Bicycle

A bicycle or bike is a machine for human transportation that employs two wheels. Unlike most modern methods of transportation, bicycles are human-powered.

Types of Bicycles

There are many types of bicycles. Most sold today fit into one of these categories:

1. Road or Racing Bicycles -- characterized by narrow tires, no shock absorbers, thin light frames, and drop handlebars. The frame generally encourages a more inclined posture.
2. Hybrid Bicycles -- a blanket term for bicycles which have characteristics of both Road and Mountain bicycles.
3. Mountain Bicycles -- characterized by wide, knobby tires, shock absorbers, heavier frames, and straight handlebars. The frame generally encourages a more upright posture.
4. Tandem Bicycles -- A bicycle with two sets of handlebars, seats and transmission, to allow two people to ride at once.
5. Recumbent Bicycles -- A bicycle in which the rider employs a horizontal, foot-first posture.
6. Faired Bicycle -- A bicycle with a partial or full aerodynamic fairing is often called a human powered vehicle.
7. Power-assisted Bicycle -- A bicycle with a motor, mostly an electric one with storage batteries.

It should be noted, however, that bicycle enthusiasts have often mixed-and-matched designs and technologies, yielding bikes that are not easily classified, for instance, a tandem bicycle in which the front rider employs the recumbent position.

Parts of a Bicycle

Despite its apparent simplicity, a modern bicycle is actually a very complicated machine. Each part of a bicycle has been repeatedly improved upon by more than a century of engineering.

The Frame

Main article: bicycle frame

The Wheels

Main article: bicycle wheel

The wheels of bicycles are very lightweight, yet strong and flexible enough to withstand bumps. A typical bicycle wheel consists of a central hub, a plethora of spokes leading to a circular rim, and a pneumatic tire fastened to the rim.

The Transmission

Main article: bicycle transmission

A human operator typically powers a bicycle by applying a force to pedals using his or her legs. The pedals are connected to a front sprocket (the chain ring) by cams (cranks or crank arms), which translate the linear force from the foot into a torque, thus rotating the chain ring. This torque is transmitted via a chain (transmission) to a rear sprocket (called a freewheel or cassette, depending on the technology), which is coupled to the rear wheel.

Alternatively, there are bicycles which use a treadle mechanism in place of the pedals.

Fixed-Gear, Fixies, and Track Bikes

In a fixed-gear bicycle, the rotation of the pedals is directly coupled with the rotation of the rear wheel. There is only one available gearing ratio. Such a bicycle cannot coast, nor can the rider adjust the gear ratio to accommodate changes in terrain. Such bicycles can also be ridden backwards.

These bicycles are perceived as being more efficient. There are arguments that one of the major sources of inefficiency experienced by a cyclist are due to derailleur transmissions. For this reason, fixed-gear bicycles are often used in races.

The rear wheel of a fixed-gear bicycle is marginally stronger than one from a multiple gear bicycle due to the wheel's dish -- an asymmetry in spoke structure necessitated by the width of multiple sprockets. This is also the case with single speed or internally geared bicycles.

Single Speed Bicycles

A single speed bicycle should not be confused with a fixed-gear bicycle. A single speed bicycle features a free wheel, which allows the rear wheel to move forward while the chain remains stationary.

The Brakes

Most bicycles feature a method for slowing the bicycle by using friction, whereby energy is converted into heat. Some electrically-assisted bicycles also allow regenerative braking, whereby energy is converted into electricy.

No brakes, a la Fixed-Gear

When the pedals are directly coupled with the motion of the rear wheel, as is the case with fixed gear bicycles, a rider can brake by simply pedaling more slowly. For this reason, experienced fixed-gear riders will often build bicycles without brakes.

However, as typical bicycle riders cannot easily cope with this sort of use and many countries legally require multiple brakes, the fixed gear design is generally employed only in certain specialized capacities, such as track (velodrome) racing.

Bicycles in Sport

Main article: cycling