Bicycle gears
Change the front and rear gears of a bicycle and see how far one turn of the pedals takes you, and how fast you go at the same pedalling. Find the easy gear for a hill and the fast gear for a flat road.
Core concept
Gear ratio = front teeth ÷ rear teeth: the back wheel turns that many times for each turn of the pedals, so a big ratio goes far and fast but is hard to push, and a small ratio is easy to push but slow
Topic: Science around us
A bicycle chain joins a big gear at the pedals to a small gear on the back wheel. Each gear has teeth, and the chain moves the same number of teeth on both. If the front gear has 42 teeth and the rear gear 16, one turn of the pedals pulls 42 teeth of chain, which turns the rear gear 42 ÷ 16 = 2.63 times. That number is the gear ratio. The back wheel turns with the rear gear, so with a wheel 0.7 m across (about 2.2 m round), one turn of the pedals carries you 2.63 × 2.2 = 5.78 m. A big ratio takes you far for each push but needs more force: A push or a pull. Forces can start, stop or turn things. All key words; a small ratio needs less force but you must pedal more turns to go the same distance. That is why riders change to a small ratio for a hill.
What you will learn
- Work out a gear ratio from the numbers of teeth.
- Find the distance for one turn of the pedals and the speed at steady pedalling.
- Explain why a small gear ratio is easier for climbing a hill.
Before you start
- Know the circumference of a circle: π × diameter.
- Know speed = distance ÷ time.
Materials
- A bicycle with gears, if there is one at school or at home
Equipment
- None
Procedure
- Record the starting gears: 42 teeth at the front and 16 at the back.
- Change to the biggest rear gear (28 teeth) and record: the hill gear.
- Change to the biggest front gear (52) and the smallest rear gear (12) and record: the fast gear.
How this simulation is simplified
- The wheel is 0.7 m across, so it travels π × 0.7 ≈ 2.2 m each turn.
- The rider pedals a steady 60 turns a minute; the chain does not slip.