Introduction to Vehicle Restraint Systems Expert Overview

In this article, the automotive engineers at Robson Forensic provide an introduction to vehicle restraint systems and how forensic experts inspect these systems as part of an investigation.

Vehicle Restraint Systems

Restraint systems and the vehicle structure are both key elements to vehicle crashworthiness. In vehicle crashworthiness, the vehicle structure acts to preserve the occupant survival space, whereas the restraint system acts to hold the occupant in place, preventing ejections, and also managing forces acting on the body to mitigate or prevent injuries.

During a collision, a vehicle will rapidly change direction and/or speed. During this "crash pulse,"  an occupant’s inertia acts to keep them moving in the pre-impact direction and speed until they contact something and/or are acted upon by an opposing force.

The purpose of restraint systems is to:

• Limit the occupant's travel to prevent ejections as well as mitigate bodily impacts with the vehicles interior
• Slow the occupant over the longest possible time frame, ride-down; absorbing energy to reduce peak forces acting upon the body
• Distribute the crash forces over the largest area possible on the occupant; reducing the risk of injury

Seatbelts

Seatbelts remain the most important restraint device. Airbags are tuned to work with seatbelt restrained occupants, as well as with unbelted occupants under lower-severity crash scenarios. Today’s lap/shoulder seatbelts are quite advanced compared to the early days of lap only belts.

Seatbelt retractors must lock before the webbing extends ~1" whenever the retractor is subjected to an acceleration of 0.7 G (similar to hard braking on dry pavement). Modern seatbelts typically include pre-tensioner devices which pull to remove ~2" of seatbelt slack early in the collision, and help to couple the seatbelt and occupant to the vehicle.

Mechanical load limiters are often installed inside the seatbelt retractor to allow “payout” of webbing during the crash to reduce the belt loads on the occupant. Most pre-tensioners, whether on the retractor or in the buckle stalk, are pyrotechnic, using a small chemical reaction to provide the force needed to provide the tension in the seatbelt webbing.

Some seatbelt retractors have electric motors providing both pre-tensioning and load limiting functions, as well as possibly implementing haptic feedback for drivers during normal operation.

Inspections of seatbelts, buckles, latches, and webbing after a crash event can provide insight into many areas, such as:

• Determining whether or not seatbelts were used
• Proper or improper performance of pre-tensioning and load limiting
• Determining or confirming crash severity
• Proper or improper design of anchor location
• Incorrect installation of retractors, buckles, anchors and other components
• Determining if inertial unlatching occurred during the collision

Airbags

Airbags are generally classified as "frontal," "side," or "curtain." Frontal airbags are typically installed in the 1st row and include driver, passenger, and knee airbags. Side airbags can be located in the seat or door and can offer head/thorax protection. Curtain airbags are installed in the roof to provide head protection during impacts and to prevent ejection of occupants during rollovers.

The two main components of an airbag are an inflator and a cushion.

There are many types of inflator devices, such as pyrotechnic, stored gas, or hybrid, but they all either generate or release gas to fill a cushion. The cushion (or bag) is constructed out of a nylon fabric material and sewn or woven into shape. These cushions are then folded or rolled into smaller packaging spaces until they are needed during an incident.

Inflator outputs, firing times, cushion size/shape, and number/size of vents are all tailored to adjust performance. Frontal airbags inflate in ~30 milliseconds, or 0.03 seconds, and side airbags can inflate in ~10 milliseconds, or .01 seconds.

Inspections of airbags after a crash can be useful to determine factors such as:

• Presence of cushion damage
• Fragmentation of the inflator
• Restriction or delay in the unfolding of the cushion
• Deployment door performance and fragmentation
• Occupant kinematics in terms of airbag interaction

Airbag Deployment Thresholds

Vehicles use data from accelerometers and other sensors processed by crash detection algorithms to determine if and when to deploy airbags. Direction and duration of impacts are important factors for this decision-making process. Although deployment thresholds can vary, frontal airbags typically deploy during collisions when the longitudinal change of velocity exceeds 10-16 mph. In side collisions, the vehicle structure has a smaller crush zone and side airbags often utilize lower thresholds.

Expert analysis can determine whether the forces involved in the crash met or exceeded the deployment thresholds for the subject vehicle's airbags, and whether the appropriate level of deployment was utilized.

Inspections of restraints and vehicle data after a crash can be useful to determine:

• The active restraints system’s response to a collision (to command deployment or not)
• Airbag or pre-tensioner failure to deploy
• Airbag or pre-tensioner late deployment
• Inadvertent deployment or use of incorrect deployment level for airbag

Vehicle Crash Expert Witness Investigations

From complex crash reconstruction to developing demonstrative evidence for court testimony, the transportation experts at Robson Forensic are well equipped to assist in your investigation. Our experts have in-depth knowledge of how vehicles are designed and built, they are fluent in the regulations affecting trucking operations, and have designed, built, and maintained our nation’s transportation infrastructure.

Common Issues for Investigation

• Determine crash severity, vehicle speeds, Delta V
• Determine whether or not seatbelts were used
• Should airbags have deployed?
• Did the restraint systems function properly?
• Interpret vehicle crash data

For more information, submit an inquiry or call us at 800.813.6736.

Featured Expert

Chase Nalls

Automotive Engineer & Crash Expert

Chase Nalls is an automotive engineer specializing in the design and performance of airbag and restraint systems. He applies his expertise to forensic casework involving crash reconstruction and… read more.