The Frame Distributes the Weight
An ingenious design, the bicycle frame consists of hollow steel, metal alloys or carbon-fiber composite tubes fashioned into two triangular shapes that distribute the weight of the rider between the front and back wheels. The seat of the bicycle is situated towards the back wheel, allowing you to lean forward to grasp the handlebars. The handlebars attach to a shaft that sits over a part of the frame secured to the axle on the front wheel that allows you to turn it in the direction you want to go.
The Wheel Machine
The bicycle wheel, the simplest of machines, revolves around an axle secured to the frame that allows it to turn, propelling the rider forward when pedaling. The size of the bicycle wheel or the gear chosen reduces or increases the pedaling needed to move the bike. Bicycle wheels typically have a larger circumference than those found on cars -- at least 20 inches across. The taller the wheels, the greater the speed when the wheels turn on their axle via pedal power. The wheels, buttressed by interior spokes, support the weight of the rider. Mounted on the wheels are the rubber tires and inner tubes filled with pressurized air. The pneumatic tires also help to cushion the ride.
Gears Multiply Speed
Bicycles have from one to 30 gears -- little sprockets with teeth attached to the back wheel of the bicycle -- linked by a chain to the crank set powered by the pedals. The multiple, different-sized gears on the back axle -- changed by a shifting lever on the handlebars or elsewhere on the bike and aligned by a derailleur -- allow the rider to go faster on straight roadways and switch to other gears to pedal uphill. When gears are shifted, the derailleur realigns its position over the different gear or sprocket, which forces the chain to move to that sprocket while changing the pedaling speed. The larger the sprocket, the harder the pedaling, the smaller sprockets require easier, but faster pedaling. The pedals attached to the crank sit opposite each other, which helps to multiply the force your legs put out when powering the bike.
Friction Braking Action
Friction keeps the bicycle moving forward on the road as it grips the pavement, and friction also stops the bike when needed. To stop a bicycle with friction involves squeezing a lever near the handle to activate the braking system. The lever pulls a cable taut when the lever is squeezed, which closes the brake caliper which presses rubber brake pads onto the wheel rim. Older single-gear bicycles required the rider to push the pedals backward to stop the bike. Not all bicycles are made the same, employing different frames, wheels, gears and tires with different treads for use while mountain biking, racing or touring.