About the efficiency of the bike. Drawing, description

PERSONAL TRANSPORT: GROUND, WATER, AIR
Free library / Directory / Personal transport: land, water, air

About the efficiency of the bike. Personal transport

Personal transport: land, water, air

Directory / Personal transport: land, water, air

The efficiency of a bicycle, both biologically and mechanically, is very high. The researchers calculated that in terms of the amount of energy that a person must expend to cover a given distance, a bicycle is the most efficient self-propelled vehicle. From a mechanical point of view, up to 99% of the energy is transferred from the pedals to the wheels, although the use of a gearshift mechanism can reduce this amount by 10-15%. In terms of the ratio of the payload that a bicycle can carry to the total weight, a bicycle is also the most efficient means of transporting goods.

Energy efficiency

A person riding a bicycle at low to medium speeds (16-24 km/h) uses the same amount of power that is required for walking, so the bicycle is the most energy efficient public transport available. Aerodynamic drag, which increases approximately with the square of speed, requires more power relative to speed due to the fact that as the speed of the bike increases, the required power increases in a cubic way, since power is equal to speed times force: P = F * v (Fig. 1.). A bike in which the rider is in a recumbent position is called a ligerad (alternatively called a rickambent), and if the bike has an aerodynamic fairing used to achieve very low aerodynamic drag, then it is called a streamliner.

Plot of required power versus bike speed

About the efficiency of the bike
Rice. 1. The dependence of the required power on the speed of the bike

On a hard, flat surface, a person weighing 70 kg needs about 30 watts of energy to move at a speed of 5 km/h. The same person on a bicycle, being on the same surface and using the same power, can move at an average speed of 15 km/h, so that the energy consumption in kcal/(kg*km) will be about three times less. The following numbers are commonly used:

1.62 kJ / (km * kg) for cycling,

3.78 kJ / (km * kg) for walking / running,

16.96 kJ / (km * kg) for swimming.

Amateur cyclists can typically develop 3W/kg for over an hour (e.g. around 210W for a 70kg rider), the best amateurs develop 5W/kg and elite athletes can reach 6W/kg over similar periods. time. Elite sprint track cyclists are able to briefly reach peak power levels of around 2000 watts, or more than 25 watts/kg; elite road cyclists can short-term peak power from 1600 watts to 1700 watts for an instant blast to the finish line at the end of a five-hour road race.

Even when moving at moderate speeds, most of the energy is spent on overcoming aerodynamic drag, which increases with the square of speed. Thus, the power required to overcome air resistance increases with the cube of speed.

Typical cycling speeds

Typical speeds for bicycles range from 15 to 30 km/h. On a fast racing bike, the average rider can ride at 50 km/h on level ground for short periods of time. The highest speed officially recorded for a vehicle driven by muscle energy while driving on a flat surface in calm weather and without external assistance (that is, no car or motorcycle was moving in front of the vehicle) was 133,284 km / h. This record was set by Sam Whittingham in 2009 in Varna. In 1989, while racing across America, a group of muscle-powered vehicles crossed the United States in just 6 days. The highest speed officially recorded when riding a bike with a normal upright rider, all other things being equal, was 82,52 km / h over a distance of more than 200 meters. This record was set in 1986 by Jim Glover on a "Multon AM7" bicycle at the third international scientific symposium for muscle-powered vehicles in Vancouver.

Weight versus power

A major competition was held to reduce the weight of racing bikes through the use of modern materials and components. In addition, modern wheels have low-friction bearings and other features to reduce drag, but in our tests, these components had little to no effect on the performance of the bike when riding on a flat road. For example, reducing the weight of a bicycle by 0,45 kg will have the same effect in a 40 km time trial on a flat road as removing any protrusion that has an airfoil area the size of a pencil. In addition, the International Cycling Union sets a limit on the minimum weight of a bicycle that will be allowed to race, to prevent bicycles from being made so thin that they are unsafe to use. For this reason, in the development of the latest models of bicycles, all efforts have been directed to reduce aerodynamic drag by using aerodynamically shaped tubes, flat spokes on wheels, and using handlebars such that the position of the rider's torso and his hands would have minimal aerodynamic drag. These changes can significantly affect the performance, reducing the time to complete the distance. Less weight results in big time savings when riding uphill in hilly terrain.

Kinetic energy of a spinning wheel

Consider the kinetic energy and "rotating masses" of a bicycle in order to study the impact of rotational energy versus non-rotating masses.

The kinetic energy of an object in translational motion is determined by the formula

E=0.5mv²

Where E - energy in joules, m - mass in kilograms, v - speed, m / s. For rotating masses (for example, for a wheel), the kinetic energy of rotation is defined as

E=0.5Iω²

Where I is the moment of inertia, ω is the angular velocity in radians per second. For a wheel with all its mass located on the outer edge (we use this approximation for a bicycle wheel), the moment of inertia is

I=0.5mr²

Where r is the radius in meters.

Angular velocity is related to forward speed and tire radius. If there is no slip, then the angular velocity will be determined by the formula:

ω=v/T

when rotating masses move along the road, then the total kinetic energy is equal to the sum of the kinetic energy of the translational and rotational movements:

E=0.5mv² + 0.5Iω²

Substituting I and ω into the previous expression, we obtain

E=0.5mv² +0.5mr² *v2/r²

The term r2 cancels out, and as a result we obtain the expression

E=0.5mv² +0.5mv² = mv²

In other words, the kinetic energy of the rotating masses of the wheels is twice as much as the energy of the stationary masses of the bicycle. There is some truth in the old adage, "A pound off the wheels equals a 2 lb off the frame."

This all depends, of course, on how accurately the thin hoop is an approximate model of a bicycle wheel. In reality, the entire mass cannot be concentrated in the wheel rim. For comparison, the other extreme would be a wheel whose mass is evenly distributed over the entire disc. In this case I = 0.5mr², and therefore the total resulting kinetic energy becomes equal to E = 0.5mv² +0.25mv² = 0.75mv². Reducing the weight of the wheel by one kilogram is equivalent to reducing the weight of the bicycle frame by 1,5 kg. The parameters of most wheels of real bicycles will be somewhere in the middle between these two extremes.

Another interesting takeaway from this equation is that for bicycle wheels that do not slip when moving, the kinetic energy is independent of their radius. In other words, the advantage of 650mm wheels is their low weight, not their smaller diameter, as is often claimed. The kinetic energy for the other rotating masses on the bike is very small compared to the kinetic energy of the wheels. For example, if you pedal at about 1/5 the speed of the wheels, then their kinetic energy will be about 1/25 (per unit weight) of the energy of the wheels. Since their center of mass moves along a smaller radius, their energy is further reduced.

Convert to kilocalories

Assuming a spinning wheel can be thought of as the sum of the masses of the rim and tire, plus another 2/3 of the mass of the spokes, this is all centered on the rim/tyres. For an 82kg rider on an 8kg bike (total weight is 90kg) at 40km/h, the kinetic energy is 5625joules for the rider plus 94joules for spinning wheels (1,5kg is the total weight of the rims, tires and spokes). Converting joules to kilocalories (for this you need to multiply the joules by 0,0002389), we get 1,4 Kcal (these are food calories).

That 1,4 kcal is the amount of energy needed to accelerate the bike from a standstill, or that is dissipated as heat when braking to a full stop. These 1,4 kilocalories are enough to heat 1 kg of water by 1,4 degrees Celsius. Since the heat capacity of aluminum is 21% of that of water, this amount of energy is enough to heat an 800 gram rim made of aluminum alloy by 8°C during a quick stop. The rims do not get very hot when stopped on a flat road. To calculate the energy expenditure of a cyclist, the efficiency factor is taken as 24%, resulting in 5,8 kcal required to accelerate the bike and the rider to a speed of 40 km / h, which takes about 0,5% of the energy needed to ride at a speed of 40 km / h. h for an hour. This energy expenditure will occur in 15 seconds, at a rate of approximately 0,4 kcal per second, while steady driving at 40 km/h requires 0,3 kilocalories per second.

Benefits of light wheels

The advantage of light bikes, and especially light wheels, with respect to kinetic energy is that kinetic energy only begins to show its effect when the speed of the bike changes, so there are two cases where light wheels are advantageous: in sprinting and when negotiating tight turns. in the criterion.

In a 250 m sprint while moving at a speed of 36 to 47 km / h, with a bike and athlete weighing 90 kg, plus another 1,75 kg of wheel weight (rims, tires, spokes), kinetic energy increases by 6360 joules (6,4 .500 kcal). If we reduce the total weight of rims, tires and spokes by 35 g, then this kinetic energy will decrease by 1 J (1,163 kcal = 500 watt-hours). The effect of this weight savings on speed or distance traveled is quite difficult to calculate, you need to know the power developed by the athlete and the length of the sprint distance. Calculations show that reducing the weight of the wheels by 0,16 grams will give the sprinter a gain in time of 188 seconds, and a gain in the distance traveled of 0,05 cm. If the wheels are made aerodynamic, then the gain will be 40 km/h at a speed of 0,6 km/h , the benefit from weight reduction will be negligible compared to the benefit gained from the aerodynamic shape of the wheels. In comparison, the best aerodynamic bicycle wheels give a gain of about 40 km/h at a speed of 500 km/h, so in a sprint it is worth using a set of aerodynamic wheels weighing XNUMX g or less.

In a criterium (group circuit race), the rider often begins to accelerate rapidly after passing each corner. If the cyclist must brake before passing each corner (rather than coast to slow down), then the kinetic energy that is added with each acceleration is lost as heat during braking. When racing criterium on flat terrain at 40 km/h, with one lap 1 km long and each lap has 4 turns, the loss of speed at each turn is 10 km/h. The duration of the race is one hour, the weight of the rider is 80 kg, the bike is 6.5 kg, the rims, wheels and spokes weigh 1.75 kg, in this race you will have to overcome 160 turns. This will require an additional 387 kcal to the 1100 kcal required to drive at a constant speed for the same distance. Reducing the weight of the wheels by 500 g will reduce the total energy consumption of the body by 4,4 kcal. If adding an extra 500g of weight to the wheels results in a 0,3% reduction in aerodynamic drag (which translates into a 0,03 km/h increase in speed while driving at 40 km/h), then the calorie burn to compensate for the extra weight will be offset by a reduction in aerodynamic drag.

Another place where lighter wheels can make a big difference is when riding uphill. You can even hear such an expression as "these wheels added 0,5-1 km / h of speed", etc. From the formula for calculating power, it follows that 450 grams of saved mass will give an increase of 0,1 km / h to speed when driving uphill with a 4° incline, and even saving 1,8 kg of weight will give an increase in speed of only 0,4 km / h for a light athlete. So, what is the significant positive effect of reducing the weight of the wheels? Some suggest that there is no savings, but there is a "placebo effect". It has also been suggested that the change in speed with each pedal stroke while driving uphill explains the resulting advantage. However, energy is conserved during changes in speed - during the pedaling phase the bike accelerates slightly, while kinetic energy is accumulated, and in the "dead zones" during the pedaling of the top point of the stroke, the bike slows down, so that kinetic energy is restored. Thus, an increase in rotating mass may reduce the variation in bicycle speed somewhat, but it does not increase the need for additional energy.

Lighter bikes climb more easily, but the "rotating mass" effect is only a problem during fast acceleration, and even then it's small.

Explanations

Possible technical explanations for the widely claimed benefits of lightweight components in general, and lightweight wheels in particular, are as follows:

1. Light weight wins in races where there are significant climbs because heavier bikes can't compensate for the energy lost downhill or flat: the rider on the lighter bike just coasts. In addition, if two identical cyclists on a heavy and light bike simultaneously reach the bottom point after climbing to the finish line, then all the advantage passes to the light bike. This is not the case in hilly time trials (or solo riding) where the advantage of heavier but more aerodynamic wheels easily compensates for the distance lost on climbs.

2. Lightweight bikes win sprints because they are easier to accelerate. But note that heavier aero wheels provide significant speed gains, and for a good part of the race, the sprinter accelerates a bit but spends most of his effort on overcoming aerodynamic drag. In many sprint situations, heavier but more aerodynamic wheels can help win.

3. Light weight provides a criterion advantage due to constant acceleration after each turn. The heavier, but more aerodynamic wheels offer a slight advantage as the riders are in a group most of the time. The energy savings from lightweight wheels are minimal, but they can be more significant because the leg muscles have to put in extra effort each time you pedal.

There are two "non-technical" explanations for the light weight effect. First, there is the placebo effect. Since the cyclist feels that he is on a better (lighter) bike, he pedals harder and therefore rides faster. The second, non-technical explanation is the triumph of hope over the cyclist's experience - because the bike's lighter weight does not significantly increase its speed, but the cyclist thinks he is going faster. Sometimes this is due to a lack of real data, such as when it took a cyclist two hours to climb a hill on his old bike, but on his new bike he did it in 01:50. Factors such as the fit of the cyclist to the bike during these two climbs, whether the weather was hot or windy, which way the wind was blowing, how the rider was feeling, etc. are not taken into account.

Another explanation, of course, could be the marketing benefits associated with promoting the idea of weight loss.

After all, the "increasing muscle energy consumption" argument is the only one that can support the claimed benefits of lightweight wheels in situations where fast acceleration is needed. This argument would have to state that if the cyclist is already at the limit of effort on every jerk or every pedal stroke, then the small amount of extra power needed to compensate for the extra weight would be a significant physiological burden. It's not clear if this statement is true, but it's the only explanation for the claimed benefits of wheel weight reduction (compared to weight reduction for the rest of the bike). For these accelerations, it makes no difference whether the wheels have become lighter by half a kilogram or the weight of the bike and the athlete has become lighter by a kilogram. The wonder of lightweight wheels (compared to the reduction in weight in any other part of the bike) is hard to see.

We recommend interesting articles Section Personal transport: land, water, air:

▪ All-terrain vehicle-pneumatic vehicle

▪ Economic bike drive

▪ Air-hydraulic glider

See other articles Section Personal transport: land, water, air.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Energy from space for Starship 08.05.2024

Producing solar energy in space is becoming more feasible with the advent of new technologies and the development of space programs. The head of the startup Virtus Solis shared his vision of using SpaceX's Starship to create orbital power plants capable of powering the Earth. Startup Virtus Solis has unveiled an ambitious project to create orbital power plants using SpaceX's Starship. This idea could significantly change the field of solar energy production, making it more accessible and cheaper. The core of the startup's plan is to reduce the cost of launching satellites into space using Starship. This technological breakthrough is expected to make solar energy production in space more competitive with traditional energy sources. Virtual Solis plans to build large photovoltaic panels in orbit, using Starship to deliver the necessary equipment. However, one of the key challenges ... >>

New method for creating powerful batteries 08.05.2024

With the development of technology and the expanding use of electronics, the issue of creating efficient and safe energy sources is becoming increasingly urgent. Researchers at the University of Queensland have unveiled a new approach to creating high-power zinc-based batteries that could change the landscape of the energy industry. One of the main problems with traditional water-based rechargeable batteries was their low voltage, which limited their use in modern devices. But thanks to a new method developed by scientists, this drawback has been successfully overcome. As part of their research, scientists turned to a special organic compound - catechol. It turned out to be an important component that can improve battery stability and increase its efficiency. This approach has led to a significant increase in the voltage of zinc-ion batteries, making them more competitive. According to scientists, such batteries have several advantages. They have b ... >>

Alcohol content of warm beer 07.05.2024

Beer, as one of the most common alcoholic drinks, has its own unique taste, which can change depending on the temperature of consumption. A new study by an international team of scientists has found that beer temperature has a significant impact on the perception of alcoholic taste. The study, led by materials scientist Lei Jiang, found that at different temperatures, ethanol and water molecules form different types of clusters, which affects the perception of alcoholic taste. At low temperatures, more pyramid-like clusters form, which reduces the pungency of the "ethanol" taste and makes the drink taste less alcoholic. On the contrary, as the temperature increases, the clusters become more chain-like, resulting in a more pronounced alcoholic taste. This explains why the taste of some alcoholic drinks, such as baijiu, can change depending on temperature. The data obtained opens up new prospects for beverage manufacturers, ... >>

Random news from the Archive

Bacteria improve plant growth and enrich the soil 12.02.2022

Cover crop plants, clover, grown in soils similar to those of Mars, have shown significant growth in the process of infection with symbiotic nitrogen-fixing bacteria. A researcher from the University of Colorado (USA) Franklin Harris, together with colleagues, presented the results of the study in the publication PLOS ONE.

As the Earth's population grows, researchers are exploring the possibility of farming Martian soil, or "regolith." Regolith lacks some key nutrients, including certain nitrogen-containing molecules that plants need to live. Therefore, agriculture on Mars will require a strategy to increase the amount of these nitrogen compounds in the regolith.

The scientists suggest that the bacteria could play a cost-effective role in increasing the fertility of Martian soils. On Earth, soil bacteria help convert or "fix" atmospheric nitrogen into molecules that plants need. Some of these microbes have a symbiotic relationship with crops, in which they fix nitrogen in nodules found on plant roots.

In order to study the possible role of symbiotic nitrogen-fixing bacteria in astro-agriculture, the researchers grew clover in artificial regolith, which is very similar to Martian. They infected some plants with the microbe Sinorhizobium meliloti, which is commonly found in clover root nodules on Earth. In previous studies, scientists have shown that clover can be grown in regolith, but inoculation with nitrogen fixers has not been studied.

The researchers found that clover inoculated with Sinorhizobium meliloti had 75% more root and shoot growth than normal clover. However, the regolith surrounding the inoculated plants did not show signs of elevated levels of NH4 (an important nitrogen-containing molecule for plants), compared to the regolith in which uninoculated clover grew.

This study suggests that the symbiotic microbes stimulated plant growth but did not lead to excess production of nitrogen compounds that could hypothetically be used by other crops growing nearby. The researchers also grew some clover in potting soil and noted some differences in symbiotic relationships compared to plants grown in regolith.

According to the results of scientific work, scientists have suggested the possibility that the symbiosis between plants and nitrogen-fixing bacteria can help agriculture on Mars.

"This study demonstrates that the nodule-forming bacteria Sinorhizobium meliloti can create nodules in Martian regolith, significantly enhancing the growth of clover (Melilotus officinalis) in greenhouse conditions. This work expands our understanding of how plant-microbial interactions will aid terraforming efforts." regolith on Mars," researchers at Colorado State University said.

Other interesting news:

▪ 5G antenna in smartphone screen

▪ Listening to birdsong can reduce anxiety and depression

▪ Canon EOS R6 Mk II Full Frame Camera

▪ Sennheiser Evolution Wireless D1 Wireless Microphone System

▪ Standard supporting 8K VESA Embedded DisplayPort 1.4a

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ site section Voltage converters, rectifiers, inverters. Article selection

▪ article Geography, ecology, geology. Crosswordist's Handbook

▪ article Where is the highest mountain? Detailed answer

▪ article Gas soldering iron. home workshop

▪ article Indicators for a paired phone. Encyclopedia of radio electronics and electrical engineering

▪ article Phase presence indicator. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages of this page

Home page | Library | Articles | Website map | Site Reviews