Knee Airbag Supplemental Restraint Systems

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).

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.