Which statement is true about driveshafts with respect to critical speed?

• A. Longer driveshaft has lower critical speed.
• B. Shorter driveshaft has lower critical speed.
• C. Critical speed is unaffected by length.
• D. Overdrive ratio changes critical speed.

Answer: (A)

Critical speed is the rotational speed at which the driveshaft’s lateral bending mode resonates, causing large vibrations. For a uniform shaft, making it longer makes it more flexible, which lowers the natural bending frequency. Since the critical speed corresponds to that natural frequency, a longer driveshaft reaches resonance at a lower rotational speed. In other words, a longer driveshaft has a lower critical speed, while a shorter one has a higher critical speed. Gear ratios or overdrive don’t change the shaft’s inherent stiffness, so they don’t alter the critical speed itself; they only shift the engine RPM at which you encounter that speed.