Driveline Components and Vibration
Driveline vibration diagnosis requires matching vibration frequency to rotating components. Components include the driveshaft (tube, slip yoke, U-joints, center bearing, companion flange) and the axle. U-joints transmit torque through an angle; because a single U-joint produces velocity fluctuation, two joints are phased so the fluctuations cancel when operating angles are equal.
Key concepts: operating angle (the angle between shaft centerlines), phasing (yoke alignment), balance, and critical speed (long shafts can whip at high rpm). A center support bearing must be shimmed so both shaft sections run at acceptable angles. Slip yokes allow length change over suspension travel and must be lubricated. Recognize that a vibration appearing only at certain speeds may be a balance or critical-speed issue, while one during acceleration may be a joint or angle problem.
Traps: installing a driveshaft out of phase (yokes misaligned) causing vibration; ignoring a worn center bearing or collapsed mount; and misattributing a driveline vibration to tires. Also beware assuming a new driveshaft needs no angle check - installation angle matters.
Worked example: After clutch service, a bus vibrates at 45–60 mph. Check that the driveshaft yokes are phased correctly and that U-joint angles match. If the vibration appeared only after service, suspect phasing or a joint installed incorrectly rather than a new balance problem.
Rule of thumb: equal and opposite U-joint operating angles cancel velocity fluctuation - if vibration began after driveline work, check phasing and angles first.