Bicycle performance

A bicycle's performance, in both biological and mechanical terms, is extraordinarily efficient. In terms of the amount of energy a person must expend to travel a given distance, investigators have calculated it to be the most efficient self-powered means of transportation.1 From a mechanical viewpoint, up to 99% of the energy delivered by the rider into the pedals is transmitted to the wheels, although the use of gearing mechanisms may reduce this by 10-15% 2 3. In terms of the ratio of cargo weight a bicycle can carry to total weight, it is also a most efficient means of cargo transportation.

Energy efficiency

A human being traveling on a bicycle at low to medium speeds of around 10-15 mph (16-24 km/h), using only the energy required to walk, is the most energy-efficient means of transport generally available. Air drag, which increases with the square of speed, requires increasingly higher power outputs relative to speed. A bicycle in which the rider lies in a supine position is referred to as a recumbent bicycle or, if covered in an aerodynamic fairing to achieve very low air drag, as a streamliner.
Enlarge picture
Racing bicycles have dropped handlebars, a narrow seat, and minimal accessories.


On firm, flat, ground, a 70 kg man requires about 100 watts to walk at 5 km/h. That same man on a bicycle, on the same ground, with the same power output, can average 25 km/h, so energy expenditure in terms of kcal/kg/km is roughly one-fifth as much. Generally used figures are
  • 1.62 kJ/(km∙kg) or 0.28 kcal/(mile∙lb) for cycling,
  • 3.78 kJ/(km∙kg) or 0.653 kcal/(mile∙lb) for walking/running,
  • 16.96 kJ/(km∙kg) or 2.93 kcal/(mile∙lb) for swimming.
For many people whose running might be limited by muscle and knee pain, cycling offers comparable outdoor exercise that can be enjoyed by people of a wide range of fitness levels: it is a "no-impact" sport that is easy on the body, especially on the knees, as long as the bike is properly "fit." In addition, since bicycling can also provide convenient transportation, less self-discipline may be required to keep to the activity, since it has a practical purpose. However, because of its efficiency, cycling requires a longer distance, and often greater time, than running to consume the same amount of energy.

The average "in-shape" person can produce about 3 watts/kg for more than an hour (e.g., around 200 watts for a 70 kg rider), with top amateurs producing 5 watts/kg and elite athletes achieving 6 watts/kg for similar lengths of time. Elite track sprinters are able to attain an instantaneous maximum output of around 2,000 watts, or in excess of 25 watts/kg; elite road cyclists may produce 1,600 to 1,700 watts as an instantaneous maximum in their burst to the finish line at the end of a five-hour long road race. Even at moderate speeds, most cycling energy is spent in overcoming aerodynamic drag, which increases with the square of speed; therefore, power needs increase approximately with the cube of speed.

Typical speeds

Typical speeds for bicycles are 16 to 32 km/h (10 to 20 mph). On a fast racing bicycle, a reasonably fit rider can ride at 50 km/h (30 mph) on flat ground for short periods. The highest speed ever officially recorded for any human-powered vehicle on level ground and with calm winds and without external aids (such as motor pacing and wind-blocks) is 130.36 km/h (81.00 mph). That record was set in 2002 by Canadian Sam Whittingham with the Varna Diablo II, a highly aerodynamic recumbent bicycle. Mr Whittingham is the holder of the IHPVA UCI Hour record as of Apr 8, 2007.

Weight vs power

There has been major corporate competition to lower the weight of racing bikes through the use of advanced materials and components. Additionally, advanced wheels are available with low-friction bearings and other features to lower resistance, however in measured tests these components have almost no effect on cycling performance. For instance, lowering a bike's weight by 1 lb, a major effort considering they may weigh less than 15 lb to start with, will have the same effect over a 40 km time trial as removing a protrusion into the air the size of a pencil. For this reason more recent designs have concentrated on lowering wind resistance, using aerodynamically shaped tubing, flat spokes on the wheels, and handlebars that allow the rider to bend over into the wind. These changes can impact performance dramatically, cutting minutes off a time trial.

Kinetic energy

Consider the kinetic energy and "rotating mass" of a bicycle in order to examine the energy impacts of rotating versus non-rotating mass.

The translational kinetic energy of an object in motion is:[1]

,


Where is energy in joules, is mass in kg, and is velocity in meters per second. For a rotating mass (such as a wheel), the rotational kinetic energy is given by

,


where is the moment of inertia, is the angular velocity in radians per second, is the radius in meters. For a wheel with all its mass at the radius (a fair approximation for a bicycle wheel), the moment of inertia is

.


The angular velocity is related to the translational velocity and the radius of the tire. As long as there is no slipping,

.


When a rotating mass is moving down the road, its total kinetic energy is the sum of its translational kinetic energy and its rotational kinetic energy:



Substituting for and , we get


The terms cancel, and we finally get
.


In other words, a mass on the tire has twice the kinetic energy of a non-rotating mass on the bike. There is a kernel of truth in the old saying that "A pound off the wheels = 2 pounds off the frame."[2] One other interesting point from this equation is that for a bicycle wheel that is not slipping, the kinetic energy is independent of wheel radius. In other words, the advantage of 650C or other smaller wheels is due to their weight savings (less material in a smaller circumference) rather than their smaller diameter, as is often stated. The KE for other rotating masses on the bike is tiny compared to that of the wheels. For example, pedals turn at about the speed of wheels, so their KE is about (per unit weight) that of a spinning wheel.

Convert to calories

Assuming that a rotating wheel can be treated as the mass of rim and tire and 2/3 of the mass of the spokes, all at the center of the rim/tire. For a 180 lb rider on an 18 lb bike (90 kg total) at 25 mph (11.2 m/s), the KE is 5625 joules for the bike/rider plus 94 joules for a rotating wheel (combined 1.5 kg of rims/tires/spokes). Converting joules to calories (multiply by 0.0002389) gives 1.4 calories.

That 1.4 calories is the energy necessary to accelerate from a standstill, or the heat to be dissipated by the brakes to stop the bike. These are kilocalories, so 1.4 calories will heat 1 kg of water 1.4 degrees Celsius. Since aluminum's heat capacity is 21% of water, this amount of energy would heat 800 g of alloy rims 8 °C (15 °F) in a rapid stop. Rims do not get very hot from stopping on flat ground. To get the rider's energy expenditure, consider the 24% efficiency factor to get 5.8 calories—accelerating a bike/rider to 25 mph (40 km/h) requires about 0.5% of the energy required to ride at 25 mph (40 km/h) for an hour. This energy expenditure would take place in about 15 seconds, at a rate of roughly 0.4 calories per second, while steady state riding at 25 mph (40 km/h) requires 0.3 calories per second.

Advantages

The advantage of light bikes, and particularly light wheels, from a KE standpoint is that KE only comes into play when speed changes, and there are certainly two cases where lighter wheels should have an advantage: sprints, and corner jumps in a criterium.[3]

In a 250 m sprint from 36 to 47 km/h to (22 to 29 mph), a 90 kg bike/rider with 1.75 kg of rims/tires/spokes increases KE by 6,360 joules (6.4 calories burned). Shaving 500 g from the rims/tires/spokes reduces this KE by 35 joules (0.04 calories = 0.01 watt-hour). The impact of this weight savings on speed or distance is rather difficult to calculate, and requires assumptions about rider power output and sprint distance. The Analytic Cycling web site (www.analyticcycling.com)[4] allows this calculation, and gives a time/distance advantage of 0.16 s/ 188 cm for a sprinter who shaves 500 g off their wheels. If that weight went to make an aero wheel that was worth 0.03 mph (0.05 km/h) at 25 mph (40 km/h), the weight savings would be canceled by the aerodynamic advantage. For reference, the best aero bicycle wheels are worth about 0.4 mph (0.6 km/h) at 25, and so in this sprint would handily beat a set of wheels weighing 500 g less.

In a criterium race, a rider is often jumping out of every corner. If the rider has to brake entering each corner (no coasting to slow down), then the KE that is added in each jump is wasted as heat in braking. For a flat crit at 40 km/h, 1 km circuit, 4 corners per lap, 10 km/h speed loss at each corner, one hour duration, 80 kg rider/6.5 kg bike/1.75 kg rims/tires/spokes, there would be 160 corner jumps. This effort adds 387 calories to the 1100 calories required for the same ride at steady speed. Removing 500 g from the wheels, reduces the total body energy requirement by 4.4 calories. If the extra 500 g in the wheels had resulted in a 0.3% reduction in aerodynamic drag factor (worth a 0.02 mph (0.03 km/h) speed increase at 25 mph), the caloric cost of the added weight effect would be canceled by the reduced work to overcome the wind.

Another place where light wheels are claimed to have great advantage is in climbing. Though one may hear expressions such as "these wheels were worth 1-2 mph," etc. The formula for power suggests that 1 lb. saved is worth 0.06 mph (0.1 km/h) on a 7% grade, and even a 4 lb savings is worth only 0.25 mph (0.4 km/h) for a light rider. So, where is the big savings in wheel weight reduction coming from? One argument is that there is no such improvement; that it is "placebo effect". But it has been proposed that the speed variation with each pedal stroke when riding up a hill explains such an advantage. However the energy of speed variation is conserved; during the power phase of pedaling the bike speeds up slightly, which stores KE, and in the "dead spot" at the top of the pedal stroke the bike slows down, which recovers that KE. Thus increased rotating mass may slightly reduce speed variations, but it does not add energy requirement beyond that of the same non-rotating mass.

Lighter bikes are easier to get up hills, but the cost of "rotating mass" is only an issue during a rapid acceleration, and it is small even then.

Explanations

Possible technical explanations for the widely claimed benefits of light components in general, and light wheels in particular, is as follows:
  1. Light weight wins races with significant climbing because the heavier bike can't make up the gap on descents or on the flats: the rider on the lighter bike just drafts. Alternatively, if the identical riders of heavier and lighter bikes simultaneously reach the bottom of a climb to the finish, all of the advantage goes to the lighter bike. This is not the case in a hilly time trials (or riding solo), where the advantage of heavier, but more aerodynamic wheels would easily make up the distance lost in climbs. In climbing, lighter wheels offer no particular advantage vs. a lighter frame, because there is no net loss of KE.
  2. Light weight wins sprints because it accelerates more easily. But note that heavier aerodynamic wheels gain significant advantage as speed increases, and for a good part of a sprint a rider is doing little accelerating but is working hard against a high-speed wind. So many sprint situations may favor heavier but more aerodynamic wheels.
  3. Light weight wins in criteriums because of the constant acceleration out of every corner. Heavier but more aerodynamic wheels offer little advantage because the riders are in a group most of the time. The energy savings from lighter wheels is minimal, but it may be more significant that the leg muscles have to put out just that bit of extra effort at each jam.


There are two "non-technical" explanations for the effects of light weight. First is the placebo effect. Since the rider feels that they are on better (lighter) equipment, they push themselves harder and therefore go faster. It's not the equipment that increases speed so much as the rider's belief and resulting higher power output. The second non-technical explanation is the triumph of hope over experience—the rider is not much faster due to lightweight equipment but thinks they are faster. Sometimes this is due to lack of real data, as when a rider took two hours to do a climb on their old bike and on their new bike did it in 1:50. No accounting for how fit the rider was during these two climbs, how hot or windy it was, which way the wind was blowing, how the rider felt that day, etc.

Another explanation, of course, may be marketing benefits associated with selling weight reductions.

In the end, the "incremental muscle power requirement" argument is the only one that can support the claimed advantages of light wheels in "jump" situations. This argument would state that: if the rider is already at the limit on each jump or each stroke of the pedals, then the small amount of extra power required for the extra weight would be a significant physiologic burden. Whether this is true is not clear, but it is the only explanation for the claimed advantage of wheel weight savings (compared to saving weight from the rest of the bike). For these accelerations, it makes no difference whether 1 lb is taken off the wheels or 2 lb off the bike/rider. The miracle of light wheels (compared to saving weight anywhere else in the bike/rider system) is hard to see.

Aerodynamics vs power

Heated debates over the relative importance of weight savings and aerodynamics are a fixture in cycling. This is an attempt to at least get the equation-based parts of the debate clarified. There will always be those who argue that "experience trumps mathematics" on this issue, so this will attempt to highlight those areas where experience might disagree with the math. From this, perhaps further discussion can focus on the topics of dispute rather than questioning known physics. To be as clear as possible, this will cover 1) the power requirements for moving a bike/rider 2) the energy cost of acceleration, and then 3) why experience and the math might disagree.

Power required

There is a well known equation that gives the power required to push a bike/rider through the air and to overcome the friction of the drive train:



Where is in watts, is Earth's gravity, is ground speed (m/s), is bike/rider mass in kg, is the grade (m/m), and is the rider's speed through the air (m/s). is a lumped constant for all frictional losses (tires, bearings, chain), and is generally reported with a value of 0.0053. is a lumped constant for aerodynamic drag and is generally reported with a value of 0.185 kg/m [5].

Note that the power required to overcome friction and gravity is proportional only to rider weight and ground speed. The power required to overcome wind drag is proportional to the cube of the air speed.

The human body runs at about 24% efficiency for a relatively fit athlete, so for every kJ delivered to the pedals the body consumes a kCal (4.2kJ) of food energy.

Obviously, both of the lumped constants in this equation depend on many variables, including drive train efficiency, the rider's position and drag area, aerodynamic equipment, tire pressure, and road surface. Also, recognize that air speed is not constant in speed or direction nor easily measured. It's certainly reasonable that the aerodynamic lumped constant would be different in cross winds or tail winds than in direct head winds, as the profile the bike/rider presents to the wind is different in each situation. Also, wind speed as seen by the bike/rider is not uniform except in zero wind conditions. Weather report wind speed is measured at some distance above the ground in free air with no obstructing trees or buildings nearby. Yet, by definition, the wind speed is always zero right at the road surface. Assuming a single wind velocity and a single lumped drag constant are just two of the simplifying assumptions of this equation. Computational Fluid Dynamicists have looked at this bicycle modeling problem and found it hard to model well. In layman's terms, this means that much more sophisticated models can be developed, but they will still have simplifying assumptions.

Given this simplified equation, however, one can calculate some values of interest. For example, assuming no wind, one gets the following results for calories required and power delivered to the pedals (watts):
  • 175W for a 90 kg bike + rider to go 9m/s (20 mph or 32 km/h) on the flats (76% of effort to overcome aerodynamic drag), or 2.6m/s (5.8 mph or 9.4 km/h) on a 7% grade (21% of effort to overcome aerodynamic drag).
  • 300W for a 90 kg bike + rider at 11m/s (25 mph or km/h) on the flats (83% of effort to overcome aerodynamic drag) or 4.3m/s (9.5 mph or 15 km/h) on a 7% grade (42% of effort to overcome aerodynamic drag).
  • 165W for a 65 kg bike + rider to go 9m/s (20 mph or 32 km/h) on the flats (82% of effort to overcome aerodynamic drag), or 3.3m/s (7.4 mph or 12 km/h) on a 7% grade (37% of effort to overcome aerodynamic drag).
  • 285W for a 65 kg bike + rider at 11m/s (25 mph or km/h) on the flats (87% of effort to overcome aerodynamic drag) or 15.3m/s (12 mph or 19 km/h) on a 7% grade (61% of effort to overcome aerodynamic drag).
Shaving 1 kg off the weight of the bike/rider would save 0.01 m/s at 9m/s on the flats (1 second in a 25 mph (40 km/h), 25 mile (40 km) TT). Losing 1 kg on a 7% grade would be worth 0.04m/s (90 kg bike + rider) to 0.07m/s (65 kg bike + rider). If one climbed for 1 hour, saving 1 lb. would gain between 225 and 350 feet (107 m) - less effect for the heavier bike + rider combination (e.g. 0.04 mph (0.06 km/h) * 1 hr * 5,280 ft (1,609 m)/mile = 225 ft). For reference, the big climbs in the Tour de France have the following average grades: The equation can be separated into level ground power

,


and vertical climbing power given by

.

Energy cost of acceleration

Why experience and the math might disagree

See also

Notes

References

1. ^ Ruina, Andy; Rudra Pratap (2002). Introduction to Statics and Dynamics (PDF), Oxford University Press, 397. Retrieved on 2006-08-04. 
2. ^ Wheels and the Myth about Rotating Weight. Retrieved on 2007-02-03.
3. ^ Technical Q&A with Lennard Zinn: The great rotating-weight debate. Retrieved on 2007-02-03.
4. ^ Analytic Cycling Long Climb. Retrieved on 2007-02-03.
5. ^ Corresponding to a surface area of 0.4m^2 with a drag coefficient of 0.7: Drag (physics)#Power
energy (from the Greek ενεργός, energos, "active, working")[1] is a scalar physical quantity that is a property of objects and systems of objects which is conserved by nature.
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Mode of transport (or means of transport or transport mode or transport modality or form of transport) is a general term for the different kinds of transport facilities that are often used to transport people or cargo.
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A bicycle pedal is the part of a bicycle that the rider pushes with his or her foot to propel the bicycle. It provides the connection between the cyclist's foot or shoe and the crankarm allowing the leg to turn the crank.
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Bicycle Wheel is a readymade by Marcel Duchamp consisting of a bicycle fork with front wheel mounted upside-down on a wooden stool.

In 1913 at his Paris studio he mounted the bicycle wheel upside down onto a stool, spinning it occasionally just to watch it.
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gearing on a bicycle is the selection of appropriate gear ratios for optimum efficiency or comfort. Different gears and ranges of gears are appropriate for different people and styles of cycling. Multi-speed bicycles allow gear selection to suit the circumstances, e.g.
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Cargo is a term used to denote goods or produce being transported generally for commercial gain, usually on a ship, plane, train, van or truck. Nowadays containers are used in most intermodal long-haul cargo transport.

Cargo represents a concern to U.S. national security.
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racing bicycle is built using lightweight, shaped aluminium tubing and carbon fiber stays and forks. It sports a drop handlebar and thin tires and wheels for efficiency and aerodynamics.
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Miles per hour is a unit of speed, expressing the number of international miles covered per hour.

Miles per hour is the unit used for speed limits, and speeds, on roads in the United Kingdom, United States and some other nations, where it is commonly abbreviated in everyday
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drag (sometimes called resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). Drag is made up of friction forces, which act in a direction parallel to the object's surface (primarily along its sides, as friction forces at the
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In physics, power (symbol: P) is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time.
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The supine position is a position of the body; lying down with the face up, as opposed to the prone position, which is face down.

Using terms defined in the anatomical position, the posterior is down and anterior is up.
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recumbent bicycle is a bicycle which places the rider in a seated or supine position (rarely, in a prone position). Recumbents hold the world speed record for a bicycle and were banned from international racing in 1934.
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fairing is a structure whose primary function is to produce a smooth outline and reduce drag. These structures are generally light-weight shapes and covers for gaps and spaces between parts of a vehicle, aircraft, or rocket to reduce form drag and interference drag, and to improve
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A streamliner is any vehicle that incorporates streamlining to produce a shape that provides less resistance to air. The term is usually applied to trains, mostly the high-speed trainsets designed in the United States in the 1930s, 1940s and 1950s, as well as successor "bullet
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kilogram or kilogramme (symbol: kg) is the SI base unit of mass. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water.
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Kilometres per hour (American English: kilometers per hour) is a unit of both speed (scalar) and velocity (vector). The unit symbol is km/h or km·h−1
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A calorie is a unit of measurement for energy. Calorie is French and derives from the Latin calor (heat). In most fields, it has been replaced by the joule, the SI unit of energy.
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The joule (IPA: [dʒuːl] or [dʒaʊl]) (symbol: J) is the SI unit of energy.
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pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called 'weight' in everyday parlance) in a number of different systems, including English units, Imperial units, and United
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Running is defined as the fastest means for an animal to move on foot. It is defined in sporting terms as a gait in which at some point all feet are off the ground at the same time. It can be a form of both aerobic and anaerobic exercise.
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Cycling is a means of transport, a form of recreation, and a sport. The bicycle carries riders across land, through tunnels, over bridges, snow, or, less frequently, over ice (icebiking).
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WATT

City of license Cadillac, Michigan
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kilogram or kilogramme (symbol: kg) is the SI base unit of mass. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water.
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Sprints are short running races in athletics.
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drag (sometimes called resistance) is the force that resists the movement of a solid object through a fluid (a liquid or gas). Drag is made up of friction forces, which act in a direction parallel to the object's surface (primarily along its sides, as friction forces at the
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Kilometres per hour (American English: kilometers per hour) is a unit of both speed (scalar) and velocity (vector). The unit symbol is km/h or km·h−1
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Miles per hour is a unit of speed, expressing the number of international miles covered per hour.

Miles per hour is the unit used for speed limits, and speeds, on roads in the United Kingdom, United States and some other nations, where it is commonly abbreviated in everyday
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racing bicycle is a bicycle designed for road cycling according to the rules of the Union Cycliste Internationale (UCI). The UCI rules were altered in 1934 to exclude recumbent bicycles. Throughout the late 1990s the rules were altered regularly to outlaw innovations.
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Sam Whittingham is a Canadian cyclist who has held several world records on recumbent bicycles.

As of 2007, he holds the following records under the sanction of the International Human Powered Vehicle Association:
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