A bicycle is a human-powered, two-wheeled vehicle for transporting the rider and cargo. Depending on the design and use, bikes can be extremely efficient in terms of the energy required to travel a given distance, although they also tend to be relatively heavy. A typical bicycle can be used to get to and from work or school for a small initial cost and very little annual running expense, making it an affordable means of transportation for the vast majority of the world’s population.
The bike consists of a frame, fork, wheels, tires and a saddle, and can be augmented with accessories such as handlebars, pedals, chains, derailleurs, seat posts, bells or horns, and cycling computers or GPS. There is a great deal of variety in the components and materials used to make bikes, and there are tradeoffs between price, weight, durability and performance. For example, a top-of-the-line carbon fiber racing bicycle can cost several thousand dollars while an inexpensive steel model may be adequate for casual riding or commuting to work and back.
It is important to find a bicycle that fits the rider well. The ideal saddle height allows the cyclist to comfortably reach the pedals at the bottom of the stroke without requiring excessive leg extension. It is also important to select a bicycle with enough gearing to allow the rider to maintain a constant pedaling rate, which reduces both fatigue and stress on the body. Most new bicycles are sold with a single speed drivetrain, but multi-speed models are available as an upgrade for those who want to increase their pedaling efficiency.
A bicycle’s brakes are a vital safety feature and are operated by levers mounted on the handlebars. The levers activate caliper brakes that clamp against the metal inner wheels. When a rider applies the brakes, kinetic energy is converted into heat that slows down the wheels and stops the bicycle.
Many bike owners add pegs (sometimes called “strike plates”) to one or both of the wheel hubs for additional traction or to provide a place for the rider to stand when stopping. Child-carrying seats, a rack and pannier bags for carrying cargo are also common accessories.
A bicycle’s frame is made of tubing, with a top tube that connects to the fork and seat tubes that connect to the handlebars. The frame is connected to the rest of the bicycle by bearings in a fork and headset assembly. There are three basic types of bicycle frames: upright, curved and dropped. Upright frames, the norm until the 1970s, curve gently back toward the rider for a comfortable upright position. Curved or drop handlebars curve forward and down, offering the rider a more aerodynamic crouched position while providing better braking control from the hands. Drop handlebars are more common on road and mountain bikes.