Knee Airbag Supplemental Restraint Systems

May 1, 2004
SUBJECT:Knee Airbag Supplemental Restraint SystemsTOPIC:Knee Airbag 2004 Model Year UpdateOBJECTIVE:Identify the various designs, locations, and function of knee airbag systems available in vehicles.TASK:Given pictures of the interior of vehicles, identify the location of the knee airbag, explain its operating features, and describe responder safety considerations for operating near an undeployed knee airbag system.

Currently, 10 models of vehicles sold in the United States are equipped with driver’s knee airbag systems or with dual, driver and front passenger knee airbags. According to one U.S.-based airbag manufacturer, 3 million model year 2004 vehicles will be produced with “inflatable knee bolsters” (see it here).

U.S. Automobile Knee Airbag Systems
Audi A8
(Driver/passenger knee bags)
10 bags total
Model year 2004
BMW 645
(Driver/passenger knee bags)
8 bags total
Model year 2004
Lexus LS 430 sedan
(Driver/passenger knee bags)
8 bags total
Model year 2004
BMW 760
(Driver/passenger knee bags)
8 bags total
Model year 2004
BMW 745
(Driver/passenger knee bags)
12 bags total in vehicle
Model year 2004
BMW Z4 convertible
(Driver/passenger knee bags)
8 bags total in vehicle
Model year 2004
(Also contains side-impact airbags in rear quarter panels of this convertible)
Chrysler Pacifica
Model year 2003
Inflatable knee blocker
for the driver only
Kia Sportage
(Driver only)
3 bags total
Model years 1996-2001
Lexus RX 330 SUV
(Driver knee bag only)
7 bags total
Model year 2004
Mercedes SL-Class SL55
(Driver knee bag only)
5 bags total in vehicle
Model year 2004

Researchers report that hip injuries accounted for 48% of car-crash leg injuries in 1993-2001 vehicles (see “Knee Protection May Cause Worse Hip Injuries” by Jayne O’Donnell and James R. Healey, USA Today, May 18, 2003). To address this injury concern and to help keep front-seat occupants away from frontal airbag inflation zones, manufacturers have dramatically increased their use of knee airbag systems in 2004 model year vehicles.

Knee airbags are installed relatively low along the instrument panel, below the glove compartment on the passenger’s side of the dash. On the driver’s side, the knee airbag is positioned beneath the steering column. This relatively new technology works simultaneously with seatbelts and pre-tensioners to improve overall occupant survival in a frontal collision. In a frontal collision that initiates deployment of one or both frontal airbags, knee airbags deploy outward from the dash toward the front edge of the seat.

There are two basic knee airbag designs. One style uses the inflated knee airbag itself to press against the knees of the front occupants. The other knee airbag design has a “knee bolster” panel permanently attached to the bag itself. It is this padded panel that presses against the occupant’s knees, not the airbag.

This pressure against the knees and legs, along with the firing of the seatbelt pre-tensioners, holds the occupants in their proper seated position at the moment of a frontal collision. An inflated knee airbag increases the chance of the driver and front-seat occupant remaining out of the 10-inch and 18-inch inflation zones of the frontal airbags. It also reduces the risk of “submarining” – the gliding of occupants under their seatbelts, crushing them under the dash and instrument panel on the floorboard.

The popularity of knee airbag systems among automakers is also due in part to the fact that having knee airbags allows the “show surface” of the instrument panel to be designed farther from the front seats, giving the front seat occupant more legroom.

The 2004 Lexus RX 330 vehicle is an excellent example of how knee air bags are being integrated into a vehicle’s total crash protection system. The RX’s safety systems include a driver’s multi-stage airbag that deploys at one of three speeds depending on many factors, including crash severity and seat track position. Similarly, the front-passenger dual-stage airbag uses weight and crash-severity sensors to monitor its deployment. For side impacts, there are seat-mounted side-impact airbags for front occupants.

There are also roof-mounted side-impact airbags along with the new driver’s knee airbag. In addition, if either frontal airbag deploys, seatbelt pre-tensioners fire to remove slack from the seatbelt system as the vehicle’s Automatic Collision Notification system engages the Lexus Tele Aid SOS telemetry system.

Ron Moore will present “Ron Moore’s Top 10 Vehicle Rescue Challenges” and “Safe Parking” at Firehouse Expo 2004 in Baltimore, July 13-18.

Photo By Ron Moore The blow-out panel for passenger knee bag on this BMW Z4 convertible shows its location above the glove compartment. The passenger frontal airbag is “hidden” inside the instrument panel, where the airbag sticker is located.
Photo By Ron Moore You can barely see the blow-out panel for the driver’s knee airbag on this 2004 Mercedes SL-Class automobile. The airbag ID is visible if you look through the bottom of the steering wheel.
Photo By Ron Moore A low-angle view shows the driver’s knee airbag blow-out panel and the small airbag ID.
Photo By Ron Moore If you are standing up at the open driver’s door of this 2004 Lexus RX 330, you may not realize that there is an undeployed driver’s knee airbag as well as a passenger knee airbag. Even the passenger’s frontal airbag is a “hidden” design with no blow-out panel.
Photo By Ron Moore By kneeling down, a rescuer may be able to “eyeball” the driver’s knee airbag blow-out panel on this dual knee airbag system. With a patient in the driver’s seat and at night, this knee airbag would be even harder to spot.
Photo By Ron Moore Believe it or not, a driver’s knee airbag is concealed behind the horseshoe-shaped panel under the steering column on this 2004 Audi A8L.
Photo By Ron Moore This sticker appears next to the passenger’s front door hinge. It depicts how the knee airbag deploys downward to completely fill the floorboard area on this Audi A8L knee airbag system.
Photo By Ron Moore The passenger’s knee airbag on this 2004 Audi A8L is located below the huge glove compartment and is hard to see unless you kneel down to inspect the lower dash area.
Photo By Ron Moore The driver’s knee airbag on the Chrysler Pacifica is housed behind the knee bolster plastic trim. Leg injury measures during this crash testing were low, but not lower than in many vehicles with conventional bolsters.Ron Moore, a Firehouse® contributing editor, is a battalion chief and the training officer for the McKinney, TX, Fire Department. He also authors a monthly online article in the “MembersZone” and serves as the Forum Moderator for the extrication section of the website. Moore can be contacted directly at [email protected].

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