When there’s no angle to the U-joint, the drive shaft, the output yoke, and the U-joint all move together at a constant speed. An operating angle at the front end of the drive shaft won’t allow for this to move in a perfect circle any longer. Instead, you have an ellipse that forces the drive shaft to slow down and speed up in order to keep pace with the output yoke.
When the output of the U-joint changes speed, the drive shaft also changes speed. This is why it’s so important to make sure that the operating angles at the front yoke and rear yoke are parallel with one another. This way, you have rotation at a constant speed for the driven member of the drive shaft.
Drive shafts that aren’t in phase won’t rotate at a constant speed. The torsion-and-inertial vibration also impacts how your drive shaft works, if you don’t have proper phasing within the components of your drive shaft system. You are recommended to mark each of the parts before you plan on putting this back together so that you maintain the correct phasing and decrease your chances of having driveline vibration.