Airbag device

1. An airbag device that is attachable to a steering wheel comprising a wheel hub and an annular wheel provided around the wheel hub, the airbag device comprising:

an inflator that generates gas;

an airbag that is inflatable by the gas and comprises a front panel, which comprises at least one fabric and is capable of restraining an occupant upon inflation under emergency of a vehicle, and a back panel, which comprises at least one fabric and is provided at a side closer to the wheel hub than the front panel; and

a retainer member that is secured to the wheel hub at a state where the inflator and the airbag are secured,

wherein in the airbag having undergone spontaneous inflation without being attached to the steering wheel,

when a size between a securing position of the airbag to the retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, the most protruding portion by the back panel is located at a position spaced from the securing position by 0.5 or greater,

a diameter of the inflated airbag of an opposing portion to an inner surface of the wheel is 0.6 to 1.2 times as large as an inner diameter of the wheel, and

a diameter of the inflated airbag of a joint portion of the front panel and the back panel is greater than the diameter of the inflated airbag of the opposing portion to the inner surface of the wheel, and

an inclination angle of the back panel is set to be substantially uniform in an entire surrounding thereof.

2. The airbag device according to claim 1 , wherein in the airbag having undergone spontaneous inflation without being attached to the steering wheel,

when the size between the securing position of the airbag to the retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, a gravity center position of the airbag is located at a position spaced from the securing position by 0.5 or greater.

3. An airbag device that is attachable to a steering wheel comprising a wheel hub and an annular wheel provided around the wheel hub, the airbag device comprising:

an inflator that generates gas;

an airbag that is inflatable by the gas and comprises a front panel, which comprises at least one fabric and is capable of restraining an occupant upon inflation under emergency of a vehicle, and a back panel, which comprises at least one fabric and is provided at a side closer to the wheel hub than the front panel; and

a retainer member that is secured to the wheel hub at a state where the inflator and the airbag are secured,

wherein in the airbag having undergone spontaneous inflation without being attached to the steering wheel,

when a size between a securing position of the airbag to the retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, the most protruding portion by the back panel is located at a position spaced from the securing position by 0.5 or greater,

a diameter of the inflated airbag of an opposing portion to an inner surface of the wheel is 0.6 to 1.2 times as large as an inner diameter of the wheel,

a diameter of the inflated airbag of a joint portion of the front panel and the back panel is greater than the diameter of the inflated airbag of the opposing portion to the inner surface of the wheel,

wherein in the airbag having undergone spontaneous inflation without being attached to the steering wheel,

when the size between the securing position of the airbag to the retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, a gravity center position of the airbag is located at a position spaced from the securing position by 0.5 or greater, and

wherein mass of the at least one fabric per unit area, which is to be included in the front panel, is greater than mass of the at least one fabric per unit area, which is to be included in the back panel.

4. An airbag device that is attachable to a steering wheel comprising a wheel hub and an annular wheel provided around the wheel hub, the airbag device comprising:

an inflator that generates gas;

an airbag that is inflatable by the gas and comprises a front panel, which comprises at least one fabric and is capable of restraining an occupant upon inflation under emergency of a vehicle, and a back panel, which comprises at least one fabric and is provided at a side closer to the wheel hub than the front panel; and

a retainer member that is secured to the wheel hub at a state where the inflator and the airbag are secured,

wherein in the airbag having undergone spontaneous inflation without being attached to the steering wheel,

when a size between a securing position of the airbag to the retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, the most protruding portion by the back panel is located at a position spaced from the securing position by 0.5 or greater,

a diameter of the inflated airbag of an opposing portion to an inner surface of the wheel is 0.6 to 1.2 times as large as an inner diameter of the wheel,

a diameter of the inflated airbag of a joint portion of the front panel and the back panel is greater than the diameter of the inflated airbag of the opposing portion to the inner surface of the wheel,

wherein in the airbag having undergone spontaneous inflation without being attached to the steering wheel,

when the size between the securing position of the airbag to the retainer member and the most protruding portion of the airbag with respect to the securing position is set as 1, a gravity center position of the airbag is located at a position spaced from the securing position by 0.5 or greater, and

wherein the front panel comprises a mass body attached to at least a part of the front panel.