The following characteristic curves are of importance in both development and production control of friction clutches:

• contact force and travel 
• release force and travel 
• disengagement stroke: clutch pressure plate 
• parallelism of pressure plate to clutch plate

For release testing the compression die is pressed against the inner diameter of the diaphragm spring and the force/travel curve recorded.  With contact testing a compression die is pressed directly against the clutch pressure plate and the spring characteristic curve of the diaphragm spring between clutch pressure plate and clutch cover is measured. 

These tests are performed in new and defined used condition. The characteristic curves for disengagement force/travel and contact force/travel are important criteria for comfortable operation  and for reliable torque transfer.