Bicycle

A bicycle, or bike, is a pedal-driven land vehicle with two wheels attached to a frame, one behind the other. First introduced in 19th-century Europe, it evolved quickly into the familiar current design. With over one billion in the world today, bicycles provide the principal means of transportation in many regions and a popular form of recreation and transport in others.

Technical aspects

Legal requirements

The 1968 Vienna Convention on Road Traffic considers a bicycle to be a vehicle, and a person controlling a bicycle is considered a driver. The traffic codes of many countries reflect these definitions and demand that a bicycle satisfy certain legal requirements, including licencing, before it can be used on public roads. In many jurisdictions it is an offence to use a bicycle that is not in roadworthy condition and which does not have functioning front and rear brakes. In some places, bicycles must have functioning front and rear lights or lamps. As some generator or dynamo-driven lamps only operate while moving, rear reflectors are frequently also mandatory. Since a moving bicycle makes very little noise, in many countries bicycles must have a warning bell for use when approaching pedestrians, equestrians and other bicyclists.

Related Topics:
Vienna Convention on Road Traffic - Brakes - Lights - Reflectors

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Construction and parts

Frame

:Main article: Bicycle frame

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Nearly all modern upright bicycles feature the diamond frame, composed of two triangles: the front triangle and the rear triangle. The front triangle consists of the head tube, top tube, down tube and seat tube. The head tube contains the headset, the interface with the fork. The top tube connects the head tube to the seat tube at the top, and the down tube connects the head tube to the bottom bracket. The rear triangle consists of the seat tube and paired chain stays and seat stays. The chain stays run parallel to the chain, connecting the bottom bracket to the rear dropouts. The seat stays connect the top of the seat tube (often at or near the same point as the top tube) to the rear dropouts.

Related Topics:
Upright bicycle - Triangle - Headset - Fork - Bottom bracket - Chain

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Historically, women's bicycle frames had a top tube that connected in the middle of the seat tube instead of the top, resulting in a lower standover height. This allowed the rider to dismount while wearing a skirt or dress. Although some women's bicycles continue to use this frame style, there is also a hybrid form, the mixte or step-through frame, which also allows easier mounting and dismounting for both male and female riders.

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Historically, materials used in bicycles have followed a similar pattern as in aircraft, the goal being strength and low weight. Since the late 1930s alloy steels have been used for frame and fork tubes in higher quality machines. Celluloid found application in mudguards, and aluminum alloys are increasingly used in components such as handlebars, seat stems (also known as seatposts), and brake levers. In the 1980s aluminum alloy frames became popular, and their affordability makes them still common. More expensive carbon fibre and titanium frames are now also available, as well as advanced steel alloys.

Related Topics:
Celluloid - 1980s - Aluminum - Carbon fibre - Titanium

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Drivetrain

The drivetrain begins with pedals which rotate the crankset, which fit into the bottom bracket. Attached to the crank is the chainring which drives the chain, which in turn rotates the rear wheel via the rear sprockets. Between the chain and rear wheel may be interspersed various gearing systems, described below, which vary the gear ratio, the number of rear wheel revolutions produced by each turn of the pedals.

Related Topics:
Pedals - Crankset - Bottom bracket - Chain - Sprocket - Gear ratio

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Since cyclists' legs produce a limited amount of power most efficiently over a narrow range of cadences, a variable gear ratio is needed to maintain an optimum pedaling speed while covering varied terrain. The gear systems are hand-operated, via cables, and are of two types. Internal hub gearing works by planetary, or epicyclic, gearing, in which the outer case of the hub gear unit turns at a different speed relative to the rear axle depending on which gear is selected. External gearing utilizes derailleurs, which can be placed on both the front chainring and on the rear cluster or cassette, to push the chain to either side, derailing it from the sprockets. The sides of the gear rings catch the chain, pulling it up onto their teeth to change gears. Internal hub gears are much less affected by adverse weather conditions than derailleurs. Internal hub gearing still predominates in some regions, particularly on utility bikes, whereas in other regions, such as the USA, external derailleur systems predominate. One such internally-geared hub, the Rohloff Speedhub, has 14 internal gears and requires much less maintenance than derailleurs.

Related Topics:
Hub gear - Planetary, or epicyclic, gearing - Derailleurs

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Road bicycles have close set multi-step gearing, which allows very fine control of cadence, while utility cycles offer fewer, more widely spaced speeds. Mountain bikes and most entry-level road racing bikes may offer an extremely low gear to facilitate climbing slowly on steep hills.

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Fixed-gear track racing bikes have transmission efficiences of over 99% (nearly all the energy put in at the pedals ends up at the wheel). Whilst variable ratio gear mechanisms are essential for human efficiency, they do reduce mechanical efficiency. The efficiency varies considerably with the gear ratio being used. In a typical hub gear mechanism the mechanical efficiency will be between 82% and 92% depending on the ratio selected. Which ratios are best and worst depends on the specific model of hub gear. Derailleur type mechanisms fare better, with a typical mid-range product (of the sort used by serious amateurs) achieving between 88% and 99% efficiency at 100W. In derailleur mechanisms the highest efficiency is achieved by the larger cogs and, generally speaking, efficiency decreases with smaller cog sizes. This is because the chain must bend more sharply as it rolls on and off the cog, and also forms a sharp angle at the chain tensioner9. Efficiency is also compromised on derailleur drivetrains if the chain is running large-ring to large-cog or small-ring to small-cog; this is called cross-chaining, and also results in increased wear because of the lateral deflection of the chain. Retro-Direct was a type of modified fixed-gear drivetrain used on some bicycles in the early 20th century that has been resurrected by bicycle hobbyists.

Related Topics:
9 - Retro-Direct - 20th century

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Steering and seating

The handlebars rotate the fork and the front wheel via the stem, which articulates with the headset. Three styles of handlebar are common. Touring handlebars, the norm in Europe and elsewhere until the 1970s, curve gently back toward the rider, offering a natural grip and comfortable upright position. Racing handlebars are "dropped", offering the cyclist either an aerodynamic "hunched" position or a more upright posture in which the hands grip the brake lever mounts. Mountain bikes feature a crosswise handlebar, which helps prevent the rider from pitching over the front in case of sudden deceleration.

Related Topics:
Handlebar - Fork - Wheel - Headset - 1970s

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Variations on these styles exist. The Bullhorn style handlebars are often seen on modern Time Trial specific bicycles, equipped with two forward-facing extensions, allowing a rider to rest his entire forearm on the bar. These are usually used in conjunction with the aero bar, a pair of forward-facing extensions spaced close together, to allow the rider a position of increased aerodynamics. The Bullhorn style handlebars are banned from ordinary road racing because it is considered there is less fine control in bike traffic.

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Seats, or saddles, also vary depending on rider preference, from the cushioned ones favoured by short-distance riders to narrower seats which allow more free leg swings. Comfort depends on riding position. With comfort bikes and hybrids the cyclist sits high over the seat, their weight directed down onto the saddle, such that a wider and more cushioned saddle is preferable. For racing bikes where the rider is bent over, weight is more evenly distributed between the handlebars and saddle, and the hips are flexed. For this style of riding a narrower and harder saddle is more efficient.

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Recumbent bicycles have more chair-like seats, and so are much more comfortable to ride, although generally slower up hills due to this positioning. The reclined, low seating position does provide increased aerodynamics over standard seating.

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Brakes

:Main article: Bicycle brake systems

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Bicycle brakes are either rim brakes, in which friction pads are compressed against the wheel rims, internal hub brakes, in which the friction pads are contained within the wheel hubs, or disc brakes. A rear hub brake may be either hand-operated or pedal-actuated, as in the back pedal coaster brakes which were the rule in North America until the 1960s. Hub drum brakes do not cope well with extended braking, so rim brakes are favoured in hilly terrain. With hand-operated brakes, force is applied to brake handles mounted on the handle bars and then transmitted via Bowden cables to the friction pads. In the late 1990s, disc brakes appeared on some off-road bicycles, tandems and recumbent bicycles, but are considered impractical on road bicycles, which rarely encounter conditions where the advantages of discs are significant.

Related Topics:
1960 - Bowden cable - 1990s - Disc brakes - Tandem - Recumbent bicycle

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The advantages of discs make them well-suited to steep, extended downhills through wet and muddy off-road terrain, which falls under the category of downhill and freeride bicycle riding. The use of tires as large as 3.0 inches in width also makes disc brakes a necessity, as rim brakes simply cannot straddle a tire that wide.

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Two main disc brake systems exist: hydraulic and mechanical (cable-actuated). Mechanical disc brakes have less modulation than hydraulic disc brake systems, and since the cable is usually open to the outside, mechanical disc brakes tend to pick up small bits of dirt and grit in the cable lines when ridden in harsh terrain. Hydraulic disc brake systems generally keep contaminants out better. However, since hydraulic disc brakes usually require relatively specialized tools to bleed the brake systems, repairs on the trail are difficult to perform, whereas mechanical disc brakes rarely fail. Also, the hydraulic fluid may boil on steep, continuous downhills. This is due to the brake losing its ability to transmit force through incompressible fluids, since some of it has become a gas, which is compressible. For these reasons, one must weigh the advantages and disadvantages of using a hydraulic system versus a mechanical system.

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Accessories and repairs

Utility bicycles have many standard features which enhance their usefulness and comfort that would be considered accessories on sports bicycles. Chainguards and mudguards, or fenders, protect clothes and moving parts from oil and spray. Kick stands help with parking. Front-mounted wicker or steel baskets for carrying goods are often used. Rear racks or carriers can be used to carry items such as school satchels. Parents sometimes add rear-mounted child seats and/or an auxiliary saddle fitted to the crossbar to transport children.

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Other accessories include lights, pump, lock, and additional (pedal or wheel-mounted) reflectors. Technical accessories include solid-state speedometers and odometers for measuring distance. Toe-clips help to keep the foot planted firmly on the pedals, and enable the cyclist to pull as well as push the pedals.

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In North America, a large minority, possibly up to 25% in the US, of bicyclists wear plastic bicycle helmets for safety. There is no US federal law requiring the use of helmets, but the majority of states require children up to a certain age to wear them; a number of cities and counties in Washington require them for riders of all ages. In most European countries, helmets are viewed as an indicator of inexperience or recklessness, and their use is considered unusual for adult utility cyclists. In Australia and New Zealand, and parts of Canada, such helmets are required by law. The use of helmets by utility cyclists is practically unknown in most other regions.

Related Topics:
Bicycle helmet - Washington - Australia - New Zealand - Canada

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Many cyclists also carry tool kits, containing at least a patch kit for tires, tire levers, and some spanners. At one time it was possible to use a single tool to carry out most common bicycle repairs. More specialised parts now often require more complex tools, including proprietary tools specific for a given manufacturer. Some bicycle parts, particularly hub-based gearing systems, are complex, and many people prefer to leave most maintenance and repairs to professionals. Others maintain their own bicycles, finding it enhances their enjoyment of the hobby of cycling.

Related Topics:
Tire lever - Maintenance and repairs - Hobby

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Performance

In both biological and mechanical terms, the bicycle 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 9. 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.

Related Topics:
1

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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

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• 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 as long as the bike is properly "fit." In addition, since bicycling can also provide convenient transportation, less self-discipline may 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.

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The average "in-shape" man 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.

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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 attained on the flat, without using motor pacing and wind-blocks, is by Canadian Sam Whittingham, who in 2002 set a 130.36 km/h (81.00 mph) record on his highly aerodynamic recumbent bicycle. This stands as the official record for all human-powered vehicles.

Related Topics:
Km/h - Mph - Sam Whittingham - 2002 - Recumbent bicycle

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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 road resistance. In measured tests these components have almost no effect on cycling performance. For instance, lowering a bike's weight by 1 kg, a major effort considering they may weigh less than 15 kg 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.

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Bicycle physics

A rider stays upright on a bicycle by steering the bicycle so that the point where the wheels touch the ground stays underneath the center of gravity. Once underway, this effort is largely replaced by physical forces generated by the rotation of the wheels which produce a remarkable "self-steering" effect.3 The angular momentum of the wheels and the torque applied to them by the ground generates a phenomenon called precession, by which the wheel turns, or trails, toward whichever side to which the bicycle tilts. Like the rider's steering adjustments, this motion automatically returns the contact point of the wheel directly under the center of gravity. These forces, perhaps aided at very high speeds by the gyroscopic effect of the spinning wheels,4 are sufficiently strong that a riderless bicycle going down a slope will stay upright by itself. Conversely, a bicycle whose steering fork is locked in a perfectly straight ahead position is virtually impossible to balance.

Related Topics:
3 - Angular momentum - Torque - Precession

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That gyroscopic effects are unimportant at normal cycling speeds was shown by physicist and researcher into bicycle stability David E. H. Jones, whose series of "URBs" ("unrideable bikes" with various modifications to the front end) included a bike which cancelled the gyroscopic effect of the front wheel by dint of attaching a second wheel to his front forks (alongside the regular wheel) whose lower edge was about an inch (25 mm) above the ground. By gearing this wheel to the regular front wheel so that it spun in the opposite direction at equal speed, the net angular momentum of both wheels together was close to zero. Jones found he could ride this bike with no difficulty, but did discover that without a rider the non-gyroscopic bike fell over much faster than a regular bike.

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Stability is also influenced by a geometric factor called trail. This is the distance between the point of contact the front wheel makes with the ground and the place the steering axis makes contact with the ground. The greater the amount of trail, the greater the reaction. One can see the effect that trail has by simply holding a bicycle by the seat and leaning it. The moment due to trail and the weight of the bicycle will turn the front wheel in the direction of the turn. Negative trail (rolling a bicycle backwards) results in immediate steering problems. Zero trail (as in a unicycle) requires constant rider adjustment. Positive trail - found on typical bicycles - creates positive stability by steering the contact patch back under the CG of the bicycle and rider. http://www.johnforester.com/Articles/BicycleEng/dahon.htm http://www.velonews.com/tech/report/articles/7322.0.html

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At higher speeds bicycles can also experience speed wobbles or shimmies, where the front wheel spontaneously oscillates to the left and right. While the wobbles can be easily remedied by slowing down, adjusting position, or relaxing one's grip on the handlebars, speed wobbles can be fatal

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This shimmy is often seen in shopping cart front wheels. Some otherwise minor irregularity accelerates the wheel to one side. The restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure. Speed changes, making the bicycle/motorcycle stiffer or lighter, or increasing the stiffness of the steering (of which the rider is the main component) can change the oscillation frequency, though only speed change is applicable in the situation.

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For more information on the technical aspects of bicycles, see also:

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• List of bicycle parts and :Category:Bicycle parts

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