Seat Belt Defects 101

Seatbelts can-and-do fail.  All too often they fail due to a poor design, faulty manufacturing or in some cases both. Typical seatbelt defects include; inertial unlatching, false latching, weakened webbing, retractor failure, poor seatbelt geometry, or vehicle system failure. 

Studies suggest that millions of cars on the road have defective seatbelts, at least to some extent. Unfortunately, the seatbelt integrity is not known to be a problem, until after a vehicle occupant is injured in an accident due to the seatbelt not providing proper restraint.

Inertial unlatching occurs when the seat belt releases during a collision. The latch button becomes "depressed" during the collision releasing the passenger. False latching occurs when the seat belt appears to have been appropriately latched, but the seat belt releases during an accident. Even small forces can cause a falsely latched seat belt to release. Inappropriate unlatching is typically noted by another passenger in the car who was a witness that an injured passenger had properly latched his or her seat belt, but the seat belt did not provide proper restraint during an accident.

Weakened webbing can occur for a number of reasons. Safety standards require that the seat belt webbing be capable of withstanding forces well in excess of those experienced during vehicle accidents. If the webbing is torn or damaged after the accident, then it may have been defective.

The seat belt retractor is the device the locks during an auto accident and holds the vehicle occupant in place. Obviously if the retractor malfunctions and does not lock properly, the occupant will be released to move forward during a collision. Seat belt retractor failure can occur in a variety of ways. In some cases, the retractor releases a small amount of slack in the seat belt webbing, before locking. Even a few inches of this type of slack can make the difference in some accidents between an injury-free event and catastrophic injuries. Some retractors have so called "window shade" designs, that allow the occupant to create a little slack in the seat belt, such as when leaning forward to adjust the radio. If the retractor allows this slack to stay in the seat belt webbing during an accident, the vehicle occupant may not be properly restrained.

For a seat belt to properly restrain the vehicle occupants, the geometry of the seat belt should be such that during a collision or rollover, the angle between belt and the person should not be too shallow. When seat belt anchors are placed away from the occupant, such as on the floor instead on the seat, this can create a shallow angle. Defective seat belt geometry can allow the occupants to move to excessively toward the roof, thus providing a higher probability of injury during roof crush in a rollover. Poor D-ring locations can also create improper geometry, again leading to shallow angles.

Door mounted seat belts can pose a significant problem, because if the door is lost during an accident, then the occupant can easily be ejected from the vehicle. Also, the so-called "automatic" seat belts can be a problem, because the occupants have the shoulder harness portion around them automatically when they sit in the car and close the door. The occupants assume that they have adequate restraint protection during an accident and they forget or neglect to attach the seat belt around the lap.