Bicycle that has a drivetrain with no freewheel mechanism
A fixed-gear bicycle (or fixed-wheel bicycle in British usage, commonly known in some places as a fixie) is a bicycle that has a drivetrain with no freewheel mechanism
such that the pedals always will spin together with the rear wheel. The freewheel was developed early in the history of bicycle design but the fixed-gear bicycle remained the standard track racing design. More recently the "fixie" has become a popular alternative among mainly urban cyclists, offering the advantage of simplicity compared with the standard multi-geared bicycle.
Most bicycle hubs incorporate a freewheel to allow the pedals to remain stationary while the bicycle is in motion, so that the rider can coast, i.e., ride without pedalling using forward momentum. A fixed-gear drivetrain has the drive sprocket (or cog) threaded or bolted directly to the hub of the back wheel, so that the pedals are directly coupled to the wheel. During acceleration, the pedal crank drives the wheel, but in other situations, the rear wheel can drive the pedal cranks. This direct coupling allows a cyclist to apply a braking force with the legs and bodyweight, by resisting the rotation of the cranks. It also makes it possible to cycle backwards.
As a general rule, fixed-gear bicycles are single-speed. A derailleur for gear selection would introduce chain slack, which would interfere with braking. Gear selection can, however, be accomplished with the use of an internally geared hub. For example, a Sturmey-Archer fixed-gear three-speed hub is a fixed-gear multi-speed arrangement. Most fixed-gear bicycles only have a front brake, and some have no brake.
Uses.
A track bicycle or track bike is a form of fixed-gear bicycle optimized for racing at a velodrome or at an outdoor track.
Some road racing and club cyclists use 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 1959 British 25 mile time trial championship was won by Alf Engers with a competition record of 55 minutes 11 seconds, riding an 84-inch fixed-gear bicycle. 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 bicycles 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 and similar areas in other Western cities, fixed-gear bicycles have achieved significant 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 price and characterized by relaxed road geometry, as opposed to the steep 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 maneuver in which the bicycle can be held stationary, balanced upright with the rider's feet on the pedals.
Advantages and disadvantages.
One of the perceived main attractions of a fixed gear bicycle is low weight. Without the added parts required for a fully geared drive train—derailleurs, shifters, cables, cable carriers, multiple chain rings, freewheel hub, brazed-on mounting lugs—a fixed gear bicycle weighs less than its geared equivalent. The chain itself is subject to less sideways force and will not wear out as fast as on a derailleur system. Also, a fixed gear drivetrain is more mechanically efficient than any other bicycle drivetrain, with the most direct power transfer from rider to the wheels. Thus, a fixed gear requires less energy in any given gear to move than a geared bike in the same gear.
In slippery conditions some riders prefer to ride fixed because they believe the transmission provides increased feedback on back tire grip. However, there is also an increased risk of loss of control in such conditions. This is especially so when taking into account the large number of riders who ride brakeless. These riders must brake entirely through the drivetrain. Ideally this is done by resisting the forward motion of the pedals, shedding speed while the bike is still moving. Alternatively, though far less efficiently, one can brake by stopping the motion of the pedals in mid-rotation, causing the rear wheel to lock in place, allowing the bicycle to skid and slow down from kinetic friction (see below).
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; however the performance boost is negligible compared to the benefits of riding a free wheel.
Riding fixed is considered by some to encourage a more effective pedaling style, which it is claimed translates into greater efficiency and power when used on a bicycle fitted with a freewheel. It allows for the rider to engage in and practice proper cadence, which is the balanced and rhythmic flow of pedaling, enhancing performance for both cyclist and bicycle.
When first riding a fixed gear, a cyclist used to a freewheel may try to freewheel, or coast, particularly when approaching corners or obstacles. Since coasting is not possible this can lead to a "kick" to the trailing leg, and even to loss of control of the bicycle. Riding at high speed around corners can be difficult on a road bike converted into a fixed-gear bicycle, as the pedals can strike the road, resulting in loss of control. Proper track bikes have a higher bottom bracket to compensate for the constantly spinning cranks and largely mitigate this problem.
Perhaps the most obvious disadvantage is the lack of multiple gears, and the flexibility in pedaling cadence and resistance made available through gear shifting. Hilly or uneven mountainous terrain with steep grades can be particularly challenging, as the rider cannot adjust the gearing to match the terrain.
Brakeless.
Many urban fixed-gear riders think brakes are not strictly necessary, and brakeless fixed riding has a cult status in some areas. Brakes and their cables are said to add extra bulk to the simple appearance of a fixed gear bicycle, and they prevent trick manoeuvres that involve spinning the front wheel in a full circle, unless equipped with special 360° freedom "detangler" system already used on BMX bicycles.
Other riders dismiss riding on roads without brakes as an affectation, based on image rather than practicality. Riding without brakes can be dangerous, is prohibited by law in many jurisdictions, and may jeopardize the chances of a claim in the event of an accident.
Technique.
It is possible to slow down or stop a fixed-gear bike in two ways. The first, most efficient, and least stressful on the rider's body is by resisting the turning cranks as they come up and around, shedding speed with each pedal rotation. The second way, less efficient but more showy, is to bump or skid the rear wheel along the riding surface. Such a move is initiated 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 body weight in opposition to the rotation of the cranks. This causes the rear wheel to skid, and slow the bike. The skid can be held until the bicycle stops or until the rider desires to continue pedaling again at a slower speed. The technique requires practice and is generally considered dangerous when used during cornering.
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, decreasing the amount of grip the rear wheel has. Transferring the rider's weight back increases rear wheel braking efficiency, but a front wheel fitted with an ordinary brake might provide 70% or more of the braking power when braking hard.
Legality.
Austria – Brakeless bicycles are illegal to ride on public roads. Every bike has to have two independent brakes, several reflectors and front and back lighting when conditions require it.
Australia – Bicycles are regarded as vehicles under the Road Rules in every state. A bike is required by law to have at least one functioning brake.
Belgium – All bicycles are required to have easy-to-hear bells and working brakes on both wheels. Lights and reflectors are not required on race, mountain, and children's bikes during daylight hours. Other (normal) bicycles need reflectors and lights. These lights may be attached to the body and may blink.
Brazil – Brazilian Traffic Law, article 105, subheading VI, says that, for bicycles, "bells, rear, front, sides and pedals signaling, and rear-view mirror on the left" are mandatory. Albeit, nothing is explicitly said about bicycle brakes.
Canada – Laws vary by province, and are controlled by highway and traffic acts. In Ontario and in Québec, a rear brake is required, and not having one may subject the rider to a fine. Alberta requires one functioning brake.
Denmark – All bicycles are required to have working brakes on both wheels, reflectors, and bells.
France – Bicycles must have two brakes, two lights, numerous reflectors, and a bell to be approved for road traffic. The laws are rarely enforced, however, and all kinds of unapproved bicycles are common.
Germany – All bicycles are required to have working brakes on both wheels, reflectors, and bells. A local court in Bonn accepted that the fixed-gear mechanism was a suitable back brake, but high-profile crackdowns specifically targeted fixed-gear bicycles in Berlin in an attempt to control what police described as a "dangerous trend"
Hungary – All bicycles are required to be equipped with two independent sets of brakes. Front and rear lamps, reflectors and a bell are also required.
Italy – All bicycles must have tires, two independent sets of brakes, a bell, a front white lamp, a rear red lamp and reflector, and amber reflectors on pedals and sides. However, these regulations are rarely enforced.
 Korea - Brakeless bicycles are illegal in the Republic of Korea. Both front and rear brakes are required to ride a bike in public.
Netherlands – All bicycles are required to have an adequate brake system.
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..."
Poland – All bicycles must have at least one functional brake.
Russian Federation – All bicycles must have working brakes, handlebars and horns 
Singapore – In a report submitted to the Singapore Ministry of Transport in December 2020, the Active Mobility Advisory Panel recommended that a new rule be put in place to allow only bicycles that have brakes installed in order for them to be used on footpaths and roads. In January 2020, a 13-year-old fixed-gear bicycle rider died after she crashed into a railing at a multi-storey car park and fell to the ground below, prompting a state coroner hearing. A court decided that such bicycles are not suitable for the young, and her death was ruled as a misadventure.
Spain – All bicycles are required to have an adequate brake system on front and rear wheels, plus a bell.
Sweden – Bicycles are required to have a brake and bell. Additionally, when it's dark outside they are required to have lights facing forward and back as well as reflectors facing forward, back and to the sides.
United Kingdom – The Pedal Cycles (Construction and Use) Regulations 1983 require pedal cycles "so constructed that one or more of the wheels is incapable of rotating independently of the pedals, be equipped with a braking system operating on the front wheel"; riding a rear-wheel drive, fixed-gear bicycle with no front brake on public highways is illegal. Following the