Lock-up piston in a torque converter

A lock-up clutch piston for a lock-up clutch of a torque converter disposed in the torque converter between a front cover and a turbine of the torque converter, said lock-up clutch piston being adapted to move in axial directions in response to changes in hydraulic fluid pressure, said lock-up clutch piston comprising:

• a disc-shaped member having a central disc-shaped plate portion;
• a flat annular portion formed on an outer periphery of said disc-shaped plate, said flat annular portion being adapted to make frictional contact with the front cover;
• said disc-shaped member further including an annular rim portion formed at an outer peripheral edge of said flat annular portion, said annular rim portion extending axially towards the turbine for providing radial outer support for torsion springs of the lock-up clutch;
• said flat annular portion having a lower rigidity than said disc-shaped plate portion;
• said flat annular portion having a thickness that is thinner than a thickness of said disc-shaped plate portion;
• a thickness of said annular rim portion being thinner than the thickness of said disc-shaped plate portion;
• and
• the thickness of said annular rim portion being in the range of 50 to 80% of the thickness of said disc-shaped plate portion.