A driveshaft exhibits vibration that occurs only at certain speeds under normal loads. The most likely cause is

• A. Driveshaft too heavy for the engine-transmission mounting system
• B. Improper phasing
• C. The driveshaft is bent or inadequately balanced
• D. One or both of the U-joints is excessively worn

Answer: (B)

When vibration shows up only at certain speeds, it’s usually a timing issue between parts rather than a simple balance problem. In a two-piece driveshaft, the two U-joints must be phased so that their cyclic variations cancel each other as the shaft spins. If the joints aren’t aligned correctly, those speed-dependent variations add up instead of canceling, producing vibration at specific RPMs. That’s why improper phasing is the most likely cause.

If the shaft were simply heavy or bent, or if a U-joint were worn, the vibration would tend to appear over a broader range of speeds or come with other symptoms like clunking, rather than appearing only at particular speeds.