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  1. #26
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    Quote Originally Posted by EvilV View Post
    I don't want to get too argumentative here, or too hair splitting, but there is something else involved. The mechanics of walking are much more energy consuming per mile than cycling on almost any bike at all. Walking, the legs carry the full weight of the body, supporting and moving it forward. Superbly efficient on the rough terrain we evolved to handle, but much less so on smooth roads. The bike allows you to sit a substantial part of your weight in the saddle and doesn't require that you take the lot on one leg at a time as walking does. Then, the bike also rolls once you have moved your body mass forward - very unlike walking, which requires constant, inefficient starts and stops at virtually every step. The consequence of this is that to cover a mile on foot, a twelve stone man (168 pounds) needs about 90 calories of fuel, whereas the same man on a bike traveling at a leisurely 12 miles an hour uses only about 35 calories. From this I conclude that a bicycle is about three times more efficient as a way of traveling on a well paved road than walking. Crossing a bog, or rough terrain, the bike would be near useless though.

    We all know this of course, after all, how far can you walk in a day on smooth roads and how far can you cycle?
    Not the mechanics, but the biomechanics. That is exactly my point, when it comes to cycling, mechanics takes a back seat and biomechanics is key.

    There are 35000 thermal calories in 35 food calories and about 4184 Joules per thermal calorie. So unless I've made a mistake those 35 food calories represent almost 146 megajoules of energy. Burning 146 megajoules while covering one mile at a pace of 12 miles per hour, means that the cyclist is burning energy at a rate of about 12 megawatts; Considering the fact that, at best, 400 of those watts make it to the pedals, the loss is absolutely staggering, completely dwarfing any of the energy that actually makes it into the mechanical system.

    And what are typical mechanical losses? 1 watt, 10 watts, maybe 100 watts for a rubbing brake? Think about it this way: a bicycle that is even 0.001% (one thousandth of one percent) ergonomically inferior is as inefficient as a rubbing brake. Muscular losses are clearly the overwhelmingly dominant source of inefficiency. Mechanical losses aren't even worth considering.
    Last edited by makeinu; 03-03-08 at 09:57 AM.

  2. #27
    Bicycling Gnome
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    Thanks for the information, but I think we have an error somewhere in those figures - probably of a level of magnitude at least. Twelve Megawatts for a mile. I doubt the French TGV uses that much power, let alone a man on a bike. Having said that - this is not my field, so I can't really argue with your figures other than to say, 'Are you sure?' because they sound unlikely.

    Many thanks for looking at them again - I'm not nit picking - honest.
    “Get a bicycle. You will not regret it, if you live." - Mark Twain

  3. #28
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    Quote Originally Posted by EvilV View Post
    Thanks for the information, but I think we have an error somewhere in those figures - probably of a level of magnitude at least. Twelve Megawatts for a mile. I doubt the French TGV uses that much power, let alone a man on a bike. Having said that - this is not my field, so I can't really argue with your figures other than to say, 'Are you sure?' because they sound unlikely.

    Many thanks for looking at them again - I'm not nit picking - honest.
    I was skeptical of the calculations too, but then I read this article by a professor who appears to know what he is talking about and the order of magnitude seems to be consistent with my calculations:
    http://www.exo.net/~pauld/activities...gcalories.html

    For example, he claims that
    -One food calorie contains the same amount of work as lifting a 155 pound person two stories up against gravity.
    -A Milky Way candy bar contains almost a megajoule of energy, which is the same amount of energy as the fundamental work required to lift a 155 pound person 1200 meters up against gravity (higher than the cliff face of Yosemite's El Capitan) and more energy than that released by exploding a stick of dynamite.
    -The body converts less than 20% of the energy in consumed food into useful work (compare that to the 90-99% efficiencies commonly cited for the mechanical bicycle).

    I know it seems like a lot of energy wasted, but given the fact that we know the body can at times waste 100% of the energy it produces (for example, when lifting weights) and the amount of body heat generated when exercising, I can't say I'm surprised. Why should we suddenly expect high levels of efficiency when cycling? Given how warm I can stay while cycling in the winter I wonder if my body generates more watts of heat when cycling than a wood burning stove. Now, I don't know much much energy is consumed by a wood burning stove, but I imagine it would take an awful lot of effort to drive an electric heater by pedal power.
    Last edited by makeinu; 03-03-08 at 11:06 AM.

  4. #29
    jur
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    Nah. Never. For cycling, we're talking several hundred watts output, so that has to be the order wasted as well. I am a power conversion engineer, I know what it takes to cool a mere several hundred watts. Megawatts has a mega too much.

    The conversion from calories to joules is probably the suspect. Calories is understood to have a kilo in it sometimes when referring to food.
    My folding bike photo essays www.dekter.net/

  5. #30
    Professional Fuss-Budget Bacciagalupe's Avatar
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    Quote Originally Posted by makeinu
    And what are typical mechanical losses? 1 watt, 10 watts, maybe 100 watts for a rubbing brake? Think about it this way: a bicycle that is even 0.001% (one thousandth of one percent) ergonomically inferior is as inefficient as a rubbing brake. Muscular losses are clearly the overwhelmingly dominant source of inefficiency. Mechanical losses aren't even worth considering.
    I think this part of your math is off, too. Sorry.

    A typical 155lb human can generate about 160 watts of usable power when cycling. If a 0.001% drop in ergonomics reduced that output by 1 watt, then a 0.16% reduction in ergonomic efficiency would bring the cyclist to a dead halt. Or am I missing something?

    Actually, the single biggest factor affecting performance is drag. Lowering your handlebars doesn't make you faster because you're in a more ergonomic position; it makes you faster because you have lowered your drag coefficient. As far as I know, you can place your body in a fairly wide variety of positions and still push close to the same amount of power to the pedals -- assuming that the saddle-to-pedal distance remains consistent, of course.

    Consider the recumbent. Recumbents, especially with fairings, are significantly faster than traditional bikes on the flats. Is it because you're in a better ergonomic position? Nope; it's almost all aerodynamics. In fact you're in a worse ergonomic position, if you haven't yet developed the different muscle groups utilized by the recumbent position.

    (Runners also have a higher drag coefficient than cyclists, by the way. A typical cyclist will have a Cd of 0.9, a runner is more like 1.0 - 1.3. I.e., it's not all just biomechanical, other factors are present as well.)

    Friction and inefficiencies in the transmission are less important than drag, but still there -- and given the latitude of riding positions that allow you to generate power, probably more important in a bike-to-bike comparison than ergonomics. I'd imagine that ergonomics have more impact for rider comfort than for power generation. Meanwhile, mechanical factors could easily soak up 5%, possibly even 10% of your power.


    So, back to the Moulton. Let's assume the smaller wheels produce less drag (performance +) but result in a harsher ride, thus necessitating suspension (performance -). I can see how these two factors can even out. Then, we have the increased rolling resistance of the tires, and in some cases increased drag of the frame. I'm guessing it's a wash.

    If anyone is willing to drop $11k for one of these bad boys, they can spring the extra few grand to stick it in a wind tunnel and test it, right?

  6. #31
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    I don't think anybody will buy a $10K moulton to ride against $10K road bikes. Moultons used to be lighter and better (In some respects) than old steel road bikes in 1960s... But nowadays you can buy some incredible parts such as stiff 700g frame, 880g aerodynamic wheelset, 5kg race-ready bike. In comparison, current moulton frameset alone weighs 4kg and it is really hard to build a moulton bike less than 9kg.

    Anyway, in a flat road moulton (and other smallwheelers) performs pretty well.

  7. #32
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    LOL - I enjoyed all that discussion. Good stuff chaps.
    “Get a bicycle. You will not regret it, if you live." - Mark Twain

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