Side airbag assembly

1. An assembly comprising:

a seatback frame having a pair of upright frame members;

each of the upright frame members having an inward side and an outward side, the inward sides spaced from each other and facing each other, and the outward sides facing away from each other;

a cross-member extending from one of the upright frame members to the other of the upright frame members; and

an airbag having a first connection fixed to the outward side, a second connection fixed to the inward side of one of the upright frame members, and a third connection fixed to the inward side of the one of the upright frame members;

the airbag having a first chamber connected to the first connection, a second chamber extending from the first chamber to the second connection, and a third chamber extending from the first chamber to the third connection.

2. The assembly of claim 1 , wherein the second connection is a tether extending from the second chamber to the inward side of the one of the upright frame members and the third connection is another tether extending from the third chamber to the inward side of the one of the upright frame members.

3. The assembly of claim 1 , wherein the first chamber is elongated along the one of the upright frame members.

4. The assembly of claim 3 , wherein the second chamber and the third chamber are elongated from the first chamber toward the inward side of the one of the upright frame members.

5. The assembly of claim 4 , wherein the second chamber and the third chamber are fluidly connected to the first chamber.

6. The assembly of claim 1 , further comprising a baffle between the first chamber and the second chamber and another baffle between the first chamber and the third chamber.

7. The assembly of claim 1 , wherein the second chamber and the third chamber are vertically spaced from each another.

8. The assembly of claim 7 , wherein the second chamber and the third chamber are elongated from the first chamber toward the inward side of the one of the upright frame members.

9. The assembly of claim 1 , wherein the first chamber is elongated along the one of the upright frame members and the second chamber is elongated from the first chamber toward the inward side of the one of the upright frame members.

10. The assembly of claim 1 , further comprising a covering, the airbag being between the covering and the seatback frame.

11. The assembly of claim 1 , wherein the cross-member extends from the inward side of the one of the upright frame members to the inward side of the other upright frame member.

12. The assembly of claim 1 , further comprising a seat bottom supporting the seatback frame, the seat bottom including a rotatable support rotatable between a vehicle-forward position and a vehicle-rearward position.

13. The assembly of claim 12 , further comprising a door, the upright frame member being between the airbag and the door when the airbag is inflated and the seat bottom is in the vehicle-rearward position.

14. The assembly of claim 1 , further comprising a computer having a processor and memory storing instructions executable by the processor to inflate the airbag in response to at least a detection that the seatback frame is in a vehicle-rearward position and detection of a sensed vehicle impact.

15. The assembly of claim 14 , further comprising a door, the upright frame member being between the airbag and the door when the airbag is inflated and the seatback frame is in the vehicle-rearward position.

16. The assembly of claim 1 , further comprising a second airbag fixed to the other upright frame member.

17. The assembly of claim 16 , wherein the other upright frame member includes an outward side and an inward side, and the second airbag has a first connection fixed to the outward side of the other upright frame member and a second connection fixed to the inward side of the other upright frame member.