea is to keep it light, as you'll accelerate faster, and corner quicker because there is less mass & force attempting to act against your actions.... (if I'm getting this BS right... lol).

    (just think F-1 Car )
    Got Nine Inches ? Cuz I do. http://67.19.50.55/forums/images/smilies/eviltongue.gif

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    My best guess at where this alleged rule of thumb come from is the formulas for potential energy, and kinetic energy. Very basic physics. Potential energy is that energy possessed by virtue of the objects configuration, for example weight. Kinetic energy is the is that energy possessed by virtue of the the objects motion.

    Specifically Potential energy Ep = Wh
    where W = Weight and h = hieght

    Kinetic energy of rotation Ke = 1/2I*w2
    Where w2 + the angular velocity squared
    I = the moment of inertia (mass) of the item.

    So if one applies the formulas at face value and assumes that I = W which it does NOT then you may think that 1 pound of rotation wieght = 1/2 pound of static wieght.

    However this is a dramatic oversimplification of a very complicated engineering problem. One really needs to consider aerodynamics, friction, average gradient of the route, and overall design efficency to really figure out what the right combination is.

    All that said. It is my opinon that a loss of body weight does not contribute as much to an increase in average speed over distance as much as a reduction in wieght of the bike. I have done both and the bike seemed to make a bigger difference. This is based on my training log. However I really do not know why.

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    before someone points out my typo. w2 = angular velocity squared

  23. #23
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    lighter wheels also require more energy to keep at constant speed. so it's a trade off (exagerated) between quicker acceleration and maintaining constant velocity.

  24. #24
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    it had never ocurred to me that there might be a momentum advantage to a heavier wheel. i have always assumed that removing an ounce of wheel weight had an equivalent affect on performance as removing two or more ounces of frame weight. i know that lighter wheels certainly feel more lively and faster. much more so than a lighter frame.

    it would be nice if someone could say definitively. are there no physicists/engineers who know?

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    Conservation of angular momentum, if i remember my physics classses. Angular momentum is proportional to the rotating mass. and the distance away from the center of rotation. It requires energy to overcome the inertia of the wheels when starting from a dead stop; it also requires the same energy energy to slow the wheels down once they are rotating. This energy (to slow the bike down) comes from wind resistance, friction of wheels over pavement, internal friction of hubs, etc.
    Any advantage of more massive rims is quite dependent on the number of start/stops and hills you have to climb. They would only help on a relatively flat course, say a TT course with no sprints or quick acceleration/deceleration, maintaining a fairly consistent speed.
    Maybe a REAL engineer or physicist could chime in.

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