One of the biggest problems in protecting occupants is keeping
them in the proper position. For example if the driver’s lower body slides under the seat belt, not only will his knees
crash into the dash, but his head and face become lower in the seat. With the airbag deploying at the angle of the steering
wheel, this put his face much closer to the bottom of the deploying airbag, endangering him of being hit. Also by his face
not landing in the center of the bag, the distribution of impact over a lager area is defeated, therefore not properly cushioning
him from the impact.
Manufacturers have developed many devices to stop this submarining
effect and are in the process of developing many others.
Knee Bolsters or Knee Airbags
Knee airbags or bolsters were some of the first of these devices to be developed and are still the most commonly used.
These devices are actually multi purpose, they not only stop the submarining effect, holding the occupant in place but, they
also protect against leg and hip injuries. Though many manufacturers used heavy padding on their lower dashes for years, in
a sever crash occupants still sustained major leg and hip injuries from their knees hitting the dash.
This sudden impact to the end of the femur (upper leg bone)
would cause either a bowing effect of the bone, causing it to break, or a dislocation of the hip joint. EMS personnel know, that because of the proximity
of the broken bone to the femoral artery, a broken femur is considered a life threatening injury.
The knee airbag is designed to deploy against the occupant’s
lower legs rather than the femur, reducing the forward momentum of their lower body during a frontal impact collision.
The airbag extends out from the dash approximately 5 inches
when fully inflated. The deployment of the bag pushes a bolster or padded panel against the occupant's lower legs, contacting
them at approximately the knee cap level and cushioning the legs down to approximately the shin and ankle area. This in turn
distributes the forces over the lower legs rather than the femur and at the same time holds the occupant properly positioned
in the seat.
The first knee airbag in the US came out on the 1996 model Kia Sportage and was only offered on the driver’s side. Today many are standard
equipment on both the driver’s and passenger’s side. These systems use an airbag attached to the back of the lower
dash panel, under the steering column or clove box.
The bag is deployed by a stored gas inflator that is triggered
by the same sensor as the driver’s and passenger’s frontal airbags but, it and the seat belt
tensioners deploy much faster than the frontal bags in order to hold the occupant in the proper position to endure
the airbag deployment.
The latest is a mechanical
system, combining the steering column, paddles and knee bolster as an energy absorbing unit.
These airbags are mounted under the carpet; they are deployed by a stored gas inflator
mounted over the center tunnel and like the knee bolster they are actuated by the same sensors as the frontal airbag, but
at a faster rate.
Also like the knee bolster they are multi purpose, providing cushioning for the occupant’s
feet and with the upward motion on the legs they are holding the occupant’s lower body in the proper position in the
These work in conjunction with another airbag mounted under the driver’s seat, that when inflated
tilts the seat back. By tilting the seat back and the upward motion to the legs, the lower body is held in place and the upper body is moved further away from the deploying frontal airbag.
Vedio- Carpet Airbag
Anti- Slide Airbags
of the newest developments is the anti slide airbag, which is not a fabric bag like most, but a thin metal envelope like structure
that is mounted under the foam cushion of the front seats.
The operating principle is identical to that of an airbag, except that
the anti-sliding airbag never comes into direct contact with the occupant.
In the event of impact it deploys in two phases:
1. A gas
generator inflates the metallic envelope compressing the foam of the cushion up in the front portion of the seat, pushing
the occupant back against the back rest, as the side foam forms around the pelvis.
2. A deflation
controller, situated beneath the module, then allows the metallic envelope to slowly deflate and the metal to deform to assume
the shape of the occupant’s pelvis. This forms like a pocket in the seat to hold the occupant in position and a protective
shield around the pelvic area to prevent injuries. It also lifts the weight of the legs, preventing the downward force of
the feet pushing into the floor.
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