Canadian Underwriter

The Science Of Seatbelts And Airbags

November 30, 2008   by Jonathan Lawrence

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An impact between two cars will cause their velocities to change suddenly. Occupants within the cars will continue to move at pre-impact speeds until they experience a force to accelerate them to the new speed of the car. For occupants who are not wearing a seatbelt, this force often occurs as they crash into the steering wheel, dashboard, and/or windshield. A seatbelt is designed to tie the occupant to the vehicle and slow the occupant at the same rate as the vehicle. Seatbelts apply collision forces to areas of the body that are able to withstand these forces without serious injury. Seatbelts also eliminate or reduce the severity of occupant contact with vehicle interior structures.

Early seatbelts were lap-only straps that were difficult to adjust. Although they were effective in preventing occupants from being thrown out of cars, they did little to restrain the torso and head in the event of a crash. The introduction of lap and torso belts provided a significant improvement in preventing head injuries in frontal impacts.

One of the key safety features in the lap and torso belt is the locking retractor. The basic function of the retractor is to allow the shoulder portion of the seatbelt to spool out and retract.

In order to restrain the wearer, the retractor must prevent more webbing from spooling out in a crash. Early seatbelts used a simple pendulum just below the retractor to detect a crash and activate a locking bar that jammed the spool. Some cars have sensitive pendulums that can lock with hard braking. Starting in the early 1990s retractors that locked if the webbing was pulled out too fast also appeared. During an examination of a crashed car it is important to check that the locking retractor is functioning properly. Many modern seatbelt retractors are equipped with pre-tensioners. These contain small explosive charges that can deploy during moderate to severe collision causing the belt to tighten if the airbags are deployed. The pre-tensioners also jam the seatbelt retractor so that webbing can no longer be spooled in or out.

When determining whether a seatbelt was worn during a crash, the following physical evidence should be investigated:

Abrasions on the D-ring, latch plate and webbing: Restraining an occupant during a crash generates large forces on the seatbelt hardware — often large enough to leave abrasions on the plastic coatings of D-rings and latch plates. Wear marks on webbing can also occur due to forces.

Activated strain relief: Some seatbelts have a loop of extra webbing sewed back on itself. The stitches that hold this section of webbing are designed to tear if the forces on the seatbelt get too high in order to relieve some of the force on the occupant.

Pre-tensioners: The length of webbing spooled out of a locked pre-tensioning retractor is a clue as to whether the seatbelt was buckled at the time of the crash.

Webbing stains: A careful examination of the seatbelt webbing can sometimes reveal blood stains on areas of the belt that would be behind a trim panel if the seatbelt was not being worn or was stowed. If the stain was the result of the accident, the webbing had to have been extended at the time of the crash.

Airbag modules: The on-board computer that controls the airbags may record information about seatbelt use and occupant positioning in the event of a crash. Currently, crash data can be downloaded from newer General Motors, Chrysler and Ford vehicles.

Occupant contact with the interior: Unrestrained occupants often hit parts of the interior that a belted occupant would not. Star shaped windshield fractures, for example, typically result from a head impact that would not occur with seatbelt use.

Occupant ejection: Seatbelts prevent ejections in all but a few unique cases, such as large rear-end impacts that cause seat back failure and rearward ejection.

Case Study: The crash of an SUV into a pole resulted in a broken femur for the driver. During examination of the vehicle to determine whether the driver was wearing a seatbelt at the time of the accident, it was concluded that the locking retractor and seatbelt were working properly. There was evidence the driver had hit his head on the windshield (a clump of long hair) and his knees on the dashboard during the crash. Using data from crash tests with seatbelted crash test dummies, the interior geometry of the vehicle and the driver’s stature, engineers were able to determine he would not have hit his head on the windshield had he been wearing his seatbelt. Data downloaded from the airbag module also indicated the driver’s seatbelt was unlatched at the time of the crash. The evidence was compelling — the driver was not wearing his seatbelt.

A biomechanical analysis was also conducted to see if the driver’s femur injury would have occurred had he been wearing his seatbelt. During a U. S. government crash test with the same SUV, a crash test dummy in the driver’s seat hit its knees on the dash even though it was properly belted. However, the force measured in the dummy’s femur was less than one-half of the generally accepted 35 per cent fracture risk level for a healthy femur. With no medical suggestion of a lowered tolerance for the driver in this case, it was concluded that seatbelt use would probably have prevented a femur fracture.


Airbags were available on some General Motors and Ford models as an extra-cost option in the mid-1970s but were discontinued due to lack of interest by the car-buying public. In 1989 the United States required all new cars to have some form of “passive restraint,” which would help drivers even if they didn’t put their seatbelts on. The airbag was the answer to this regulation. In 1998 regulators required an airbag for the front passenger and allowed manufacturers to put de-powered airbags in their cars to reduce potential harm to small drivers who sit close to the steering wheel.

Airbags are designed to inflate almost instantaneously in the event of a crash and provide a relatively soft surface for an occupant’s head to hit. They function best when an occupant’s head hits them after they have fully inflated. As a rule of thumb designers want their airbags to inflate fully before the occupant has moved six inches forward.

The decision to deploy an airbag has to be made well before a crash is over. During a typical high-speed crash vehicle deceleration lasts for about 150 milliseconds (literally the blink of an eye). Occupants have moved forward six inches after about 50 milliseconds. It takes about 30 milliseconds to inflate an airbag. Therefore, the decision to deploy an airbag has to be made in the first 20 milliseconds of a 150 millisecond event i. e. before the crash is 15 per cent complete.

Because of the predictive nature of airbag systems, it is impossible to determine the precise threshold speed that will cause a deployment. Other factors like the weight and stiffness of the object being hit also affect the threshold. Some general guidelines published by the U. S. government suggest airbags should not deploy with a speed change below 22 km/h and should deploy with a speed change above 29 km/h. A speed change of 22 km/h will cause some, but probably minor, structural damage to the front of a car. A speed change of 29 km/h can cause significant structural damage to the front of a car.

Manufacturers are continuously updating and redesigning their airbag systems. Because a deployment decision needs to be made in the early phases of a crash it can sometimes be wrong. Short, sharp impacts (like hitting a curb with a suspension part or a frame cross-member) can look severe for the first 20 milliseconds and inappropriate deployments are possible. To reduce these kinds of events, manufacturers are continuously developing their airbag systems by tailoring specific decision algorithms for each type of vehicle sold.

Once airbags have deployed,
occupant factors can influence airbag effectiveness in a given collision. Occupants can be injured by airbags if they are close to the dashboard. To address this problem manufacturers are introducing “smart” systems which can detect occupant position, weight and seatbelt use and incorporate this information into the decision about whether, and how, to deploy an airbag. The following case study illustrates some of the issues surrounding an occupant seated close to the dashboard/airbag at impact.

Case Study: The front passenger in a large sedan suffered a cervical spine fracture. She admitted she was not wearing her seatbelt and had her seat fully forward at the time of the crash. The medical reports described her injury as a hangman’s neck fracture, which is caused by an upwards force to the chin. In a crash test with an unrestrained dummy, the dummy moved forward at impact and received a blow under the chin from the airbag while it was still inflating. This contact represents an injury mechanism for the occupant’s neck fracture. In a test with a seat belted dummy, airbag contact under the chin did not occur, but rather the face of the belted crash test dummy struck the airbag after it was fully inflated. As this interaction would not apply the necessary upward forces required for injury, engineers were able to conclude that the cervical spine fracture would probably have been prevented with seatbelt use.

Seatbelts and airbags are important automobile safety features. Their improper use or malfunction can dramatically alter injury outcomes for car occupants involved in a crash. Often evidence gathered during the examination of a crashed car will indicate to a forensic engineer how the seatbelts and airbags functioned during a crash. Combined with a bio-mechanical assessment of injury, this evidence can provide a compelling picture of how a particular injury occurred and how different circumstances would have changed things.

Jonathan Lawrence, P. Eng. is a principal and senior engineer of MEA Forensic Engineers & Scientists. He leads the Transportation Group in British Columbia, conducting technical investigations typically focusing on issues such as speed, impact severity, seat belt use, driver evasion potential, visibility and the interpretation of crash data recorders.

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