Seatback and vehicle seat

1 . A seatback configured to support a back of an occupant, the seatback comprising:

a pressure-receiving member including:

a first pressure-receiving portion configured to support a lumbar region of the occupant, and

a second pressure-receiving portion joined to an upper part of the first pressure-receiving portion and configured to support the back of the occupant in cooperation with the first pressure-receiving portion; and

a drive mechanism configured to displace the pressure-receiving member between a support position that is on the occupant's side and a non-support position that is rearward of the support position, and

a support member that rotatably supports a pressure-receiving plate at a first end thereof in an up-down direction,

wherein the pressure-receiving member is configured, when in the support position, to be bent at a joining portion where the first pressure-receiving portion and the second pressure-receiving portion are joined together to become convex toward a seat front side,

wherein the pressure-receiving member comprises the pressure-receiving plate in which the first pressure-receiving portion, the second pressure-receiving portion, and the joining portion are integrated in one piece,

wherein the joining portion has a lower flexural rigidity than the first pressure-receiving portion and the second pressure-receiving portion and is elastically deformable,

wherein, when a part of the pressure-receiving plate that is rotatably supported by the support member is referred to as a rotation center, and when a second end, in the up-down direction, of the pressure-receiving plate is referred to as a rotation end, and when a distance from the rotation center to the rotation end is referred to as a pressure-receiving plate length, the rotation end when the pressure-receiving plate is in the support position is located on the occupant's side relative to the rotation end when the pressure-receiving plate is in the non-support position, and

wherein the pressure-receiving plate length when the pressure-receiving plate is in the support position is smaller than the pressure-receiving plate length when the pressure-receiving plate is in the non-support position.

2 . The seatback according to claim 1 ,

wherein the drive mechanism comprises:

a rotation arm, a first end of which is rotatably coupled to the pressure-receiving plate and a second end of which is rotatably coupled to the support member, the first end being coupled at a position, in the pressure-receiving plate, closer to the rotation end than to the joining portion; and

an actuator configured to cause the rotation arm to pivotally rotate about a coupling part to the support member,

wherein, when a distance from a coupling part where the rotation arm is coupled to the pressure-receiving plate to the rotation center is referred to as an arm end radius, and when a distance from the coupling part where the rotation arm is coupled to the support member to the rotation center is referred to as a center end radius, a difference between the arm end radius when the pressure-receiving plate is in the support position and the center end radius is smaller than a difference between the arm end radius when the pressure-receiving plate is in the non-support position and the center end radius.

3 . The seatback according to claim 2 , further comprising:

a bending regulator configured, while the pressure-receiving plate is being displaced from the non-support position to the support position, to cause the pressure-receiving plate to be bent so as to become convex toward the seat front side.

4 . A vehicle seat comprising a seatback according to claim 1 .