A ring binder mechanism includes a housing and a ring support supported by the housing for movement relative thereto. Each ring of a plurality of rings includes first and second ring members. The first ring member is mounted on the ring support for movement with the ring support relative to the housing between a closed position and an opened position. An actuator is mounted for pivotal movement relative to the housing for moving the rings from their closed position to their opened position. The actuator includes a lower arm having a contact surface engageable with a lower surface of the ring support when the rings are in their opened position and out of engagement with the ring support when the rings are in their closed position. The contact surface defines an angle between its outer surface and a horizontal plane between about 16 degrees and about 55 degrees.