A bicycle is a two-wheeled steerable machine that is propelled by the action of a rider’s feet on pedals. It is the most efficient means of human energy conversion to motion yet devised, and has revolutionized transportation and outdoor recreation in many countries. Bicycles are categorized by function, number of riders, frame construction and gearing, with more common types including utility bicycles, racing bicycles, touring bicycles, hybrid bicycles, cruiser bicycles, and BMX bikes. A unicycle, tricycle and quadracycle are also technically bicycles, but differ in that they have one, three or four wheels instead of two.

A modern bicycle has a frame made of tubular steel or aluminium, with forks that connect the head tube to the seat tube and the rear axle. The front fork may have a stem that adjusts to vary the height of the handlebars. The handlebars may have a fixed position that curves back toward the rider, drop handlebars that curve forward and down for more aerodynamic control from a “crouched” position, or riser bars that move in between these two styles.

Brakes slow the bike by applying pressure to the friction pads on the wheel hubs. The force is transmitted through brake levers mounted on the handlebars and then transferred to the hubs by either a Bowden cable or hydraulic lines. A hand-operated front hub brake and a pedal-operated rear hub brake are often standard on a bike.

More elaborate bikes may have hubs with multiple axles, which allow the same wheel to be used for both coaster and derailleur brakes, and which can be fitted with gearing to increase speed range. Accessories such as baskets, racks, fenders, reflectors and lights are useful to improve riding comfort and safety. Technical accessories include cyclocomputers to monitor and record data such as speed, distance and heart rate.

Bicycles are relatively simple compared with automobiles, so some cyclists choose to perform at least some maintenance and repair themselves. A basic set of tools is typically needed, and some components may require special manufacturer-dependent tools to access and replace. A bicycle’s tires are subject to punctures from sharp objects such as thorns, nails and glass. Puncture repair kits are available to patch or replace a tire, and some tires are designed to be more puncture resistant than others.

A bike may be equipped with a basket or pannier racks for carrying cargo, and a kickstand to hold the bicycle upright when not being ridden. For safety, most cyclists wear a helmet. A bike may also be equipped with a rear rack to hold a luggage carrier or a child seat. A hitch can be attached to the rear of a bike to tow a trailer. Bicycles are a popular mode of transport in cities and towns, especially in developing countries, where they can reduce congestion and air pollution and provide a form of cheap, sustainable transportation for people who do not have access to motorized vehicles. It is estimated that a bicycle can reduce the need for a car for trips to work, school and shopping by about ten percent.