If a chain drive uses a 24-tooth driver and a 72-tooth driven, what is the sprocket ratio?

• A. 3
• B. 1/3
• C. 24/72
• D. 72/24

Answer: (A)

In a chain drive, the sprocket ratio comes from how many teeth each sprocket has. The driven sprocket has 72 teeth and the driver has 24 teeth, so the ratio is 72 divided by 24, which equals 3. This is a 3:1 reduction, meaning the driver must turn three times for the driven to complete one turn. Physically, the output rotates more slowly (one-third the input speed) but with about three times the torque (ignoring losses). The other way of looking at it would give the inverse (1/3), but the standard way this question uses the ratio is driven over driver, yielding 3.