Licensed Professional Engineers

FORENSIC CLUES # 41- "Modern Safety Devices of Automobiles—Part 1: Seatbelts" by John L. Ryan


A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.

Issue 42: Vol. 1 March/April 2011

© 2011 M.A.S.E. LLC

( 479) 549-4860

Automotive technology has advanced quickly over the past few decades. Safety technology has also advanced quickly; lap belts were developed into shoulder/lap belts, frontal airbags and seatbelt pretensioners were developed, and later side airbags. This Clues begins a series dedicated to automotive safety technology. This issue will cover seatbelt pretensioning and load limiting devices.

What Causes Injury in a Collision

 Humans have had to adapt to a wide variety of environmental conditions over time. Unfortunately our bodies have yet to catch up with technology – our bodies are not equipped to experience high rates of acceleration

or deceleration. Acceleration and deceleration occur when there is a change in the velocity of an object, and results in force being applied to the object. No force acts on an object unless that object is subject to acceleration. As acceleration/deceleration increase, the quantity of force increases. Problems caused by forces from high rates of acceleration are really only a problem for jet fighter pilots – the rest of us must be concerned with high rates of deceleration.

Internal blood vessels and organs can rupture and tear at high rates of deceleration, and the human spine will break at a certain deceleration. The key to surviving collisions is keeping deceleration levels within survivability thresholds.

Pretensioning Devices

Seatbelt pretensioners are devices that are designed to reduce the amount of slack that may be present in a seatbelt in a collision. Seatbelts are most effective when there is minimum slack present in the seatbelt. Slack can be introduced for numerous reasons, from normal use, from leaning forward or to the side, from the seatbelt snagging or becoming pinned, from bulky clothing, etc. With slack in a seatbelt, when a collision occurs the belted occupant begins to accelerate forward relative to the vehicle.

This may result in the occupant impacting the interior of the vehicle, including the dash or windshield. The occupant then is decelerating rapidly upon impact with the loose seatbelt, or interior of the vehicle. This impact may cause serious injury or death. When a seatbelted vehicle occupant is properly belted with minimum slack, the occupant decelerates at the same rate as the vehicle. Deceleration rates are reduced by the crushing of the front of a vehicle.

Without pretensioners, a seatbelt can at best stop and prevent further webbing from unspooling. Pretensioners take up slack in seatbelts at the beginning of a collision sequence. Whenever a collision with a high enough deceleration is detected, the control module of the vehicle will trigger the pretensioning devices. An explosive charge is triggered by this signal. This explosive charge tightens the seatbelt rapidly. This explosive charge is usually located in the retractor mechanism, although it can be located in the buckle assembly. Once an explosive pretensioner is triggered, the seatbelt will mechanical lock to prevent webbing from spooling out.

If a pretensioner fails to fire, vehicle occupants may incur significant injury if there is slack in the seatbelt system. Pretensioners are not designed to engage in all collision scenarios. Occupant injury without pretensioner firing may or may not indicate a faulty system.

Load Limiting Devices

Load limiting devices are designed to deflect in the event of a collision, reducing the deceleration as well as the forces experienced by a seatbelted occupant. This is done by extending the time of deceleration by allowing a small amount of webbing to unspool.

There are several methods of load limiting that manufacturers use. A simple method used is seatbelt webbing that is stitched back on itself. At a certain predetermined load the stitching will fail, theoretically reducing the deceleration on the occupant. Other seatbelt manufacturers design the retractor spindle so

that when a seatbelt is loaded with collision forces, internal frame members of the retractor twist and permanently deform.

Another method of load limiting involves physical deflection of seat belt mounting. The anchors are designed to deflect upon collision loading.

Before frontal airbags were standard equipment on automobiles, few load limiters were in use. The reason for this is the increased excursion associated with load limiting devices. In other words, load limiters allow occupants to be thrown further forward, and without a frontal airbag to inflate and catch them they are better off without the load limiter. A vehicle with frontal airbags that do not deploy in an accident is functionally no different than a vehicle without frontal airbags.

In an article by Matthew Brumbelow entitled "Effects of Seat Belt Load Limiters on Driver Fatalities in Frontal Crashes of Passenger Cars", he states that ideal airbag protection has often been assumed. He quotes Mertz et. al. 1995 as writing, "The dilemma of using a shoulder belt force limiter ... is selecting a load limit that will balance the reduced risk of significant thoracic injury ... against the increased risk of head injury due to the greater upper torso motion allowed by the shoulder belt load limiter. However, with the use of air bags, this dilemma is more manageable since it only occurs for non-deploy accidents where the risk of significant head injury is low even for the unbelted occupant." Brumbolow also quotes Petitjean et. al. 2003 to the same point, "The lowering of the shoulder belt load allows the thoracic injury risk to be reduced. At the same time, the airbag avoids the increase of the head and neck injury risks due to greater torso motion." Again there is the assumption that the airbag will always deploy as designed.

Potential problems of seatbelt load limiting devices include reliance on airbag deployment, which may not occur in certain crash circumstances or if there is a sensor malfunction. Increased slack in the system increases the risk of injury due to impact with the steering wheel or other parts of the automobile interior. Additionally, after a load limiting device has deployed, the occupant will be not be restrained effectively for secondary impacts.

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