Roof-mounted occupant restraint system

1 . A restraint system for helping to protect an occupant of a vehicle having a roof and a cabin with a seat for the occupant, comprising:

an airbag mounted to the vehicle roof, the airbag having a stored condition and being inflatable to a deployed condition, the airbag when deployed having a front-facing occupant receiving portion and an opposite rear-facing portion facing away from the occupant;

an inflator configured to direct inflation fluid into the airbag to inflate and pressurize the airbag;

wherein the airbag has a generally U-shaped configuration with a first leg comprising the occupant receiving portion and a second leg comprising the rear-facing portion, the first leg having a terminal end portion with a connection to the vehicle roof, wherein the airbag is configured to pivot about the connection to the vehicle roof in response to the occupant receiving portion being acted upon by an impacting occupant;

wherein a terminal end portion of the second leg is free from connections to the vehicle and configured to move into engagement with the vehicle roof in response to the airbag pivoting about the connection to cause the vehicle roof to act as a reaction surface for resisting pivoting movement of the airbag about the connection in response to the impacting occupant so that the airbag cushions the impacting occupant without relying on the rear-facing portion of the airbag engaging vehicle structure other than the roof as a reaction surface for supporting the airbag;

wherein the restraint system further comprises at least one tether that connects the end portion of the first leg to the end portion of the second leg, wherein the first and second legs are configured to move away from each other in response to pressurization and the at least one tether is configured to limit the first and second legs from moving away from each other in response to pressurization so that pressurization of the first and second legs tensions the at least one tether, which pre-loads the first and second legs against movement toward each other;

wherein the inflator is configured to deliver a volume of inflation fluid to the airbag sufficient to pressurize the first and second legs such that the pre-load resists movement of the first leg toward the second leg to a degree sufficient to absorb impact forces and provide a ride-down effect to the occupant due to the vehicle roof receiving the end of the second leg and acting as a reaction surface that resists further pivotal movement of the airbag about the connection of the first leg to the vehicle roof.

2 . The restraint system recited in claim 1 , wherein the at least one tether is configured to enforce a curved configuration of the airbag when deployed.

3 . The restraint system recited in claim 2 , wherein the curved configuration enforced by the at least one tether comprises a curved configuration of a front-facing surface of the airbag.

4 . The restraint system recited in claim 2 , wherein the at least one tether is configured to restrict movement of the end portion of the second leg away from the roof.

5 . The restraint system recited in claim 1 , wherein the restraint system is configured to use only the roof as the reaction surface.

6 . The restraint system recited in claim 1 , further comprising:

a sensor configured to sense the occurrence of an event for which deployment of the airbag is desired and to produce a signal indicative thereof; and

a controller operatively connected to the sensor and to the inflator, the controller being configured, in response to receiving the signal from the sensor, to actuate the inflator to inflate the airbag to the deployed condition.

7 . A roof-mounted airbag for helping to protect an occupant of a vehicle having a roof and a cabin with a seat for the occupant, comprising:

an inflatable U-shaped chamber having a first leg with an upper end configured to be connected to the vehicle roof, a second leg with an upper end free from connections to the vehicle and configured to engage the vehicle roof, and a third leg that extends between and connects lower ends of the first and second legs, wherein the first leg is configured to receive the occupant; and

a tether that connects the first leg to the second leg;

wherein the first leg is configured to allow the airbag to pivot with respect to the roof about a connection of the upper end of the first leg to the roof in response to the first leg being acted upon by an impacting occupant;

wherein the upper end of the second leg is configured to move into engagement with the vehicle roof to cause the vehicle roof to act as a reaction surface for resisting pivoting movement of the airbag about the connection in response to the impacting occupant;

wherein the first and second legs are configured to move away from each other in response to pressurization of the U-shaped chamber;

wherein the tether is configured to limit the first and second legs from moving away from each other in response to pressurization so that pressurization of the U-shaped chamber tensions the tether, which allows the U-shaped chamber to be pressurized in order to pre-load the first and second legs against movement toward each other.