Imagine you no longer have a spangly bike with dozens of gears. Instead, you have a model with only two chainrings at the front, which you turn by pedalling, and two sprockets on the back, which push the back wheel around when they are turned. This leaves you with a 10-tooth and 20-tooth sprocket at the back, and a 20-tooth and a 40-tooth sprocket at the front. These combinations aren’t useful for cycling, but they make the math easier.
Start with your chain running between the 40 at the front and the 10 at the back. Begin with one pedal crank pointing upwards, in line with the seat-tube. Turn the cranks round exactly once. Each link of the chain gets picked up in the valley between two teeth. Since there…
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Bike Maintenance Tips, Tricks & Techniques (Digital) - 1 Issue, Bike Maintenance Tips, Tricks & Techniques 3rd Edition