Kamis, 24 Maret 2011

Fixed-gear bicycle

In Australia, "fixed-wheel" is the normal term for the subject of this article — meaning the opposite of freewheel, and "fixed-gear" usually refers to a single-speed bicycle.

A fixed-gear bicycle (or fixed-wheel bicycle, sometimes known in the USA as a fixie) is a bicycle that has no freewheel, meaning it cannot coast — the pedals are always in motion when the bicycle is moving.
The sprocket is screwed or bolted directly onto a fixed hub.
When the rear wheel turns, the pedals turn in the same direction. This allows a cyclist to stop without using a brake, by resisting the rotation of the cranks, and also to ride in reverse.


A fixed/freewheel rear hub (flip-flop)

The track bicycle is a form of fixed-gear bicycle used for track cycling in a velodrome. But since a "fixed-gear bicycle" is just a bicycle without a freewheel, a fixed-gear bicycle can be almost any type of bicycle.
Traditionally, some road racing, club cyclists used a fixed-gear bicycle for training during the winter months, generally using a relatively low gear ratio, believed to help develop a good pedalling style. In the UK until the 1950s it was common for riders to use fixed-gear bicycles for time trials. The fixed-gear was also commonly used, and continues to be used in the end of season hill climb races in the autumn. A typical club men's fixed-gear machine would have been a "road/path" or "road/track" cycle. In the era when most riders only had one cycle, the same bike when stripped down and fitted with racing wheels was used for road time trials and track racing, and when fitted with mudguards (fenders) and a bag, it was used for club runs, touring and winter training.By the 1960s, multi-gear derailleurs had become the norm and riding fixed-gear on the road declined over the next few decades. Recent years have seen renewed interest and increased popularity of fixed-gear cycling. In urban North America fixed-gear bicycles have achieved tremendous popularity, with the rise of discernible regional aesthetic preferences for finish and design details.

Dedicated fixed-gear road bicycles are being produced in greater numbers by established bicycle manufacturers. They are generally low in priceand characterized by a very forgiving, slack road geometry, as opposed to the steep, aggressive geometry of track bicycles.

Fixed-gear bicycles are also used in cycle ball, bike polo and artistic cycling.
A fixed-gear bicycle is particularly well suited for track stands, a manouver in which the bicycle can be held stationary, balanced upright with the rider's feet on the pedals.
The fixed-gear bicycle is often identified with the hipster subculture.

Advantages and disadvantages

In slippery conditions some riders prefer to ride fixed because they believe the transmission provides increased feedback on back tire grip.
Descending any significant gradient is more difficult as the rider must spin the cranks at high speed (sometimes at 170 rpm or more), or use the brakes to slow down. Some consider that the enforced fast spin when descending increases suppleness or flexibility, which is said to improve pedalling performance on any type of bicycle.
Riding fixed is considered by some to encourage a more effective pedaling style, which is claimed translates into greater efficiency and power when used on a bicycle fitted with a freewheel.
When first riding a fixed gear, a cyclist used to a freewheel may have a tendency to try to coast, particularly when approaching corners or obstacles. Since freewheeling, or coasting, is not possible this can lead to a 'kick' to the trailing leg, or even to loss of control of the bicycle.
Riding at high speed around corners can be difficult for the novice rider, as the pedals can strike the road, resulting in possible loss of control.
A fixed gear bicycle has fewer moving parts than a multi-gear bicycle and requires less maintenance.


Some fixed-gear riders think brakes are not strictly necessary, and brakeless fixed riding has a cult status in some areas. For the image-concerned cyclist, brakes and their cables add extra aesthetic "bulk" to the simple, minimalist look of a fixed gear.

Other riders dismiss riding on roads without brakes as an unnecessary affectation, based on image rather than practicality. Furthermore, riding brakeless can be very dangerous, and may jeopardize the chances of a successful insurance claim in the event of an accident — and is against the law in many jurisdictions.
Physics and technique

It is possible to slow down or stop a fixed-gear bike by resisting the turning cranks, and a rider can also lock the rear wheel and skid to slow down or completely stop on a fixed-gear bicycle, a maneuver sometimes known as a skid stop. It is initiated by unweighting the rear wheel while in motion by shifting the rider's weight slightly forward and pulling up on the pedals using clipless pedals or toe clips and straps. The rider then stops turning the cranks, thus stopping the drivetrain and rear wheel, while applying his or her body weight in opposition to the normal rotation of the cranks. This action causes the rear wheel to skid, which acts to slow the bike. The skid can be held until the bicycle stops or until the rider desires to continue pedalling again at a slower speed. The technique requires a little practice and using it while cornering is generally considered dangerous. A wet surface further reduces the effectiveness of this method, almost to the point of not reducing speed at all.

On any bike with only rear wheel braking, the maximum deceleration is significantly lower than on a bike equipped with a front brake As a vehicle brakes, weight is transferred towards the front wheel and away from the rear wheel, decreasing the amount of grip the rear wheel has. Transferring the rider's weight back will increase rear wheel braking efficiency, but normally the front wheel might provide 70% or more of the braking power when braking hard

United States — The use of any bike without brakes on public roads is illegal in many places, but the wording is often something along the lines of "...must be equipped with a brake that will enable the person operating the cycle to make the braked wheels skid on dry, level and clean pavement..." which some have argued allows the use of the legs and gears. The retail sale of bikes without brakes is banned by the U.S.Consumer Product Safety Commission – but with an exception for the "track bicycle" (...a bicycle designed and intended for sale as a competitive machine having tubular tires, single crank-to-wheel ratio, and no free-wheeling feature between the rear wheel and the crank....

UK — The Pedal Cycles Construction and Use Regulations 1983 requires pedal cycles "with a saddle height over 635 mm to have two independent braking systems, with one acting on the front wheel(s) and one on the rear". It is commonly thought that a front brake and a fixed rear wheel satisfies this requirement .
Germany — All bicycles are required to have working brakes on both wheels, as well as reflectors and bells. In Bonn a local court accepted that the fixed-gear mechanism was suitable back brake, but high-profile crackdowns specifically targeted fixies in Berlin, in an attempt to control what police described as a "dangerous trend"

Australia — In every state, bicycles are regarded as vehicles under the Road Rules. By law, a bike is required to have at least one functioning brake.
New Zealand — By law all bicycles must have a minimum of "...a good rear brake..."—and those made since 1 January 1988 must also have"...a good front brake..."

France — To be approved for road traffic, a bike must have 2 brakes, 2 lights, numerous reflectors, and a ringer. However the laws are rarely enforced, and the sight of all kinds of non officially approved bikes is quite common.
Denmark — All bicycles are required to have working brakes on both wheels, as well as reflectors and bells.


Many companies sell bicycle frames designed specifically for use with fixed-gear hubs. A fixed-gear or track-bike hub includes special threads for a lockring that tightens in the opposite (counter-clockwise) direction compared with the cog. This ensures that the cog cannot unscrew when the rider "backpedals" while braking.

A horizontal dropout on a steel frame road bicycle converted to a single-speed. The derailleur hanger (below the axle) and an eyelet (above the axle) for mounting a fender or rack, both integral parts of the original frame, are now unused.
For a variety of reasons, many cyclists choose to convert freewheel bicycles to fixed gear. Frames with horizontal dropouts will be straightforward to convert, frames with vertical dropouts less so.One method is to simply replace the rear wheel with a wheel that has a track/fixed hub. Another is to use a hub designed to be used with a threaded multi-speed freewheel. Such a hub will only have the normal right-handed threads for the cog and not the reverse threads for the lockrings used on track/fixed hubs. There is the possibility that the sprocket on a hub without a lockring will unscrew while back pedalling. Even if a bottom bracket lockring is threaded onto the hub along with a track sprocket, because the bottom-bracket lockring is not reverse threaded, the possibility still exists that both the sprocket and locknut can unscrew. Therefore it is recommended to have both front and rear brakes on a fixed-gear bicycle using a converted freewheel hub in case the cog unscrews while back pedaling. It is also advisable to use a thread sealer for the cog and bottom bracket lockring. Therotafix (or "frame whipping") method may be helpful to securely install the sprocket.
Bicycles with vertical dropouts and no derailleur require some way to adjust chain tension. Most bicycles with horizontal dropouts can be tensioned by moving the wheel forward or backward in the dropouts. Bicycles with vertical dropouts can also be converted with some additional hardware. Possibilities include:
 An eccentric hub or bottom bracket allows the off center axle or bottom bracket spindle to pivot and change the chain tension.
 A "Ghost" or "floating" chainring. An additional chainring placed in the drive train between the driving chainring and sprocket. The top of the chain moves it forward at the same speed that the bottom of the chain moves it backwards, giving the appearance that it is floating in the chain.
 A "Magic gear". With some math you can calculate a gearing ratio to fit a taut chain between the rear dropout and bottom bracket. Also, using a chain half link and slightly filing the dropouts to increase the width of the slot will increase the chances of finding a "magic gear." It is worth noting that the "magic gear" setup is controversial, due to inevitable chain stretch and subsequent slippage that can lead to serious injury.
Separate chain tensioning devices such as the type which are attached to the dropout gear hanger (commonly used on single speed mountain bikes) cannot be used because they will be damaged as soon as the lower part of the chain becomes tight.
Additional adjustments or modification may be needed to ensure a good chainline. The chain should run straight from the chainring to the sprocket, therefore both need to be the same distance away from the bicycle's centerline. Matched groupsets of track components are normally designed to give a chainline of 42 mm, but conversions using road or mountain bike cranksets often use more chainline. Some hubs, such as White Industries' ENO, or the British Goldtec track hub, are better suited to this task as they have a chainline greater than standard. Failure to achieve good chainline will at best lead to a noisy chain and increased wear, and at worst can throw the chain off the sprocket. This can result in rear wheel lockup and a wrecked frame if the chain falls between the rear sprocket and the spokes. Chainline can be adjusted in a number of ways, which may be used in combination with each other:
 Obtaining a bottom bracket with a different spindle length, to move the chainring inboard or outboard
 Choosing a bottom bracket with two lockrings, which gives fine adjustment of chainring position
 Respacing and redishing the rear wheel, where permitted by the hub design
 Placing thin spacers under the bottom bracket's right-hand cup (Sturmey-Archer make a suitable 1/16" spacer) to move the chainring outboard
 Placing thin spacers between the chainring and its stack bolts to move it inboard (if the chainring is on the inside of the crank spider) or outboard (if the ring is on the outside of the spider)
 Placing thin spacers between the hub shoulder and the cog- only recommended in the case of a freewheel-threaded hub, which has sufficiently deep threads for this
There are many forms of competition using a fixed gear bike, most of the competitions being track races. Bike messengers and other urban riders may ride fixed gear bicycles in alleycat races, including New York City's famous fixed-gear-only race Monstertrack alleycat.
There are also events based on messenger racing such as Mixpression which has been held 9 times in Tokyo. Trick demonstrations have been held since the late 1800s in the US and Europe; while they continued into a competitive form in Europe (Artistic Cycling), subsequent to the recent widespread popularity and advancement of fixed gear bikes, trick competitions have also now established themselves at venues in the US and Asia. European competitions include solo and team balletic movements on a controlled, flat surface; US and Asian competitions often include "park" and "flatland" styles and venues, a la BMX. Other competitions include games of "foot down" and bike polo.
In 2006, Adventures for the Cure made a documentary film on riding across the United States on fixed gears; they repeated this feat as a 4-man team at the 2008 Race Across America.
Fixed gear riders sharing the specific philosophy are also seen at the Single Speed World Championships.
Maintenance and upkeep
Maintaining a fixed gear is relatively easy because it has fewer parts than a geared bicycle. The sprocket should be checked regularly to make sure there is no damage to any teeth and that no object is grinding it as it turns with the rear wheel. The chainring should be checked similarly for any damage.
As with any other bicycle, the chain should be checked making sure the master link is securely latched. The chain can be lubricated monthly for smooth riding.Also, as needed, the brakes should be tightened as they wear and tire condition observed for possible puncture locations. Air pressure in the tires, tire alignment, brake handle placement, and rust should be monitored on a daily basis because they can change very easily during a jarring ride.

source : original text source here

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