The Traffic Accident Reconstruction Origin -Article-

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Injury Mitigation Devices: Seat Belts.

By Ed Phillips, B.A., BScEng.


Imagine you are standing on a tennis court smashing tennis balls as hard as you can into a wall. Just a few feet in front of the wall is a net. You strike a tennis ball as hard as you can. It crushes into the wall and then bounces back to you. You drive the tennis ball again, but this time it goes into the net and slows until it loses all its forward velocity. The net cords stretched out and arrested the speed of the tennis ball. The time the tennis ball was interacting with the net was much longer than the time it took to bounce off the wall.

In a severe collision, the occupant is like the tennis ball. The occupant can either strike the dashboard, which is analogous to the tennis ball hitting the wall, or strike the seat belt which is like the tennis ball hitting the net. How much trauma the body of the occupant experiences will depend on the time period over which the force is applied and the stiffness of the body parts absorbing the force. Is it going to be applied quickly, or a longer period of time? To reduce the peak force, the occupant needs the force to be applied over a longest period of time possible.

Stretching the time epoch of the collision for the occupants and redistributing the crash forces to the stiffest parts of the human anatomy is the duty of the seat belt. Equally important, seat belts are the best way to prevent ejection from the vehicle.

Brief History of Seat Belts:

1903 A patent for a sophisticated system of seat belts and upper torso restraint integrated into a seat back was obtained by a French automobile manufacturer.

1910 A seat strap was introduced to airplanes, mainly to keep the pilot strapped to the plane in cases of inverted flying.

1949 Production installed seat belts are offered as an option.

1966 Congress required lap belts (pelvic restraints) for front seat outboard and rear seat outboard positions.

1967 Congress adopted Federal Motor Vehicle Safety Standards (FMVSS) 208, 209, and 210 dealing with restraint systems.

1968 Congress requires lap/shoulder belt combinations for all vehicles sold.

1971 FMVSS 208 was amended to specify occupant crash protection and establish a criterion for occupant crash injury.

1972 Seat belts became mandatory.

1973 The shoulder belt becomes permanently attached to the lap belt.

1974 Ignition interlock systems were required that would not allow the vehicle to be started unless the seat belt was in use. Requirement rescinded one year later.

1977 Airbags are to be phased in by car size.

1981 Airbag requirement is rescinded.

1987 The passive restraint systems are required with a phase-in.

1993 Mandatory phase-in of dual airbags.

Seat Belt System:

The seat belt restraint system contains some or all of these components;

1 - shoulder guide loop,

2 - webbing,

3 - non-locking retractor,

3a - retractors (generally),

4 - automatic locking retractor,

5 - emergency locking retractor,

6 - vehicle sensitive retractors,

7 - webbing sensitive retractors,

8 - three point belts,

9 - upper torso webbing,

10 - pelvic webbing,

11 - buckle,

12 - buckle release,

13 - tongue (latch plate),

14 - selvage,

15 - anchorages,

16 - mouse (carrier and track),

17 - adjustment hardware.

Components and Usage:

(1) The shoulder guide loop, often spoken of as the "d" ring, is the oval guide that the webbing travels through between the retractor and the buckle. Typically attached to the 'B' pillar, or roof line, the shoulder guide loop is often made of metal with a plastic coating, or completely of a plastic. During collisions with speed changes greater than about 12 mph, the rapid pay out of webbing sliding through this ring can burn and abrade the nylon covering and leave telltale marks of usage of the belt.

(2) The webbing is a narrow fabric woven with continuous filling yarn and finished with selvages. During a significant collision, great enough frictional forces can be generated to melt nylon material from the d-ring onto the webbing, skin can also be abraded or melted onto the webbing.

The belt (now typically a polyester weave with less elastic property but higher tensile loading capabilities) length and a coating of foreign materials like dirt or blood can also be indications of usage. The federal law mandates the width of the webbing to be not less than 1.8 inches except those portions that do not come into contact with the 95 percentile test dummy.

(3) The non-locking retractor allows the webbing to be spooled out at any time without locking and stopping this from happening. This is an older form of the retractor (the original form) and the occupant was responsible for adjusting the webbing length once the belt was buckled. The retractor would spool in excess webbing for stowage inside the retractor housing.

(3a) The retractor itself consists of various components (see illustration to left). The primary parts are the frame, which houses the mechanism; the spool, which winds the webbing up; the ratchet is located on the sides of the spool and are a portion of the mechanism to stop the spool-out; the spring assembly which acts to put tension on the spool to allow it to draw up the webbing; the lock pawl (A4) is a bar that is forced into position across the teeth of the ratchets; the G-sensor is an inertial device that when free to move, releases or applies a force to a spring which moves the lock pawl or allows the lock pawl to move into position across the ratchets. For comfort, to maintain a certain length, a ratchet lever will engage with the teeth of the ratchet.

Also, in webbing sensitive mechanisms, pins on the side of the ratchet stick out. Inertial bars, attached to light springs and mounted to the outer sides of the spool hub, spin freely under normal rotations. Under rapid rotation, with a sudden spool out the inertia of the bars pull them outward elongating the spring and causing the bars to quickly engage with the pins and lock the retractor. Some design have multiple retractors, often in door mounted passive/active systems.

(4) The automatic locking retractor allows the webbing to be with drawn and then rewound but will not permit a second withdrawal until the belt is nearly completely rewound. This device allowed for the installation of a locking device within the mechanism located inside the retractor housing. The mechanism was typically a clutch and locking bar combination which would engage if the belt spooled up an amount when it was in use.

(5) The emergency locking retractor allows the belt to spool out and rewind freely except when a vehicle acceleration demands it locks up. The mechanism may be of an inertial reel nature or vehicle sensitive type.

(6) The vehicle sensitive retractors act similar to the emergency locking retractors when the vehicle tilts at greater than a 15-degree pitch (0.26 G) or decelerates at greater than or equal to 0.7 G in ANY direction. This system allows for less spool out of the webbing.

The mechanism for the lock up is a pendulum or other inertial device which resists the acceleration of impact and causes a bar to lock across the ratchets of the retractor. The pendulum hangs freely and moves slightly in relation to the frame with changes in acceleration. An accelerations of more than 0.7 G is great enough to overcome the inertial force and swing the pendulum fully which, in turn activates the locking bar or pawl which stops the motion of the teeth of the retractor ending spool out. Another form of the inertial device is a steel ball which moves under a sufficiently large acceleration and causes the engagement of the lock pawl or a clutch which stops the spool out.

The distance of travel between the pawl and the teeth are less than or equal to 1/10 of one inch. The lock up force is along the order of 0.3 G to 0.5 G, and occurs in about 15 ms to 30 ms (0.015 - 0.030 seconds), but may be as rapid as 9 ms. At the first lock up the reel will engage and halt the pay out of webbing and then, with rebound into the seat the spring tension on the reel will wind up any additional slack created.

Older models of the pendulum were free to drop through a hole in the roof of the G-sensor housing if the vehicle over turned and thus, releases the ratchet allowing free spool out of the webbing. Newer devices have a ring which denies the pendulum from dropping through the top plate of the G-sensor housing should the vehicle overturn and prevents the pawl from disengaging.

(7) Webbing sensitive retractors automatically lock when the webbing is suddenly withdrawn, they do NOT lock when the webbing is slowly spooled out. The mechanism of lock up was a mass attached to a light spring. When the retractor began to rapidly spin with sudden pay-out, the mass would lock up the spool and prevent further withdrawal of the webbing from the spool hub.

(8) Three point restraints are seen in many vehicles today, the pelvic webbing and torso webbing are sewn together at the buckle area to maintain one triangular shaped belt. One end may be fixed to the vehicle near the door sill without a retractor or both pelvic and torso webbing may operate with separate retractors (often seen in the door mounted systems).

A continuous loop belt is a one retractor system. Typically, a floor anchor is used for the buckle and one portion (the distal end of the pelvic webbing)of the loop. The torso webbing feeds up from the retractor through a guide loop typically mounted on the 'B' pillar and the latch plate slides on the webbing via a guide loop. Movement of the latch plate may be limited by a button or build up of material sewn onto the webbing.

(9) The upper torso webbing (shoulder belt or shoulder strap) may be a part of a continuous belt or may be separately attached to a motorized mouse that runs along a track. The upper torso belt typically spools from a retractor and locks into a buckle with/or as part of a separate system with the lap belt.

(10) The pelvic webbing or lap belt may be part of a continuous system or may be separate in the case of some active/passive design systems. It runs from a retractor to a buckle that it latches into, often in conjunction with the torso webbing.

(11) The buckle is the one piece housing that surrounds the release and may extend up from the floor mounted anchorage. The system is functioning or "buckled" when the latch plate has clicked home into the receiving portion of the buckle.

(12) The release is the button in the portion of the buckle housing that allows the tongue to disengage from the buckle receiver. The release is designed to minimize unintended activity and should not release with a force less than or equal to 30 pounds.

Releases may be on an end or located near the center of the buckle.

(13) The tongue is the metal plate that is surrounded by, but extends from the conjunction of the webbing or may slide somewhat freely on the continuous loop webbing. Typically the tongue (latch plate) has a slot in it for engaging with the receiving end of the buckle mechanism. The latch plate often has its own guide loop which may be constructed from metal or plastic.

(14) The selvages are the woven edges of the webbing.

(15) Anchorages are technically defined as "attachment hardware" and must be manufactured to meet certain loading requirements.

(16) Mouse. (Carrier and track assembly). The mouse is a small motorized unit that attaches the distal end of the upper torso restraint for those types of passive systems that use this mechanism. When the door is opened, the mouse runs forward on a track along the head liner or sill and when the door is closed, the mouse runs rearward positioning the torso belt in place. The upper torso restraint is sometimes anchored to the floor with its own buckle. A buckle and emergency release is designed into the mouse in that device. Some systems employ separate buckles for the upper torso (passive) and the pelvic (active) webbing.

(17) Adjustment hardware. This type of hardware is designed to fit the belts to the individual user and includes the buckle, attachment hardware and the retractor.

Load Limiters:

Load limiters are simple mechanisms to increase the ride-down time of the event for the occupant while staying below the limits of torso over load. Systems manage the energy by deforming at the anchorage or the use of a metal honeycomb covering that deforms prior to the onset of extreme tensile loading on the webbing.

Energy management loops are a series of webbing folds, located near the anchorage, sewn together accordion fashion. As the tensile loading on the webbing approaches a limit (typically less than 1,500 lbf - which has been shown may cause serious chest injuries) the stitches break releasing some of the folds and absorbing additional kinetic energy through the webbing.


Most manufacturers report that having in excess of 1.5 inches of slack in the belt will create excessive webbing and will allow for too great a payout of the belt, increasing the travel distance (excursion) of the occupant.


Excessive slack may be accumulated due to bulky clothing or manipulation of the webbing for comfort. The purpose of the pre-tensioner is to take up excess webbing that has spooled out immediately before the collision forces act on the occupant (15 ms - 30 ms). This allows for a tighter couple between the car/restraint and occupant system and reduces the length of the travel for the occupant with a better apportioning of the collision forces.

One system also works with a cable and pulley system affixed to the engine that tugs the cable and tightens the webbing as the engine of the car is moved rearward, or more commonly it may operate on a sensor and explosive charge that fires a short bolt that takes the slack out of the webbing.

Inertial devices also are employed as pre-tensioners and grabbers. Grabbers grip the webbing and stop the spool out of webbing slack that is not tightly spooled around the hub. Typically, pre-tensioners and grabbers work in the 12-mph delta v range and may remove as much as six inches of slack from the system. The pre-tensioner typically operates in the range of 17 ms (0.017 seconds) and may activate prior to the airbag beginning to deploy.

Explosive pre-tensioners DO NOT WORK if the seat belt is not buckled.

Types of seat belts:

Type 1: A lap belt only for pelvic restraint.

Type 2: Is a combination of pelvic and upper torso restraint.

Type 2A: A separate shoulder belt intended to be used in conjunction with a lap belt to form a type 2 assembly.

Passive restraints: The type of restraint that requires NO action on the part of the occupant to put the device in place. Passive restraints were designed to increase the lack of compliance with seat belt usage.

Active restraint: The occupant is required to take some action to ensure the restraint is worn.

Passive/Active: There are combinations that have a passive torso restraint and an active pelvic restraint.

Pay-Out or Spool Out:

This is the webbing length that is created by the loosely wrapped webbing around the spool hub having the slack removed, as well as the amount that is pulled from the retractor prior to its locking.

FMVSS 208 - Occupant Crash Protection:

Passenger cars manufactured since 1972 have been required to meet specifications for crash protection at certain locations in the car. The manufacturer could comply with these demands in three ways:

(1) First Option: - Complete Passive Protection. The vehicle shall meet crash protection requirements as set forth by means that require no action by vehicle occupants (passive restraints).

(2) Second Option: -Lap Belt Protection System With Belt Warning. Have a type 1 or type 2 belt (with a detachable shoulder portion) at designated seating positions with a warning (audible or visible to driver) device.

(3) Third option: - Lap and Shoulder Belt Protection With Belt Warning. Standards similar to the second option.

More recently the standard has shifted to primarily passive systems.

Crash protection:

The standards set forth these criteria: The vehicle, at any speed up to and including 30 mph, is crashed into a fixed barrier that is perpendicular to the line of travel of the vehicle, or at any angle up to 30o; the instrumented test dummy must not show accelerations, or loading of specific areas, higher than set forth in FMVSS 209; no part of the system may fail.

Side Impact:

A barrier weighing 4,000 lbs. with a vertical, rigid, flat, rectangular surface, 6.5 feet wide by 5.0 feet high, is accelerated in a straight line and crashed into the side of the car. The barrier cannot undergo any significant static or dynamic deformation and absorb no significant portion of the energy resulting from the impact, save rebound velocity.

Injury Criteria:

( A ) The anthropometric test dummies shall be completely contained during the test.

( B ) The dummy shall not record a resultant head acceleration such that the equation:

shall exceed 1,000, where a is the resultant acceleration expressed as a multiple of the acceleration of gravity (g) and t1 and t2 are any two points in time during the crash of the vehicle which are separated by not more than a 36-ms time interval (0.036 seconds).

( C ) The resultant acceleration of the c.g. of the upper thorax shall not exceed 60 G's, except for intervals whose cumulative duration is not more than 3 ms.

( D ) The compressive force transmitted axially through each upper leg shall not exceed 2,250 lbs.

FMVSS 209 - Seat belt assembly requirements:

Webbing breaking strength:

6,000 lbf Type 1

5,000 lbf Type 2, 2a - pelvic*

4,000 lbf Type 2, 2a - shoulder*

(* together)

Elongation percentage belt type load

20% Type 1 2500 lbf

30% Type 2 2500 lbf - pelvic

40% Type 2 2500 lbf - torso

Emergency Locking Retractor:

Shall lock before webbing extends one inch when the retractor is subject to a deceleration 0.7 G. Shall NOT lock before the webbing extends more than two inches when the retractor is subject to a deceleration of 0.3 G.

Attachment hardware:

Anchorages and all hardware must be able to withstand the same loading as the webbing. The performance of the system shall not fail at 20 G. The hardware must withstand a force of 9,000 lbs. except that in which the ends of two or more assemblies cannot be attached to the vehicle by a single bolt shall have a breaking strength of not less than 5,000 lb. Hardware designed to receive the ends of the two seat belt assemblies shall withstand a tensile force of at least 6,000 lbs.

Buckle Release:

The buckle of a type 1 or type 2 seat belt shall release when a force of not more than 30 lbs. is applied.


The buckle of a type 1 or type 2 seat belt assembly shall NOT release under a compressive force of 400 lbs. Metal to metal buckles when in PARTIAL engagement shall separate by a force of not more than 5 lbs.

Structural Components:

Type 1: The assembly loop shall withstand a force of not less than 5,000 lbs., and each component shall withstand a force of not less than 2,500 lbs. The loop shall not extend more than 7 inches when a force of 5,000 lbs. is applied. The length of the assembly between anchorages shall not increase more than 14 inches under a force of 5,000 lbs. Any webbing cut by hardware shall have a breaking strength at the cut of not less than 4,200 lbs. Complete fracture of any solid section of metal attachment shall not occur during the test.

Type 2: The structural components of the pelvic restraint shall withstand a force of not less than 2,500 lbs. The structural components of the upper torso restraint shall withstand a force of not less than 1,500 lbs. The structural components of the assembly that are common to the pelvic and the upper torso shall withstand a force of not less than 3,000 lbs. The length of the pelvic restraint between anchorages shall not increase more than 20 inches when subjected to a force of 2,500 lbs.

The length of the upper torso restraint between anchorages shall not increase more than 20 inches when subjected to a force of 1,500 lbs. Any webbing cut by hardware during the test shall have a breaking strength of not less than 3,500 lbs. for the pelvic webbing and not less than 2,800 lbs. for the upper torso webbing. No complete fracture through any solid section of metal attachment hardware is allowed.

FMVSS 210:

Seat belt anchorages, attachments, hardware and attachment bolts must withstand a 5,000 lb. force. Type 1 and type 2 seat belt anchorages must withstand a 5,000 lbf load for 10 seconds. The load is applied at an onset rate of not more than 50,000 pounds per second, and attaining the 5,000 lbf load in not more than 30 seconds. Type 2 (automatic) seat belt anchorages must withstand a 3,000 lbs. for 10 seconds when that load is applied with an onset rate of not more than 30,000 lb/second, and attaining the 3,000 lbf load in not more than 30 seconds. The load for each test must be applied in the direction in which the seat faces to a pelvic block, at an angle of not less than 5 degrees or more than 15 degrees.

Mandatory Usage Laws:

Mandatory use laws immediately increased compliance in many areas by about 15% of the driving population, but passive restraints alone tend to receive nearly 80% compliance.

Primary Direction of Force:

The PDOF is a physical property imparted to an object during the impulse as a result of being involved in a collision. The normal and tangential forces at a contact area combine to yield a single resultant force at their peak or limit, often spoken of as maximum engagement. This force, imparted by a component of the momentum change of each car (which is relative to the mass of the effected vehicle) can be thought of as acting in a single direction. The occupants will "appear" to move opposite of the PDOF when, in reality, the vehicle is suddenly accelerated in the direction of the PDOF and the inertia of the occupants causes the car to move toward them.

Delta v:

By definition:

Delta v, means generally a change of velocity, the term has a specific meaning in the accident reconstruction community. It is identified as the instantaneous change in speed that occurs during the momentum exchange at Impulse (F dt).

As the definition of acceleration (above) indicates, the instantaneous change in the magnitude and direction of the velocity is equivalent to the instantaneous change in the magnitude and direction of the acceleration during the time period of the momentum exchange. This change in velocity has been shown to be a good indicator of the level of injury an occupant may receive.


This introductory paper has covered basic seat belt nomenclature. Types of seat belt systems and how they function were also presented. Finally some basic theory was discussed. In the next installment a general method for examining and testing restraint systems will be developed.


SAE TOPTECH - Passive Restraints

SAE 840396 - Diagnosis of Seat Belt Usage in Accidents.

SAE 840392 - Historical Perspective on Seat Belt Restraint Systems

NHTSA - FMVSS 208, 209, 210

Texas A & M - Biomechanics Course

Biomechanics of Trauma, by Nahum and Melvin

CHP Academy - lecture notes from TAR

Ed Phillips was a police officer for the City of Vacaville, California from 1979 - 1987. In 1987 he accepted a position as an Investigator with the County of San Diego Engineering section.

The position provides on-scene documentation through total station survey and photography of traffic collisions involving severe injuries and death. The entire County population is over 3 million residents, and the County maintains over 1,800 miles of roadways. Mr. Phillips has investigated hundreds of fatal collisions during his time with the County. He is used as a consultant for allied agencies, County Counsel and the District Attorneys' Office.

Mr. Phillips has consulted with Law Firms through out the western United States as well as the State Attorney Generals Office and the Department of Transportation.

Mr. Phillips holds Bachelor Degrees in Criminal Justice and Engineering (Manufacturing) and has attended many hundreds of hours of specific technical training in the field of collision reconstruction.

He can be contacted at

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