Drive Shafts and Wheel Bearings

Driveshaft Angles, Phasing, and Vibration Analysis

On a conventional rear-drive vehicle whose propeller shaft drives the differential pinion directly, shaft speed during straight-line travel is approximately wheel speed multiplied by the final-drive ratio; use the actual drivetrain layout and gear ratios for other designs. In comparison, a drive axle shaft generally turns at wheel speed. Driveshaft imbalance produces a vibration at driveshaft rotational speed, which is typically felt through the floor or seat and becomes most noticeable at certain vehicle speeds. Imbalance can be diagnosed through road testing, electronic vibration analysis, or checking for lost factory balance weights.

A universal joint's operating angle is the angle between the centerlines of the two shafts it connects. A single Cardan joint operating at an angle produces rotational-speed variation twice per revolution. In a conventional two-joint shaft, the second joint cancels the first joint's speed variation when the joints are correctly phased and their operating angles are equal and in the proper plane; verify the required geometry against manufacturer service information. On a conventional two-Cardan-joint shaft, the yokes at opposite ends of the same shaft section must have the phasing specified by the manufacturer, commonly with their ears aligned in the same plane. Incorrect driveshaft phasing causes a torsional vibration at twice driveshaft rotational frequency, which technicians diagnose by checking phasing and operating angles against service information.

Measure driveshaft working angles with an inclinometer or digital angle gauge with the vehicle at the ride height and loading condition specified in the manufacturer's service information. For a conventional two-single-Cardan-joint driveshaft, the opposing operating angles should be closely matched in the arrangement specified by the manufacturer, using its limits rather than assuming a universal tolerance. Rear axle pinion angles can be adjusted using tapered spring shims, adjustable suspension links, or eccentric cam bolts according to manufacturer procedures.

Both excessive and insufficient operating angles cause driveline problems. An excessively small universal-joint operating angle can limit needle-bearing movement and lubricant distribution, potentially shortening joint life. Conversely, excessive universal joint operating angles cause rapid joint wear, bearing overheating, and severe torsional vibration. Manufacturer service information is the final authority for working angle limits and balance limits.