Listen to a bike. Turn a bike upside down. Spin the back wheel and listen for the click. Now turn the pedals backwards: the wheel stays still. That one-way clicking part is a ratchet. Count the teeth on the front chainring and on one back cog, and divide to see how many times the wheel turns for one turn of the pedals.

Before chapter 3. Beto wants to tighten the chain. Will that stop the pedals pushing back?

The answer. No. The chain was fine. The freewheel in the back hub is a ratchet that lets the wheel run on when you stop pedalling. Its catches were glued by old grease, so the wheel dragged the pedals round. Oil freed them. Then Pinion showed that 42 teeth over 14 turns the wheel 3 times a pedal turn, which is hard uphill, while the 28 cog turns it 1.5 times.

Pause to think. Beto fixed two things in this book. What was the second one?

The grown-up layer. "Find out why before you fix" is the habit good engineers have. MachineForge has gears and ratchets to try.

For the ride home. Ask: which other things only turn one way?