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Clutch drag is caused by friction between clutch parts and adjacent components having different rotational speeds when the clutch is released. The root cause of drag is the presence of normal reactions between friction surfaces when the clutch is disengaged. In most cases, especially in multiple disc clutches, these reactions cannot be eliminated but they can and should be minimized by proper design. The major difficulty in dealing with drag problems is associated with the relatively small magnitudes of drag torque in comparison to useful torques carried by the drivetrain. From the history of drag problems, especially in the automotive industry, it appears obvious that the problem of dragging clutches has no single solution. There are many potential sources of friction forces contributing to drag torque. In addition, contributions from particular sources are very small and not necessarily consistent.

Recent technological advances in electronics have strongly affected the medium and heavy duty truck industries. Modern truck engines supply much higher torques over a wider range of rotational speeds than their predecessors of just a few years ago. Drivetrains with “new generation” engines and “old generation” clutches most often have resonances that are spread over the entire operating range. Consequently, these powertrains experience large torsional oscillations accompanied by considerable dynamic overloads. This implies a special responsibility binding clutch manufacturers. New clutches must not only have higher torque capacities, much lower damper rates and longer working travels, but they also must be packaged in the very limited environment allowed by the old designs. Newly redesigned LTD discs produced by the Meritor Clutch Company meet these new demands. As a modular product the LTD disc has been designed to comply with today's rigorous dynamic requirements.

Ceramic, twin disc, manual clutches, that are common in the North American heavy duty truck market, require frequent adjustments in order to function properly and perform according to their specifications. Wear on disc facings and friction surfaces of the flywheel, intermediate and pressure plates causes changes in the thickness of these components, resulting in a change of the release bearing position. As wear increases, the driver experiences a decrease and finally a complete loss of free play in the clutch pedal. The clutch is then out of adjustment. By manually adjusting the clutch, its efficiency can be restored, but this maintenance activity incurs labor costs and causes downtime losses. A self-adjusting clutch is a clutch containing a mechanical, automatic control system that measures instantaneous wear and then compensates the release bearing position. In this way the clutch is always in proper adjustment.