Rocket cuts two paper gears. One is big. One is small.
He links them with a paper strip and turns the big one once.
"Whoa! The little one went around twice!"
Raven leans in. "What do you notice about the bumps on each gear?"
Nova blinks. "Count the teeth on each one," she says. "Then compare."
Gears have teeth. The chain fits over the teeth.
Here is a real example from a museum scientist.
His front chain ring had 54 teeth. His back cog had 27 teeth.
One pedal turn moves the chain 54 teeth.
The 27-tooth cog needs 27 teeth to go around once.
So 54 teeth of chain turn it around twice.
The back wheel turns twice for one pedal turn.
| Front teeth | Back teeth | Back wheel turns per pedal turn |
|---|---|---|
| 54 | 27 | 2 |
The rule: front teeth divided by back teeth.
That tells how many times the back wheel turns for one pedal turn.
These are made-up practice numbers. Use the rule for each one.
| Made-up gear set | Front teeth | Back teeth |
|---|---|---|
| Set A | 40 | 20 |
| Set B | 30 | 10 |
| Set C | 48 | 12 |
| Statement | True or false? |
|---|---|
| The chain fits over the teeth of the gears. | ? |
| A 27-tooth cog turns once when the chain moves 27 teeth. | ? |
| The rule is back teeth divided by front teeth. | ? |
| With 54 teeth in front, an 11-tooth cog turns fewer times than a 27-tooth cog. | ? |
Terrific tooth counting! Tomorrow we build a model chain drive with no bike at all.