How a Bicycle Functions
As you pedal a bicycle, the crank arms rotate in a circular motion. A chainring is attached to the rightmost crank arm. As the chainring spins, the teeth of the chain ring grasp the chain and pull it forward.
Insert picture of crankset and chainring pulling on the chain
In the rear of the bicycle, there is a cog with teeth. This cog is attached to the wheel, and the chain is wrapped around the cog. When the chain is pulled forward, the cog spins, and thus, the wheel spins. The bicycle is now moving forward.
Insert picture of chain pulling cog and wheel
Notice how power is converted into motion. The cyclist pushes on the pedals. The crankset and chainring rotate. The chainring teeth pull on the chain. The chain pulls on the teeth of the rear cog. The wheel spins.
Alternatively, the circular motion of the crankset and chainring is transformed into the forward pull of the chain. The forward pull of the chain is transformed into the circular motion of the cog and wheel.
Insert picture of drive train in action
As you speed down the busy street, you come to a busy intersection. It is time to stop. To do this, you squeeze the brake lever. When you squeeze the brake lever, it pulls on the brake cable.
Insert picture of brake lever pulling on the brake cable
This cable is attached to the brake, and the pulling action causes the brake arms to come together and squeeze the wheel rim with the brake pads.
Insert picture of cable pulling the brake arms together
Notice that the brake cable runs through cable housing in various places. The housing constrains the motion of the brake cable to a specific path. This has the effect of controlling precisely the amount that the brake arms come together, as well as ensuring that cable moves smoothly.
Insert narrative explanation of shifting action