Driveline Geometry 101 - Continued
General Application

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THE KINKS

When perfect cancellation is not achieved, the regular motion of the transmission output shaft is still translated into acceleration/deceleration cycles in the driveshaft but it is not translated back to regular motion at the second U-joint. In this case, the alternating cycles of acceleration /deceleration are first taken up in any free-play that exists in the driveline. The resulting noise and vibration you will likely experience under these conditions are the feel and sound of bearings being hammered to death. Although this action is most often first seen in the premature failure of U-joints, pinion and transmission output shaft bearings, gears and splines take a beating as well.

In the case where the free-play isn't absorbing all of the hammering, components not designed to flex begin to do so anyway. In many cases the driveshaft literally begins to twist and untwist (torsion). This is quite easily felt and heard by anyone inside the truck, and is obviously very destructive. The only prevention or cure is to correct the driveline geometry to achieve cancellation.

PRE-LOADING THE PINION

The one major variable affecting driveline geometry is the tendency for the pinion to rotate up under vehicle acceleration and down under deceleration. Although all trucks have some pinion movement, the more weight that is carried, torque applied, and built-in suspension flex is present, the more the pinion will rotate. To compensate for pinion rotation when pulling loads for long distances or under constant heavy acceleration you may want to rotate the pinion downward so that under normal driving conditions (whatever they are) the pinion will rotate into a position resulting in good cancellation.

Given that the condition of the suspension components and the amount of torque being applied will vary, all over the place from truck to truck, there is no hard and fast rule to how much pinion pre-load is correct. However, most chassis and suspension people I've run across believe that no more than three additional degrees of pinion pre-load are advisable. For high stress, momentary or short-term pinion rotation beyond this, a pinion snubber is a good way to limit rotation, prevent banging, and preserve cancellation. CTS

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