Bicycle Parts That Turn the Wheel

When you push down on the pedal, the crank turns the big toothed ring at the front. The chain carries that turning force back to the small cog on the rear wheel, so the wheel spins and the bike moves.

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In the picture

Pedal — your foot pushes down here, and the crank arm carries it around in a circle.
Chainring — the large toothed ring at the front that the crank turns.
Chain — the loop of steel links that carries the force from the front to the back.
Rear cog — the small toothed wheel that the chain pulls around.
Rear hub — the centre of the back wheel, which the cog turns along with it.

Examples

Standing up on the pedals to climb a steep hill makes the chain pull harder on the rear cog.
When the chain slips off the chainring, the pedals spin freely and the bike stays still.

Key takeaways

Force travels pedal to chainring to chain to rear cog to wheel.
A big chainring with a small rear cog makes the wheel turn more times per pedal stroke.