Myth or Truth?

[TheScrutineer]

Push bike is surely is the most efficient form of transport available? Pretty much anything using an engine will expend energy while not in motion (idling, clutch down etc). Obviously, it has its limits on mileage and the ability of the rider - so then if these limits are ignored...isn't walking the most effective method?

[Cyco]

Walking will never get close to cycling for efficiency. Walking requires the body's muscles to support its whole weight whilst the bike does much of the support Also the bike gives the human muscles a mechanical advantage. This results in the bike being (terrain dependant) about 5 times as fast (or travelling 5 times further) for the same energy burnt.

[MilesR]

I think that the efficiency comparison does depend entirely upon the criteria used. From memory, a well-maintained bicycle drivetrain (both derailleur and Sturmey-Archer internal type gears) has a mechanical transmission efficiency of about 95%, which is similar to a manual gearbox in a car, and better than a conventional automatic, or flappy paddle gearbox. Any type of four-wheel drive system is considerably worse, due to the excessive number of differentials, seals etc. that create drag. Therefore, the difference in efficiency, in thermodynamic terms, is largely determined by the difference in the efficiency of the engines - internal combustion in a car, and biological metabolism on a bicycle.

Human metabolic efficiency has been calculated to be about 30%. Simply existing requires a dietary intake of about 2,200 kcalories, per day, for an average chap, who is not exercising. Most of this energy eventually is lost as heat. When the same average chap performs vigorous exercise, he may eat around 4,800 kcalories each day, with the additional 2,600 being dedicated to doing the work involved in his choice of exercise. At 30% efficiency, this translates (after a bunch of conversions) to a power output of 200-300 watts, for two to three hours. This is about consistent with the output of a fast cyclist, as measured by a bicycle power meter. Depending on conditions (wind, bicycle, etc.) I would estimate that this would translate to a speed of around 40km/h.

By comparison, internal combustion petrol engines are, at best, about 25% efficient, while diesel engines range from about 35% to 45%, depending upon design and application (slow-turning truck engines are more efficient than relatively fast car diesels). Thermodynamically, then, car engines are fairly comparable with human metabolism, with petrol being slightly worse, and diesel slightly better.

The energy content of petrol is about 32 megajoules per litre, and diesel about 38MJ/L. A sensible car travelling 100km, at 100km/h, will use about 6L of petrol, or 4L of diesel. This translates to 192MJ of petrol, or 152MJ of diesel. A cyclist, as described above, will take about 2.5 hours to cover the same distance. Assuming a power output of 250 watts (riding fast), this converts to 2.25MJ. Even at 30% metabolic efficiency, this translates to only 7.5MJ of food (about 1,800kcal). A fast runner, with the same power output, might be expected to maintain 15km/h, thus covering only 37.5km in the same time, for the same energy cost.

Purely energetically, the bicycle comes out on top by almost an order of magnitude, even if there are five people in the car. However, as mentioned, the bicycle is slower, taking 2.5 times as long to complete the same distance, so if time is a concern, the car may be better. Cyclists also have a much higher coefficient of aerodynamic drag, because the person, which makes up the majority of the cyclist, was not designed in a wind tunnel. The total drag on the vehicle depends also upon its size, and its speed. The cyclist, being smaller, and much slower, will feel far less aerodynamic resistance than the car does. Driving the car at 40km/h, with appropriate gearing, might bring its fuel consumption close to that of the cyclist. However, it will never match it, because even if the car was driven at walking pace, the engine would still be consuming fuel to keep itself and its accessories working. By comparison, if the runner walked, his energy use for 100km would be less than that of the cyclist, but at the cost of taking at least 2 days to cover the same distance, and if the cyclist slowed to the same pace, his energy usage would be reduced further than the walker's would be - walking pace on a bicycle requires about one turn of the pedals, every minute or so.

Finally, on the small, south-western Pacific island that I occupy, it is nearly impossible to buy a new car that weighs less than one tonne - there are only two such models that I am aware of. Most common cars are in the area of one to two tonnes. I weigh 70kg, or so, and I have an objection to dragging about 20 times my own weight of car, just to get 70kg of me from one place to another. A bicycle, weighing 10-15kg, makes much more sense to me, and between my ability to avoid traffic (or play with it) and speed limits, I am normally not much slower than a car, to and from work, or the shops. Sometimes I am even faster, and I get to enjoy the ride. The light weight and simplicity of the bicycle also means that the cost of production, in terms of resource use, environmental impact, and economic cost, is much, much lower than any car. For the same reason, parking, repairs, etc. are also much less expensive and inconvenient.

I think I will stop now. The longer I keep going, the more arguments I will come up with.....

[f6fhellcat13]

Informative first post! Welcome to UCP, Miles!

[LeonOfTheDead]

Mad props Miles and welcome here!

I'd add that modern diesel car engines barely tops a 30% efficiency.

On the other hand it is pretty obvious that the bicycle is more efficient than walking or running as even a trained athletic can't run for more than about 40 km without being just dead-tired at the end, while pro-cyclists can cover distances of about 200 km each day for a month.

I used to play with mileage figures when I was a kid , trying to figure if it was more efficient traveling for 200 km at 80 km/h and taking more time, or at 120 km/h, using more fuel, but less time. Of course at that time I didn't a have a clue on dimensional analysis was. :P

[henk4]

Welcome Miles, rarely have I seen such an informative and well considered post as an opener. Averaging 40 on a bike is bit on the optimistic side though, that is clearly the territory of well trained semi-pro cyclists. I am always happy when I manage an average over 30, but that takes into account normal traffic circumstances, which is not always favourable to cyclists, even in the small (and flat) North Western European country that I live in.
Having that said that, the little difference will not affect the basis for your calculations only the results. And my wife, a dietician, always tells me that 2000 kcal is enough for non-exercising day in the office....

[Fleet 500]

As many who have replied have already said, I agree that for relatively short distances, bicycles are the most efficient, but not for long distances.

[IBrake4Rainbows]

Well Efficiency doesn't generally change over distance, so I'm not sure about that. A bike thats more efficient over 20km is likely to remain as efficient over 200km.

But what I'd suggest is that if you use a time/efficiency cross table there would be a swing against the bicycle.

[henk4]

As many who have replied have already said, I agree that for relatively short distances, bicycles are the most efficient, but not for long distances.

that is not the bike's problem, but more of the rider....if you change riders every twenty km, you won't loose the efficiency of the machine itself.
(Also car drivers wear out after long trips)

[LeonOfTheDead]

that is not the bike's problem, but more of the rider....if you change riders every twenty km, you won't loose the efficiency of the machine itself.
(Also car drivers wear out after long trips)

That may have also to do with the car they are driving.
Otherwise I agree.
As temperature is an important factor in the overall efficiency of every machine, I can't see a man being just fine after 4 hours either on a bike or running.

[MilesR]

Thank you, good folk, for your kind welcome. Always happy to be of service. Yes, 40km/h is fast for a bicycle. I based my power output on figures for competition cyclists. A Tour de France cyclist will typically generate a maximum average of about 400 Watts of power, on a hard hillclimb. I am a less than competition standard rider, but on a good day, I will cruise at 35km/h or so. A competition rider may be expected to generate 200-300 Watts while cruising, so I estimate that an output of 250 Watts on a good bicycle, will give rise to about 40km/h cruise speed.

To fill a few gaps, and consider a few other possibilities, a Segway scooter, while technically quite impressive, depends entirely upon being recharged to work. Fossil-fuelled electricity stations typically claim somewhere in the area of 40% efficiency. Factoring in 20% electrical losses (in transmission and transformation) for recharging, and something similar for discharging and electric motor conversion efficiency, the Segway quickly falls below the 30% efficiency of human metabolism. Even with totally renewable power, there are 30-40% losses in the recharging and discharging processes, so the efficiency cannot exceed 60-70%. I am neglecting the upright riding position, which increases aerodynamic drag. Batteries and electronics are not cheap, and factoring in the environmental, economic and resource costs of design, construction, and maintenance, the case quickly becomes less than compelling for a Segway. The lower top speed, shorter range, and oddball design of the scooter make clear that it is intended as an alternative to walking (which I read as laziness), does not improve health and fitness, and makes the rider look like something of a prat. I prefer a bicycle.

Electrically boosted bicycles suffer the same electrical efficiency limitations, with the added drawback (in the United States of Australia, anyway) that they are limited to a maximum output of 200 Watts, and a maximum assisted speed of 28km/h. Above this speed, the weight and drag of the motor, drive and control systems, and the battery, turn it into an inefficient conventional bicycle. I usually ride faster than 28km/h, so the assistance is of no real use to me.

Skateboards and rollerskates also both have and upright position, and, like walking and running, require some energy to be used to support the weight of the user, and to maintain balance. The tiny, fast-turning wheels, generate more mechanical drag, from bearing and rolling resistance, and there are twice as many wheels, on skateboards and blades, or four times as many on conventional skates, than on a bicycle. While the skates have a gearing effect, because drive comes from pushing outwards, like a skier, rather than backwards like a runner, it is not realistic to expect to compete with a bicycle for speed. By comparison, a skateboard is driven by pushing backwards. This means that, as for a runner, energy must be used to bring the driving leg forwards between each push, without contributing to propulsion, hence wasting energy. Consequently the speed is limited by how fast the rider can push, limiting the speed effectively to that of a runner.

Fuel consumptions range from about 2L/100km, at 50km/h or so, for a 100cc four-stroke motor scooter, to about 8L/100km for a performance motorcycle. Compared with a diesel car, this is varies from twice as good (admittedly at half the speed), to twice as bad. Considering that both carry only one or two people, this leaves them at best 4-9x worse than a cyclist, and in the most favourable case, not all that much faster.

According to the source of all knowledge ([ame=http://en.wikipedia.org/wiki/Fuel_efficiency_in_transportation]Fuel efficiency in transportation - Wikipedia, the free encyclopedia[/ame]) a diesel bus may be expected to average 39L/100km, while carrying 55 seated passengers. Even stretching this to 70 seated and standing, we find a fuel consumption of a little over 0.5L/100km per passenger. Compared with the car, that is 1/8th the fuel use, or about 20MJ/100km, which is about 3x worse than the bicycle, and, of course, most buses that I see do not contain 70 people. Also, while cycling, I can usually outrun a bus in the city, and just about match the bus in the suburbs, barring substantial hills or headwinds. Combined with the ability of the cyclist to travel as shortly as possible from origin to destination, while the passenger is bound to the whims of the bus route, the speed advantage of a bus within a city is small or non-existent, and the distance travelled by the bus passenger is likely longer than the cyclist.

From the same source, the energy usage of trains varies from 35MJ/100km per passenger, according to East Japan Railway Company, to 8.5MJ/100km per passenger, according to a study of light rail in Basel, by Siemens. Superficially, this makes passenger rail 1-5x as inefficient as cycling, but neither of these values accounts for electric generation or transmission inefficiencies. Diesel railway tends to be less efficient, due to the weight of the engine and fuel, which must be self contained, and the inability to regenerate electricity during braking, as electric rail can. However, diesel rail, for freight purposes, still tends to use 70% less energy than trucks, according to an English study.

At risk of becoming rather less objective and rigorous, I also believe that cycling has other benefits. I enjoy cycling, so I do not suffer the frustrations of motorists who are stuck in traffic, or building up road-rage. It may take longer than driving, but I enjoy the time, rather than suffering it. Likewise, while cycling requires more food to sustain, I love food, so my enjoyment of life increases again.

On the subject of food, the point was made, by Aiasib, that factoring the cost of food production into the equation could change things. However there is so much variation in diet that comparisons are hard to make. Animal products, such as meat and dairy, are expensive to make, as vegetable matter must be grown, then fed to animals. Vegetable diets tend to be quite vastly less energy and land intensive. Likewise, heavily processed and packaged foods, such as frozen meals, cups of soup, biscuits etc., tend to be far more costly to produce. I eat a diet composed mainly of minimally processed grains, fruit and vegetables, with rather occasional pies, or packets of crisps. Considering that there is still an overproduction of certain foods, and that distribution remains a greater problem than supply, there is significant capacity for everyone to eat more, without threatening supplies. Also consider that while fossil fuels are not renewable, the energy content of food is derived from the sun, directly or indirectly, so food powered bicycles are more sustainable than fossil powered combustion engines. The conversion from plant to ethanol also wastes some energy, even before the 30% efficient combustion in a car, so a food powered bicycle is also more efficient than a bioethanol powered car, as long as the bicycle is mostly vegetable powered (foods like potatoes and rolled oats are good power sources, in case you were wondering). People eat whether cycling or not, so the time taken to "refuel" should not represent a major problem. In addition, my energetic calculations for fossil fuels neglect the cost of extracting, refining and transporting the fuels, which can be a considerable fraction of the energy available from their combustion, so food is not at such a clear disadvantage, on this point.

I like driving, too, but I rather think that my enjoyment of driving would seem most incomprehensible to the majority of fellow members of my curious species. I enjoy driving my hilariously tiny car very sedately and carefully. I enjoy achieving as much as possible, while using as little as possible, so knowing that I am getting from origin to destination with minimal noise, cost, pollution and car brings a degree of satisfaction. In addition, achieving this without inconveniencing other motorists, and also not thrashing my delightfully diminutive drivetrain, requires a degree of attention that seems to be lacking from most people who I know, who drive cars larger and faster than mine. The car just makes concentration seem unnecessary, because speed comes so quickly and quietly.

While I am impressed by the technical achievements that are the Bugatti Veyron and the 9ff GT9R, they look to me like an exceptionally dangerous sledgehammer being used to kill a fly. In my interpretation, the pinnacle of sportscars is somewhere between the Honda Beat and the Lotus type 14 Elite - minimum excess producing maximum satisfaction. The smallness, lightness and quickness of cars like this make slow feel fast, and everyday driving engaging. Going fast requires skill, just like bigger cars, but fast in a slow car is safer than fast in a fast car. By the same logic, motorcycles are even more engaging, because they feel faster, and more dangerous. This is because they are more dangerous, and I have decided that I prefer my spine intact and my legs with the skin on, hence my four wheels and a roof.

[henk4]

^^
Hear Hear....very good. Just a question, did you ever do calculations for a speed skater on ice? Not a very popular way of moving forward in your areas, but ever so popular here. Speeds achieved are well over 50 kph, so what about energy inputs? Of course available tracks are limited and highly deviate in quality, but for an academic excercise it might be interesting if they could beat the cyclist. And for your info, speed skaters are often very good bike riders and vice versa, as they mostly use the same muscles.

[Kitdy]

And for your info, speed skaters are often very good bike riders and vice versa, as they mostly use the muscles.

In fact, a Canadian speed skater named Clara Hughes actually won multiple medals in both the winter and summer Olympics (the only person to do so) in the speed skating and road biking disciplines.

She was our flag bearer at the opening ceremonies for Vancouver 2010.

[henk4]

Yes, we have Jan Bos, mainly a skater, but together with his brother Theo (5 times world sprint champion) he also participated in the 2004 Athens olympics, but without winning a medal. (He won a couple on the ice though)

[MilesR]

Good thought, Henk. I was wondering what other possibilities there would be. I have not specifically measured or calculated anything for ice-skates, but I can venture a few thoughts.

The coefficient of friction of steel on ice is the lowest of any two materials, as far as I am aware. The rolling resistance of a good road racer, with good tyres, bearings, wheels and correct setup, is likewise sod all. I would estimate that the bicycle and the ice skates would be equivalent on this front. Ice skates would not have the 5% or so of mechanical losses, from the drivetrain of a bicycle, and the skates would be lighter than the bicycle, translating to less useless weight being carted about. The position of a speed skater is similar to that of a cyclist, and, as mentioned, so are the muscle groups used. A skater has a larger frontal area than a cyclist, as the skater can have both legs fully extended at once, while the cyclist cannot have both more than half-extended at the same time. As a rough estimate, I imagine that this would be pretty much offset by the absence of drag from the bicycle itself.

In terms of performance, a professional velodrome sprinter will reach speeds of 70km/h, while finishing sprints in the Tour will reach 60km/h. Wikipedia's page on velodrome cycling claims that speeds can reach 85km/h, which seems optimistic to me, but I cannot exclude the possibility. It is surprisingly hard to find a number for the speeds reached by speed skaters. Records are for sporting events, and quote an average speed, calculated from the event distance/time. This does not tell us how fast the skater can go, because it is averaged from a standing start, a series of corners, and a sprint to the finish. The quoted world record averages range from about 48km/h to about 52km/h, depending upon distance, gender etc.

To try to remove some of these uncertainties, I used a video of South Korean Lee Kang-Seok, setting a world record over a 500m, long track race, at the world championships in Salt Lake City, in 2007. The course was a 400m oval, and using a stopwatch, while watching the video, I measured 25.03 seconds for the final 400m of the race. The long course has wider corners than a short track, hopefully minimising speed lost in the corners. Also, by taking the final 400m, most of the acceleration phase was out of the way before I started timing, so the time should be only for the skater effectively at his top speed. Throwing it into a calculator gives a speed of 57.6km/h.

The results are a bit ambiguous. A sprinting cyclist is significantly faster than a sprinting skater, however, based on mechanical efficiency, weight etc. the skater should be very slightly more efficient. Clearly the skater is more efficient than a pedestrian. I think it will depend upon speed. A cyclist can select his bicycle gearing to allow his muscles to operate at their optimum speed, whereas the speed skater has no gears, as such. His adaptation to speed comes from providing a sideways force, as well as a backwards force, and turning the skate to make the best use of the force, in the same way that sail trim can be used to optimise the performance of a yacht.

Initially, the skater starts with his skate transverse to his direction of travel, and pushes backwards, as a runner would, to propel himself from rest. At higher speed, he cannot move his leg backward fast enough to keep accelerating this way, at which point the force acquires a sideways component, and the skate is rotated to produce a forward force vector, as well as a sideways force vector. As the speed increases, the magnitude of the sideways vector increases, relative to the forward vector, resulting in more energy being expended in side-to-side movement, and less in thrust, reducing the efficiency of this mode of propulsion. Conclusion? I think that at low speeds, ice skates and bicycles will be effectively equivalent, while at high speeds the bicycle will gain the upper hand. Practically, however, it is worth bearing in mind that the ice skates tend to be at their most useful when there is suitable ice around, while the bicycle tends to handle better when there is not. I read these two techniques as functionally equivalent, but suitable exclusively for their own conditions.

One other option is that of the land yacht, solar car, or other vehicle or method that can scavenge power from its environment. The efficiency calculation becomes awkward, as a horribly inefficient method may be quite desirable, as long as the energy comes for free. Wind power, for example, would need no energy input from fuels, electricity, or the user, so even if the efficiency of energy recovery from the wind is 5%, you still end up travelling for free. Likewise with solar power. In these cases, the practical efficiency calculation must account for the cost of production and maintenance, and the general usability of the technique - highly efficient land yachts are useless in the face of unfavourable wind conditions, and solar cars are useless at night.