The transmission of power from the connecting rod to the crankshaft is most effective when the angle between the connecting rod and the crank-web is at what value?

• A. 0 degrees
• B. 90 degrees
• C. 180 degrees
• D. 45 degrees

Answer: (B)

Torque about the crankshaft is maximized when the angle between the connecting rod and the crank radius is 90 degrees. The force transmitted through the connecting rod has a tangential component that actually drives the crank to rotate. This tangential component is proportional to sin(theta), where theta is that angle. At 90 degrees, the tangential component is at its maximum, so the crank receives the greatest turning moment for a given piston force. At 0 or 180 degrees, the force is aligned with the crank center and contributes little to rotation, while at intermediate angles only part of the force contributes to turning the crank. Thus, 90 degrees yields the most effective power transfer.