Forensic Clues: Seat Belt Failure

In 2007, an estimated 15,147 lives were saved from seat belt use. Seat belts save five times more lives than airbags, according to statistics released by NHTSA. Seat belt use has increased over the years, due to cultural trends, and possibly due to the enactment of seat belt use laws. Seat belt systems do not always function as predicted. Seat belt systems can fail during a collision, often resulting in serious injury or death to the vehicle occupant. This Clues issue will examine how seat belt systems can fail in different ways.

Inertial Unlatching

Inertial unlatching is a disputed phenomenon that is purported to occur when forces from the accident cause the release mechanism to disengage. Forces from the accident act on the spring mechanism in seat belt release buttons, and any impact between the latching housing and a person's pelvis or hard object will increase the forces acting on the release mechanism. This can result in unlatching of the seat belt. Private laboratories claim to have recreated inertial unlatching, but the automotive industry insists that it cannot occur during a real world accident.

False Latching

False latching occurs when the seat belt user attempts to engage the seat belt latching mechanism without actually engaging it. This can occur due to internal friction in the fastening system, and can sometimes occur when there is an alternate path that the seat belt can be pushed into that is not the locking mechanism.

Accidental Release

Certain release mechanisms can be triggered easily during a collision by an occupant's arm or elbow. These are typically release mechanisms with a vertically oriented release button that is raised above the surface of the seat belt latch housing.

Seat Belt Webbing Failure

Seat belt webbing is normally rated between 5,000 and 6,000 pound breaking strength. Seat belt failures will often involve other parts of the seat belt mechanism, but manufacturer defects can result in webbing failure below the rated strength. Seat belt failure can also occur due to sharp edges in the car.

Webbing failure also occurs due to dynamic or "shock loading" of the seat belt. This occurs if the seat belt has not retracted properly and there is excess slack in the seat belt. During a frontal collision, the occupants continue to move forward until they are stopped by a seat belt, air bag, or the car itself. If there is slack in the seat belt, the occupant will already be moving relative to the seat belt, and will impact the seat belt, creating impact forces that can cause a failure in the webbing material. The nylon material seat belts are made of is extremely strong statically, but due to its inelastic nature cannot effectively withstand dynamic forces. The excess slack can be the result of mechanism failure, or the seat belt can become lodged externally so that the retractor cannot maintain tension in the seat belt.

Anchor Failure

Lap Only

Lap-only seat belts are a known hazard that can cause severe internal injury, spinal injury, brain damage, and death. Despite knowing of the hazards of this restraint system from the 1960's, the automobile industry did not start putting full lap and shoulder seat belt systems in automobiles until the late 80's. This means there are many cars still on the road today that have lap only restraints.

Shoulder Only

There are many cars on the road that have automatic front seat belt systems where the shoulder belt either opens automatically, or is attached to the door frame and opens with the door. Volkswagen led the way with this seat belt. These cars have manual lap belts, which are not always used. Without the lap belts, during a collision the occupant slides under the shoulder belt, since the center of mass of humans is near the belly button. The occupant is now not restrained, and is impacting the frame of the car with his or her legs. The shoulder belt now becomes a liability as it becomes taut, and can cause severe or fatal neck and back injury.

Retractor Failure

For seat belts to work, they must remain snug against the vehicle occupants. Any slack in the seat belt will increase the likelihood of injury to the occupants. Slack in the seat belt can cause failure of the seat belt, or cause the occupants to collide with the interior of the vehicle. Retractor failure can occur if the locking mechanism does not seat properly. A lock bar that normally engages a tooth on a disc when the seat belt is loaded can fail to engage the teeth. This occurs when the lock bar impacts the very tip of a sprocket tooth, knocking the lock bar away from the sprocket, which results in the retractor mechanism allowing slack to unspool. The occupant is no longer restrained by the seat belt when this occurs. At some point, the locking bar will engage the sprocket and lock the seat belt, but any amount of slack can result in serious injuries due to impacting the interior of the car.

Excessive Slack

When the occupant of a vehicle leans over or otherwise moves, the seat belt will pay out slack as long as the occupant moves with an acceleration less than what will cause the seat belt mechanism to lock. Seat belt retractors may not pull this excessive slack in without input from the occupant. The seat belt user must pull the seat belt to disengage the retractor teeth, and allow the retractor to function properly.

Seat belt system defects may be identified after an accident. Give us a call at (855) 627-6273 if you have a seat belt case you would like to discuss.