Section I Risk Factors For Torsional Vibration

The following vehicle configurations and/or usage patterns singularly or in combination have been identified as susceptible to or the cause of excessive torsional vibrations if a soft damped clutch is not incorporated in the drivetrain. The dynamic pulses from engines or the torsional oscillations from drivelines make these vehicles prone to component failure and/or driver discomfort (experienced as noise and vibration).

Readers of this RP should assess their torsional vibration risk by reviewing the risk factors listed in Section I. If the reader finds that one or more of these factors apply to their fleet, he or she should consider following the recommendations for corrective actions offered in Section III.  The more risk factors in Section I that the reader can associate with his or her fleet, the more susceptible the reader's vehicles are to the negative effects caused by torsional vibration. Conversely, if the corrective actions listed in Section III are taken, (i.e., if the vehicle is configured and operated properly) the risk associated with certain vehicle configurations and usage patterns is either reduced substantially, or in most cases eliminated. The risk factors are:

1. Wheelbase Length of Less Than 150 Inches -  Engine firing pulses excite torsional resonances in these vehicles at or slightly below peak torque speeds (1200 RPM). Excessive noise and vibration and destructive dynamic torque can cause premature failures of the clutches, transmission shafts, transmission synchronizers (manual transmission only) and transmission gears. Extra driveline inertia in longer wheelbase vehicles reduces risk.
2. 4x2 Vehicle Configuration -  As in short wheelbase trucks, torsional resonances occur at engine speeds near peak torque in 4x2 vehicle configurations. Operation near these speeds almost certainly will cause noise, vibration, and component durability problems. 6x4 configurations have less risk of resonance-related problems because of power divider inertia.
3. Direct Drive and Overdrive Transmissions -  Direct drive transmissions are more prone to engine firing excited resonance than overdrives. Overdrive transmissions are more susceptible to U-joint excited vibrations than directs. Individual vehicle configuration and use habits can influence which of these power trains is at greater risk to vibration damage.
4. High Torque Engines -  Engines with 5-9 liter displacements, up to 330 horsepower, and/or 1050 Ib.-ft. of torque have firing pulses that can be highly destructive to drive train components if resonance is excited. In fact, their very size and power encourage the driver to operate the vehicle in the engine's sensitive low speed range, in which they get the best fuel economy. In addition, these engines have pulsations at half-firing frequency that can excite torsional resonances within the highly used speed range of the engine, particularly in transmission ratios around 1.35. All these factors raise driver discomfort and component durability issues.
5. Use of Four-Cylinder Engines  in conjunction with automatic transmission. These engines have firing pulses that excite destructive resonances 50 percent higher in the engine speed range than do six-cylinder engines. Some automatic transmissions have specific shift calibration requirements with Four-Cylinder Engines. Contact your automatic transmission supplier for more details.
6. Up shifting at 1500 RPM or Less In High Range -  Normal up shifting can put the engine at speeds that jeopardize drive train durability due to resonance excitation. If a torsional resonance is excited, damage can accumulate at a rate of nearly 50 times per second. Transmissions with ratio steps of less than 20 percent incur risk of this phenomenon at 1350 RPM and below.
7. Operating Engine at Less Than 1200 RPM -  Modern engines - because of their high torque, improved low-speed performance, and their inherent low-speed fuel economy - are prone to usage at or below peak torque speed. If this is the case, torsional resonance can be excited by engine firing pulses that can be at more than twice the rated input torque of the transmission. This situation is very destructive to drive train components and may be uncomfortable to the driver.
8. U-Joint Working Angles Not Cancelled -  If driveline-working angles are not canceled, non-uniform motion is created in the components, which generates vibration and dynamic torque. Driveshafts with joint working angles above six degrees and not cancelled within two degrees generate potential durability, noise and vibration problems.
9. Air Suspension Ride Height Varied From OEM Specification -  Improper driveline angularity and the destructive torsional effect that it creates are very sensitive to air suspension ride height. Because ride height is relatively easy to adjust (and there are many perceived benefits of adjusting it), ride height is often changed in the field from the OEM specification. For every inch of ride height change, there could be up to four degrees of angularity mismatch induced in the driveline, which negatively influences component durability and potentially driver comfort.