The sprocket ratio for a chain drive system having a driven sprocket with 54 teeth and a driver sprocket with 18 teeth is __________.

• A. 3
• B. 1/3
• C. 6
• D. 2

Answer: (A)

In a chain drive, the speed relationship between sprockets is set by their number of teeth because the chain moves the same length on each sprocket per revolution. The sprocket with more teeth must rotate more slowly to take up the same chain length.

Here, the driven sprocket has 54 teeth and the driver has 18 teeth. The ratio of driven to driver teeth is 54 divided by 18, which equals 3. So the sprocket ratio is 3:1. This means the driver must turn three times for the driven to complete one revolution, and the driven runs at one-third the speed of the driver. Torque on the driven increases by roughly that same factor (ignoring losses).