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Bike Shop 3-5 / Week 05 / Bike Shop Day
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Week 05 · Gears and the Chain

Bike Shop Day

Bike Shop Day: Tooth Count
// Counting teeth to predict turns
⏱ about 15 min

Bike Shop Day: Tooth Count

Rocket holds a pencil like a pointer. "No fingers near the chain today!"

Raven grabs the shop notebook and a strip of tape.

"What do you notice?" she asks. "Which gear has more teeth?"

Nova hovers by the back wheel. "Put the tape on the tire," she says. "It will help you count turns."

The family gets ready.

  1. The chain drive connects the pedals to the back wheel.
  2. Pedals turn the chain ring, the chain turns the cog, and the wheel turns.
  3. Front teeth divided by back teeth tells the back wheel turns per pedal turn.
  4. Easy gears help on hills. Other gears go farther on flat ground.
  5. Hands stay off the chain. Wash hands if you touch it.
◇ COUNT TEETH AND TURNS
A grown-up holds the bike still. Choose paper gears if there is no bike.
Point with a pencil and count the teeth on the front chain ring.
Count the teeth on the back cog the chain is on.
Write both numbers in your shop notebook.
Put a strip of tape on the top of the back tire.
Predict how many times the tape will go around for one pedal turn.
A grown-up lifts the back wheel and turns the pedals one full turn.
Count the tape's turns from a safe spot. Compare with your prediction.
Safety first
The bike stands still while you count teeth.
A grown-up turns the pedals. Your hands stay far from the chain, gears and spokes.
Point with a pencil, not a finger.
Wash your hands if you touch the chain.
For a grown-up
Lift the back wheel off the ground before turning the pedals.
The back wheel may keep spinning. Slow it gently before counting again.
Paper gears work just as well if there is no bike.
What we countedOur number
Front teeth
Back teeth
Predicted wheel turns
Counted wheel turns
SHOP DAY CHECK
  • Read the question.
  • Tap your answer.
Why use a pencil to count teeth?
A made-up bike has 36 front teeth and 12 back teeth. How many wheel turns per pedal turn?

Great counting, crew! Next week we use wheels to find patterns of motion and make predictions.

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