Kneebag and occupant leg protection system

1. A kneebag to be inflated upwardly from below in front of legs of a vehicle occupant, comprising:

an upper portion having a substantially rectangular shape when viewed in front and a first thickness in an inflated state,

a middle portion below the upper portion in a vertical direction having a generally triangular shape oriented downwardly when viewed in front, said middle portion extending downwardly from the upper portion with the rectangular shape to gradually reduce lateral length thereof and having a second thickness in the inflated state, said second thickness being smaller than the first thickness except for lateral end portions of the kneebag,

a lower end portion located at a bottom of the middle portion,

an upper tether located at a boundary between the upper portion and the middle portion to have vent spaces at two lateral sides thereof, and

a lower tether located between the upper tether and the lower end portion to form a narrow width portion with respect to the upper tether and having a lateral length shorter than that of the upper tether to have vent spaces at two lateral sides thereof, said vent spaces of the upper and lower tethers being formed obliquely between respective lateral ends of the upper and lower tethers and lateral edges of the kneebag, said vent spaces being substantially constant so that gas supplied at the lower end portion first flows obliquely upwardly to the upper portion through the vent spaces along the lateral edges formed with the triangular shape to thereby inflate the vent spaces and the narrow width portion upwardly; and then, the upper portion is inflated from two lateral sides thereof by the gas supplied from the vent spaces.

2. A kneebag according to claim 1 , wherein said upper and lower tethers are connected between a front side of the kneebag and a rear side thereof.

3. A kneebag according to claim 2 , wherein said upper tether has a first length in a thickness direction of the kneebag smaller than a half of a second length of the kneebag extending from a connected portion between the upper tether and the front side adjacent to the occupant to a connected portion between the upper tether and the rear side passing through an upper rim of the kneebag, said first length being smaller than a half of a third length surrounding one vent space from a connected end between a lateral front end of the upper tether and the front side adjacent to the occupant to a connected end between a lateral rear end of the upper tether and the rear side passing through a side of the kneebag.

4. An occupant leg protection system comprising the kneebag according to claim 1 and an inflator for inflating the kneebag.

5. An occupant leg protection system according to claim 4 , wherein said lower end portion has a lateral size to receive the inflator therein so that the gas from the inflator is quickly lead laterally upwardly along the lateral edges formed with the triangular shape.

6. A kneebag according to claim 1 , wherein said upper tether has a length in a thickness direction longer than that of the lower tether so that thickness of the kneebag is gradually increased from the lower end portion to the upper portion when inflated.

7. A kneebag according to claim 1 , wherein a distance forming the vent space, extending from the lateral end of the upper tether to the lateral edge of the knee bag nearest to said lateral end of the upper tether is generally same as a distance forming the vent space, extending from the lateral end of the lower tether to the lateral edge of the knee bag nearest to said lateral end of the lower tether, to thereby form the vent space generally constant.

8. A kneebag to be inflated upwardly from below in front of legs of a vehicle occupant, comprising:

an upper portion having a first thickness in an inflated state,

a middle portion below the upper portion in a vertical direction having a second thickness in the inflated state, said second thickness being smaller than the first thickness except for lateral end portions of the kneebag, and

a plurality of tethers disposed in the kneebag to be spaced apart from each other in a vertical direction and extending continuously laterally for connecting a front side of the kneebag adjacent to the occupant and a rear side thereof such that vent spaces are formed between respective lateral ends of the tethers and lateral edges of the kneebag,

wherein an uppermost tether in the plurality of tethers has a first length in a thickness direction of the kneebag between 20 and 150 mm, and a lower tether lower than the uppermost tether in the plurality of tethers has a length in the thickness direction less than that of the uppermost tether and between 10 and 130 mm,

total area of the vent spaces formed on two sides of the tethers is between 3000 and 7850 mm 2 , and

said uppermost tether is located close to a boundary between the upper portion and the middle portion, the first length of the uppermost tether being smaller than a half of a second length of the kneebag extending from a connected portion between the uppermost tether and the front side adjacent to the occupant to a connected portion between the uppermost tether and the rear side passing through an upper rim of the kneebag, said first length being smaller than a half of a third length surrounding one vent space from a connected end between a lateral front end of the uppermost tether and the front side adjacent to the occupant to a connected end between a lateral rear end of the uppermost tether and the rear side passing through a side of the kneebag.