1What a gear really is
Teeth that march past each other
A gear is just a wheel of teeth. Where two gears touch, their teeth lock and pass by one-for-one — so the same number of teeth has to go by on both. That one rule decides who spins faster.
Teeth count
Big gear = lots of teeth. Small gear = few teeth. To send the same teeth through, the small one must spin around more times.
Locked together
Their teeth march in step. One tooth passes on this gear, one tooth passes on that gear — they can never slip past each other.
2The two ways to pair them
Speed-up pair vs strength-up pair
Same two gears, but which one you crank flips the result. Watch them turn at their own steady speeds:
Crank the big gear, read the small one. The small gear whirls faster than your hand.
Crank the small gear, read the big one. The big gear turns slower than your hand.
3Your turn — be the crank
Crank the amber gear and resize the teal one
You always crank the amber gear at one steady speed. Drag the slider to grow or shrink the teal driven gear, and watch its spin-speed change as the teeth stay locked.
4Now try to cheat the trade
Make it spin 3× faster — does it still push hard? 🏋️
Shrink the teal gear so it spins about 3 times faster. Then hang a weight on its axle and see how high it can lift it.
Guess before you run it
You shrink the teal gear so it spins 3× faster. What happens to how hard it can push — the weight it can lift?
5So which pairing wins?
Neither — each one trades something away
A tiny driven gear whirls around quickly — great when you want lots of spin.
A big driven gear pushes hard — enough to climb a hill or haul a weight up.
Gears don't make power from nothing. They swap speed for strength and back again — and speed times strength always comes out the same. That's the deal inside every bike gear and clock.
Psst, grown-ups: meshing gears share the same tooth speed at the contact point, so their angular speeds are inversely proportional to their tooth counts — that ratio (N₂/N₁) is the gear ratio. Ignoring friction, an ideal gear train conserves power: torque rises by the same factor speed falls (τ₁ω₁ = τ₂ω₂), so torque scales with the ratio while output power equals input power. A gear train is a rotational lever, and the speed-for-torque trade is just conservation of energy — the same rule behind every lever and pulley. Real gears lose a little to friction, so output power is always slightly less than input, never more.