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. 2
• C. 1
• D. 0.5

Answer: (A)

In a chain drive, the sprocket ratio is determined by how many teeth each sprocket has because that tells you how far the chain moves per revolution and how many driver revolutions are needed to turn the driven sprocket once. The ratio equals driven teeth divided by driver teeth.

With a driven sprocket of 54 teeth and a driver sprocket of 18 teeth, the ratio is 54 / 18 = 3. That means three turns of the driver are required to rotate the driven sprocket once. The system therefore provides a speed reduction of 3:1 (the driven turns at one-third the driver speed) and, ignoring losses, increases torque by about the same factor.