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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 1 Motor Vehicle Accident Reconstruction & Biomechanical Physics Robert C. McElroy, Ph.D. www . ForensicAccident . Com ABSTRACT Accident reconstructionists rely on a wide range of methods to record and analyze motor vehicle accident information. This paper addresses con- temporary methods of obtaining and analyzing collisions with emphasis on G force explanation for biomechanical analysis. INTRODUCTION Traffic accident reconstruction is the effort to determine, from whatever resources are available, how an accident happened. A traffic accident reconstructionist must be familiar with the application of a wide range of mathematics and specialized aspects of vehicle technology. Because of the wide range of knowledge required by the accident reconstructionist, voluntary certification is available through the Accreditation Commission for Traffic Ac- cident Reconstruction. ACTAR certification includes education, work experi- ence, and successful completion of a comprehensive examination. MATHEMATICS FOUNDATION Mathematics are at the core of traffic accident reconstruction. Many different equations are used to determine different aspects of an accident. Sir Isaac Newton developed three mathematical laws of motion which provide the foundation for traffic accident reconstruction. In Newton’ s first law of motion an important property of matter ap- pears. It is known as inertia, that property of matter by which an object main- tains a constant velocity in the absence of an unbalanced external force. When an automobile is suddenly stopped, the passengers obey Newton’ s first law and continue in their motion with constant velocity until some external
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N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

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Technical paper addressing impact and collision forces incorporated into masters degree program dealing with biomechanical trauma for physicians conducted by Lynn University and the University of Miami Medical Center.
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Page 1: N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 1

Motor Vehicle Accident Reconstruction

& Biomechanical Physics

Robert C. McElroy, Ph.D.

www . ForensicAccident . Com

ABSTRACT

Accident reconstructionists rely on a wide range of methods to record

and analyze motor vehicle accident information. This paper addresses con-

temporary methods of obtaining and analyzing collisions with emphasis on G

force explanation for biomechanical analysis.

INTRODUCTION

Traffic accident reconstruction is the effort to determine, from whatever

resources are available, how an accident happened. A traffic accident

reconstructionist must be familiar with the application of a wide range of

mathematics and specialized aspects of vehicle technology. Because of the

wide range of knowledge required by the accident reconstructionist, voluntary

certification is available through the Accreditation Commission for Traffic Ac-

cident Reconstruction. ACTAR certification includes education, work experi-

ence, and successful completion of a comprehensive examination.

MATHEMATICS FOUNDATION

Mathematics are at the core of traffic accident reconstruction. Many

different equations are used to determine different aspects of an accident. Sir

Isaac Newton developed three mathematical laws of motion which provide the

foundation for traffic accident reconstruction.

In Newton’s first law of motion an important property of matter ap-

pears. It is known as inertia, that property of matter by which an object main-

tains a constant velocity in the absence of an unbalanced external force. When

an automobile is suddenly stopped, the passengers obey Newton’s first law

and continue in their motion with constant velocity until some external

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 2

force changes their state of motion. Seat belts in a automobile can provide

such an external force which is much preferred to that exerted by the wind-

shield or dashboard. Another statement is the following:

A body at rest remains at rest, and a body in motion remains in motion with

constant velocity along the same straight line unless acted upon by some outside

force.

Newton’s second law states that if a body is acted upon by an unbal-

anced force F, its center of mass will accelerate in the direction of the force.

The acceleration, a, is proportional to the force, F, and the constant of propor-

tionality, m, is called the mass of the body. Another statement is the following:

The acceleration of a body is directly proportional to the resultant force action upon

the body and acceleration is inversely proportional to the mass of the body.

Newton's second law provides the key relationship between force and

acceleration since force and acceleration are vectors and vectors have both

magnitude and direction. Mass is only a magnitude, so it follows that the

magnitude of force equals the magnitude of acceleration times the mass.

Unification of these concepts reveals that force direction must be the same as

the direction of the acceleration because mass does not have a directional

property.

Newton’s first law states that a body at rest

remains at rest unless acted upon by some

external unbalanced force.

Newton’s first law also states that a body in

motion remains in motion with constant velocity

unless acted upon by some resultant force.

Newton’s First Law of Motion: Inertia of Rest & Motion

Page 3: N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 3

The second law is written F = ma where the unit of force is the new-

ton. One newton produces an acceleration of one meter per second, per

second, in a mass of one kilogram. One newton has a value of .2248 lb.

Newton’s Third Law of Motion: Action = Reaction

Action (force exerted on the trailer) is equal to the

reaction (force exerted on the car by the trailer).

Newton's third law is equally valid in dealing with bodies at rest or

in motion, either uniform or accelerated. The wheels of an automobile in

motion push backward on the road, but the road pushes forward on the

wheels with an equal force during acceleration. Another statement is the

following:

Whenever one body exerts a force upon a second body, the second body exerts an

equal and opposite force on the first.

Newton’s Second Law of Motion: F = m a

Acceleration due to a given result-

ant force is inversely proportional to

the mass accelerated.

Acceleration of a body is directly

proportional to the resultant force

acting on the body.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 4

SPEED CHANGE

Prediction of what happened during a collision by examination of what

remains in the form of residual damage can be used to calculate speed change

or Delta Velocity (DV) experienced by the vehicles in the collision. DV is one

of the best available measures of accident severity.

DV assumes that collision stopping force on a vehicle is a linear function

of residual crush depth. Up to a certain force level, there is no permanent

damage and beyond that point, permanent damage increases with increased

force. Two stiffness coefficients, A and B, define the force-damage curve.

Fundamental to a solution for speed change are appropriate A and B values

for a specific vehicle. A and B values are derived from the collision damage

sustained from known velocity changes of a vehicle into a barrier. Therefore,

A and B values can be used to calculate speed change based on permanent

vehicle crush (see chart).

Vehicle tests sponsored by the

National Highway Traffic Safety

Administration resulted in a series of

computer programs released to the

public called CRASH. The last public

version CRASH3 was revised and

released in 1982. Several computer

based accident analysis programs are

available for the accident

reconstructionist, each ultimately

stems from this background.

A collision analysis project is

defined as a series of step by step

calculations. The investigator will organize the project into separate events

which require solutions for specific unknowns. Normally, each event or

step will consist of an equation with only one unknown. The art of collision

analysis is to determine which event to solve first and then how to proceed

with the next calculation to develop a unified collision sequence analysis.

Calculations from a popular AI Tools computer program help to address

specific information needed in order to piece together the accident

reconstruction. When properly used each calculation will help to fill in a missing

piece of information. Here is a list of AI Tools calculations:

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 5

Special 45 Tangent Offset

46 Radius from Chord and Mid-Ordinate

47 Critical Curve Speed

48 Combined Speed from Drag Surfaces

49 Combined Speeds

Airborne 50 Horizontal Launch and Fall Speed

51 Small Angle Speed at Launch Equation

52 General Projectile Speed Equation

Energy 60 Speed from Linear Kinetic Energy

61 Kinetic Energy from Speed and Weight

62 Force from Kinetic Energy and Distance

63 Distance from Kinetic Energy and Force

Motorcycle 70 Lateral Acceleration Factor from Speed and Radius

71 M/C Lean angle from Speed and Radius

72 Turning Radius From Speed and lateral acc

Animation 80 Final Speed from Start Speed, Drag and Time

Assist 81 Distance from Start Distance, Speed, Drag and Time

Reaction 90 Total D from P/R time, Speeds & Drag f

Times 91 Total D from Start Speed, Times & f

92 P/R Time from Speeds, Total D & f

93 P/R Time from Start Speed, Total D, f & Time

94 Start Speed from Total D, Times & Drag

95 Start Speed from Total D, Final Speed, P/R Time & f

96 Drag from Speeds, P/R Time & Total D

97 Drag from Start Speed, Total D & Times

98 Total Time from P/R Time, Speeds & f

99 Total Time from P/R Time, Brake D & Drag

Eq Group Eq # Problem Description

Speed 1 Speed from Distance & Drag

2 Constant Speed from Distance & Time

3 Speed from Drag & Time

4 Final Speed from Start Speed, Drag & Time

5 Final Speed from Start Speed, Drag & Distance

6 Start Speed from Final Speed, Drag & Time

7 Start Speed from Final Speed, Drag & Distance

Time 10 Time from Constant Speed & Distance

11 Time from Speed & Drag

12 Time from both Speeds & Drag

13 Time from Distance & Drag

Distance 20 Distance from Speed & Drag

21 Distance from Constant Speed & Time

22 Distance from Two Speeds & Drag

23 Distance from Initial Speed, Drag & Time

Acceleration 30 Drag from Speed & Distance

Factor 31 Drag from Speed & Time

32 Drag from both Speeds & Time

33 Drag from both Speeds & Distance

34 Drag from Distance & Time

35 Drag from Initial Speed, Distance & Time

36 Drag from Road Friction, Brake Efficiency & Grade

37 Drag from Horizontal Force & Weight

Linear 40 General Two-Dimensional Momentum

Momentum 41 Inline - V1 from V2, V3, and V4

42 Inline - Coefficient of Restitution

43 Inline - Plastic, V3 = V4

44 Inline -Elastic

Equation #1 Speed from Distance &

Drag. Calculation of vehicle speed made

from skid distance and drag factor.

Measured skid distance is 120 feet and

deceleration factor for the vehicle &

road surface combination is .7 G. The

vehicle is calculated to have been going

50 mph when the brakes were applied.

MOMENTUM

Momentum is a restatement of Newton's laws in a form that is useful for

the collision analysis or any event which involves very short periods of time.

The second law for a single object would be rewritten as F Dt = m DV.

The left side of the equation is the Impulse and the right side is the Change in

Momentum. This relationship is still a vector relationship. Force has the

same direction as the change in momentum which has the same direction as

the change in velocity. In summary:

Impulse = Change in Momentum

If two vehicles collide, the force or impulse on one of the vehicles is equal

and opposite to the force on the other. This is a consequence of Newton's third

law. Changes in momentum for both collision vehicles cancel. There is no gain

or no loss of momentum during a collision. Momentum before the collision

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 6

equals the momentum after the collision and because weight is propor-

tional to mass the final equation can be rewritten as:

W1V1 + W2V2 = W1V3 + W2V4.

On the left is a three car collision. AI

Tools Linear Momentum module below,

calculates that the Chevy Lumina came

into the collision at 27 mph and that the

Dodge had an entry speed of 15 mph.

Equation #1 was initially used to deter-

mine slide to stop, or departure, speeds

for each vehicle.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 7

HUMAN DATA

Medical personnel involved in an accident investigation can provide

valuable injury information that assists in cause analysis. Where possible in

fatality accidents, autopsies should be

performed to determine the cause of death

and record information about the injuries.

Nonfatal injury information is also useful

to the investigator. Location of broken

bones is especially useful when

graphically represented in a skeleton

diagram in a seated position, on left. Injury

illustration could be done for each

occupant to produce a composite occupant

diagram for the vehicle. Injuries will

indicate the direction of crash loading for

the vehicle. Location of bruises and

contusions can also be addressed, since

these injuries can sometimes indicate use

or nonuse of a seat belt or shoulder

harness.

Head injuries are important clues. If an instrument panel, roof pillar,

steering wheel or glass has evidence of a head strike (i.e., blood, skin, hair,

dent, teeth), that spot should be documented. With a specific location for the

strike and the relationship to the occupant’s body, the investigator can evalu-

ate the angle of head impact, body position, and restraint system function, see

below. It is important to remember that an occupants head motion is exactly

opposite to the crash loading direction.

Injury location identifica-

tion from bone fractures

in illustration.

Correlation of head strike information between passengers and vehicle’s interior.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 8

DECELERATION LOADS

In a biomechanical investigation approach, the most important task is

to determine occupant crash loads and probability of serious injury. Other

investigative tasks help the biomechanical investigator to understand physi-

cal relationships in the accident which lead up to either receiving a serious

injury or not. To calculate an average G force crash pulse, the following equa-

tion is used for each axis of occupant travel.

V2

Gavg

=

2gS

In this equation G = the average force on the specific occupant and is

expressed as a multiple of occupant weight. Because of crash dynamics, peak

G figures will typically be twice the average G.

V = velocity change at the major impact, expressed in ft/s

g = acceleration of gravity, 32.2 ft/s2

S = deceleration distance, expressed in ft

The biomechanical investigator should look for clues in each axis (i.e.,

roll, pitch, yaw) for velocity change and stopping or deceleration distance to be

able to determine a unique crash loading for each occupant.

An extreme example of how the different G loading is experienced by

different people decelerating over different distances is found in this illustra-

tion. To understand the concept, consider a long uniform airplane fuselage

that crashes head-on into a cliff at 200

mph with Persons A, B, and C who decel-

erate in the crash distances of 5 ft, 20 ft,

and 45 ft, respectively. A calculation of

the average G-loading experience by Per-

sons A, B, and C would be 266G, 66G, and

29G, respectively. The average G calcu-

lation for passenger C is:

The average forward load on Persons

A, B, and C can be calculated by multi-

Page 9: N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 9

plying each persons weight by each persons average G load. If the weights of

Persons A, B, and C were 200 lb, 170 lb, and 100 lb, respectively, the average

loading experienced would be 53,395 lbf, 11,346 lbf, and 2,966 lbf respectively.

The weight load calculation for passenger C would be:

This airplane example assumed seats and restraints that held through-

out the crash sequence. For such a severe crash, it is not likely for all of the

seats and restraints to remain attached. A seat will fail when its maximum

load capability is exceeded. Assume seats with integral shoulder harness and

lap belts were designed for 25G static loads in the forward direction. Person

C of our airplane example would have a seat where the minimum force before

seat separation can be expected of 25 times their weight of 170 lb x 25G or

4,250 lbf.

The load experienced by Person C was 2,966 lbf average or 5,932 lbf

peak. Thus, the seat for Person C would separate when its peak load exceeded

the design load capability of the seat. The more damaging loads for Person C

will occur later, when he or she undergoes major deceleration.

AUTOMOTIVE LOADS

A vehicle traveling 35 mph sustained two feet of uniform crush. Substi-

tution into the formula reveals the G forces for this accident.

V2 352

Gavg

= = = 9.5 Gavg

x 2 = 19 Gpeak

2gS2 x 32.2 x 2

Federal Motor Vehicle Safety Standard, FMVSS

207 Seating Systems. Under this standard the seat

must be capable of withstanding a force “20 times the

weight of the seat applied in a forward (S4.2.a) or rear-

ward (S4.2.b) longitudinal direction.” Thus if the seat

weighs 30 lb then it must not fail at a level below 600 lb

when calculated as 20 x 30 lb = 600 lb. It is important to

note that occupant weight is not considered. S4.3.2.1

Static force specifies 20 times the weight of the seat back.

Page 10: N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 10

VERTICAL CRASH ANALYSIS

A more detailed analysis will reveal that the G loading will change

throughout the crash sequence. For example, crash loads experienced by an

occupant of an extremely high vertical velocity impact are shown on page 10.

From point A to B, the crash loads are low (typically 2 to 3 G) as the landing

gear deforms. Point B is where the fuselage lower skin contacts the terrain.

Loading from point B to C to D to E is extremely high as the aircraft floor

comes to rest at point F.

If an occupant is sitting on the floor, the loading experienced would have

been points B to C to D to E to F. Note the horizontal line, which is an injury

load threshold (within time duration limits) above which severe injury is ex-

pected. If the occupant is sitting on a seat, the vertical crash loading experi-

enced will rise from point B to C and then drop to point G. Loads do not exceed

point C, as this is the maximum strength of the seat which fails at Point C.

The occupant’s load is about zero from point G to H because the occupant is

basically free-falling from a seated position until contacting the floor at point

H.

Vertical G loading during a vertical

crash.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 11

However, the aircraft floor is just about to come to rest at point F. Thus,

the occupant impacts a nearly stationary floor and the crash loads experi-

enced by the occupant will go from point H to I to J. Unfortunately, occupant

load penetration above the injury zone threshold line would indicate that a

severe injury would be expected for the example occupant. It is obvious that

an average G loading for an entire aircraft is inaccurate and misleading.

Understanding the crash loads on an occupant is not possible without good

information from the crash survival investigation on injuries, restraints, seat

damage, and fuselage damage.

AERIAL CRUSH PHOTOGRAPHY

On-site aerial photographs taken with the aid of a boom and perimeter

grid capture collision data. This system has proven to be extremely useful in

addressing collision dynamics. A brief synopsis of this method reveals its use-

fulness to the accident reconstructionist and biomechanical expert.

Top View of Camera & Grid

Front View of Camera & Grid

Securely place the base against a tire.

Elevate & lower the boom gently.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 12

COMPOSITE

PHOTOGRAMMETRY

At night a westbound pickup

truck was towing a 2700 pound air

compressor. The air compressor dis-

connected from the pickup, crossed

the centerline of a two lane roadway

and struck an eastbound 3,100

pound automobile. The speedometer

of the car was “crushed” and read 55

mph. Delta V calculated at 108 mph.

Below is a composite photogrammet-

ric assembly for this collision. Aerial

photographs of the car and air compressor were placed together in order to

illustrate maximum engagement at collision. A perimeter grid has been placed

around the car. Black markings one foot apart are used for post collision

analysis by permitting lines to be drawn across the photograph for documen-

tation of crush sustained by the vehicle.

Page 13: N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 13

VEHICLE CRUSH DAMAGE

Analysis of photographs reveals that the subject vehicle sustained 6.3

feet of crush as a result of the collision.

Subject Vehicle With Roof Folded Back

Exemplar Vehicle With Pre Crash and Maximum EngagementDriver to Bumper & Air Compressor Illustration

Page 14: N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 14

VEHICLE CRUSH DAMAGE - SIDE VIEW

These CAD illustrations

show the vehicle side view

before and after impact and

the relative position of the

driver inside the automobile.

Post-collision vehicle defor-

mation at right illustrates the

vehicle contact with the

driver. It can be clearly seen

that this collision was not

survivable.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 15

COLLISION SURVIVABILITY

Linear momentum calculations show a closing speed for this head-on

collision of approximately 108 mph and Equivalent Barrier Speed (or EBS) of

59 mph. EBS is when a vehicle impacts a massive barrier which absorbs no

energy of collision. It is a convenient concept to compare the energy absorbed

in crushing vehicles. The National Highway Traffic Safety Administration

annually releases its New Car Assessment Program (NCAP) crash test results

for current model year vehicles. These tests give occupant injury criterion

values for collisions at the 35 mph EBS. In the EBS crash of an exemplar Buick

Century, approximately 2' of uniform crush was sustained.

As previously addressed, an increase in

velocity at impact results in an increase in the

energy of collision. The subject vehicle had an

EBS of 59 mph which results in an increase of

the energy involved in this collision of 184%, or

almost three times as much energy being pro-

duced, as in the 35 mph crash tests. This

massive increase in energy is translated di-

rectly into an increase in the forces making

this collision unsurvivable.

The air compressor has an overall width of 56 inches, 13" less than the

Buick, and weighed almost as much. It approached from the driver's side at

a slight angle of about 8°. The net result of these factors was primary damage

and collision force concentration in an area directly in front of the driver. Force

concentration is shown in illustrations included in this report. The previous

page however, best illustrates the total amount of intrusion into the driver's

side area. Measurements are shown of the distance from the driver to the left

front of the vehicle both before and after collision. These measurements

reveal that the initial 9.2 ft. of vehicle in front of the driver had been crushed

to 2.9 feet. Thus, the area immediately in front of the driver sustained a total

crush of approximately 6.3 feet.

Extremely high EBS loads coupled with massive collision damage con-

centrated directly in front of the driver, made this collision unsurvivable under

any circumstances by the driver of the Buick.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 16

LOW SPEED COLLISIONS

According to General Motors, in 1986, more than 33% of all automobile

injuries occurred in low-speed collisions where the speed difference between

the vehicles was less than 20 mph. It has also been estimated that more than

75% of low-speed collisions resulting in injury are rear-end collisions.

Over the past forty years, there has been a significant amount of research

into the effects of vehicle collisions and resultant occupant movement. However,

the majority of this research concentrates on the effects of high collision speeds,

typically 30 mph or above. Most staged collisions performed by car

manufacturers, insurance institutions (Insurance Institute for Highway

Safety), and the United States Government (National Highway Traffic Safety

Administration) are frontal collisions at speeds of 30 to 35 mph into a rigid

barrier.

The four main types of low-speed collisions are 1) rear-end, 2) front-end,

3) lateral, and 4) side-swipe.

Rear End Side

Front End Lateral Swipe

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 17

In many low-speed collisions, occupants claim common “whiplash”

symptoms such as pain in the neck, shoulders, arms, and low back, despite the

absence of vehicle damage. Investigators are frequently asked whether the

claimed injuries could have resulted from what appears to be a trivial event.

To address this question, investigators must first determine the severity of

the collision and then compare that value to human injury tolerance levels.

IMPACT SEVERITY

Before looking at the ways to determine the severity of a low-speed impact,

it is first important to understand the difference between high-speed and low-

speed impacts. When two vehicles collide at a high speed, they essentially act

like two balls of clay. They deform on impact and remain deformed with little

or no crush energy being released after impact. There is little or no “bounce”

or elastic behavior during the impact. The engineering term for this “bounce”

or elastic behavior is restitution. In high-speed collisions, the restitution

approaches a minimum value of zero and, therefore, when reconstructing high

speed collisions, restitution is usually ignored.

When two vehicles collide at a very low speed, they act more like two

tennis balls. A large percentage of the deformation of the vehicles is elastic in

nature and is released after impact. Only a small percentage remains as

permanent crush damage. In low-speed collisions, the restitution approaches

a maximum value of one. Therefore, when reconstructing low-speed collisions,

the effects of restitution cannot be ignored.

The severity of a collision is quantified by the acceleration (or

deceleration) experienced by a vehicle during impact. Acceleration is speed

change divided by time. If collision duration is assumed essentially constant,

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 18

then velocity change can be used to quantify collision severity. This is a valid

assumption for most collisions where the time duration is approximately 1/

10th of a second. However, in collisions such as underride impacts, the

In general terms, severity of an impact is related to vehicle damage. A

vehicle that has sustained several inches of rear-end crush has experienced a

more severe impact than the same vehicle that has less or even no permanent

rear-end damage. However, there are significant differences between the

relative strengths of different surfaces of the same car, and between the same

surfaces of different cars. For example, the rear ends of two different cars will

not be equally strong, so that two different cars with similar damage may not

have experienced the same impact severity.

BUMPER TECHNOLOGY

Where there is bumper engagement with no damage in a rear or front

impact, it is often possible to determine impact severity from an inspection of

the vehicle bumpers. In many cases, the amount of compression of bumper

isolators can be correlated to the vehicle’s V (velocity change) in a minor

front or rear impact. In non-isolator equipped cars, which are increasingly

more common, the task of determining severity is more difficult. For many

passenger cars in North America, there is often little or no damage after a

minor impact. Some Asian cars in particular have quite robust foam-core

bumpers that are more damage resistant. Attached is a list of some vehicles

equipped with foam core bumpers.

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Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 19

In lateral and side swipe impacts, damage is more noticeable, since body

panels, which are much weaker than bumpers, are involved. Body panels dent

and horizontal scuffs and creases are easily produced with very minor impacts.

Currently in the United States, a Federal safety standard requires that

automobile bumpers keep damage away from car bodies in 2½ mph front and

rear into flat barrier impacts. Damage is allowed to the bumper itself. These

requirements are much weaker than the stronger 5 mph no-damage bumper

rule that was in effect during the 1980 to 1982 model years. Neither strong

nor weak bumper requirements have ever applied to trucks or vans.

Low speed crash tests indicate that many bumpers are built to exceed

the standard and in some cases are undamaged at speeds well in excess of

those set out in the standards.

DAMAGE & SEVERITY ASSESSMENT

Damage sustained by vehicles during impacts varies greatly among

models and manufacturers. Certain vehicles show no evidence that an impact

occurred, even after impacts with severities as high as 10 mph V while others

have incurred structural damage during very low severity impacts. These

differences in vehicle behavior help explain instances where one vehicle will

show large amounts of deformation while the other vehicle will appear

undamaged.

Often, investigators will underestimate the impact severity because no

damage was observed during the vehicle examinations. Similarly,

overestimates have also been made when the vehicles show obvious damage.

Closer investigation into vehicles’ properties will provide insight into

relationships between vehicle damage and the corresponding impact severity.

Results from staged human volunteer collisions is significant. From this

data, it can be seen that the type of vehicle engagement during collision and

type of impact both need to be considered when determining collision severity.

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It should be noted that the severity given in the table represents the

probable maximum severity. This maximum value is only used if there exists

no test data for the same or similar model vehicle, or any other evidence, to

indicate a lower damage threshold.

Collision severity can also be determined from physical evidence at the

scene of an accident, such as vehicle final rest positions and pre-impact

acceleration distances, skid marks, etc. Unfortunately, this physical evidence

will typically not be recorded by police because the accident is viewed as a

“minor collision.”

COMPUTER MODELING

Most computer programs used to determine speed from crush damage

(eg. SLAM, EDCRASH) are written for high-speed collisions and assume a

linear (straight line) relationship between crush and speed. They use vehicle

crush characteristics, or stiffness coefficients, obtained from crash tests typically

performed in the 30 to 40 mph range. In this speed range, for most vehicles,

there is essentially a linear (straight line) relationship between crush and

speed. Consequently, these computer programs give good results for barrier

equivalent speeds of 20 to 50 mph.

When these computer programs are used to analyze low speed collisions

the same linear relationship is assumed to exist at low speeds. The results

obtained from 30 to 40 mph staged collisions are extrapolated backwards.

Results from staged low speed collisions indicate that this assumption is

incorrect. For most vehicles there is not a linear relationship between speed

and crush at low speeds, note the dashed lines above. Therefore, these computer

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programs should not be used to analyze low speed collisions unless great care

is used to modify the vehicle crush coefficients.

HUMAN INJURY TOLERANCE LEVELS

When trying to understand the motion of an occupant subjected to a low

speed front- or rear-end collision it is useful to visualize the occupant as a

simple head-on-a-stick. Depicted below is a rear-end collision.

R o t a t io n

T o r q u eA r m A c c e le r a t io n

Whenever a vehicle is rear-ended, everything moves forward. This in-

cludes the vehicle, the seat, the occupant’s torso and the occupant’s head.

However, there is differential motion. The car and seat move fastest, the torso

initially moves more slowly, and the head moves slowest of all. Consequently,

the occupant suffers the sensation of “sinking back” into the seat, while their

head suffers a rearward rotation (this is the origin of the so-called “whiplash”

mechanism). As the torso sinks back into the seat (but actually moving for-

ward), the seat compresses like a spring. Ultimately, this compression stops

and the torso reaches the same speed as the car.

Thereafter, the seat “unloads” and acts as a damped spring, resulting in

the torso moving forward faster than the car. As a result of this “seat bounce,”

the torso can move forward up to about 1.3 times the speed change that the car

experiences due to impact. Thus, if the car experiences a velocity change of 5

mph due to a rear impact, the torso can end up moving at up to 6.5 mph.

Whether the occupant actually experiences this increase in speed depends on

the lockup behavior of the seat belt, especially the shoulder belt.

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This composite illustrates the motion of an occupant during a rear-end

impact. A potential for injury occurs when the head is fully rotated backwards

[6], known as hyperextension, or fully rotated forward [8], known as

hyperflexion. A properly positioned head rest can reduce the amount of

backward head rotation and hence reduce the potential for head injury.

Once the severity of the collision has been determined, then this value

can be taken and compared to human injury tolerance levels. Results from

daily activities or volunteer exposure to staged low-speed collisions can be

used.

1. Daily Activities: The loads that an occupant was subjected to during

a collision can only be compared to daily activities if the direction, duration,

and magnitude of the loads are the same. For short time durations, less than

a second, human injury tolerance is sensitive to the time duration that the

load is applied. A higher load can be applied without injury if the time duration

is shortened.

If an individual jumps off of a table on to a solid floor, then he or she will

be subjected to a fairly high deceleration on impact. But injuries typically do

not occur because time duration of the impact is of the order of a 1000th of a

second.

Tests carried out with amusement park bumper cars reveal that, during

collisions, vehicle velocity changes as high as 5 m.p.h. can occur. The time

interval of these impacts is comparable to the typical duration of a low speed

automobile impacts.

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2. Volunteer Exposure: Low-speed collisions with human volunteers

have been quantified in the chart below.

It should be noted that in the staged rear-end and lateral collisions,

none of the reported symptoms lasted longer than three days. Several

interesting trends can be observed from their results. In the staged rear end

collisions, symptoms started to be reported at collision severities in the 4 to 5

mph range. In the frontal and lateral staged collisions, symptoms started to

be reported at about 10 mph. Results indicate that the injury threshold level

for a frontal collision is greater than for a rear end collision. This is consistent

with typical real life low speed rear-end collisions where occupants of the struck

vehicle report injuries, but the occupants of the striking vehicle do not.

When using results from volunteer exposure tests, an investigator should

be aware of the following limitations:

1. Most of the staged collisions are bumper to bumper impact and not

override/underride impacts.

2. Most of the volunteers have been male.

3. Most of the volunteers face forward at the time of impact.

4. Most of the volunteers are under 50 years of age.

5. Most of the volunteers are seat belted.

6. Most of the volunteers do not have preexisting conditions.

7. Most of the volunteers are mentally prepared for the impact.

8. It is difficult but not impossible to insure that a volunteer is

subjected to an unexpected impact.

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The table below combines MacInnis Engineering's vehicle damage

threshold and volunteer exposure results. From the table it can be seen that

in a rear-end impact it is possible for occupants to sustain symptoms when the

vehicle has no visible damage.

SUMMARY AND CONCLUSION

Use of results from staged low speed collisions, conducted at known

speeds, it is possible to determine collision severity of a real life low speed

collision. The determined impact severity can then be compared to results

from staged low speed collisions to determine if an injury threshold has been

reached.

Volunteer exposure tests give a good indication of the collision severity

at which symptoms, typically lasting less than 2 to 3 days, start to appear (i.e.

the injury threshold). Because human volunteers are used, the severity of the

staged collisions is typically not increased beyond the injury threshold severity.

Therefore, there is currently little or no data to indicate the relationship between

injury severity and collision severity above the injury threshold.

Accident reconstruction and biomechanical analysis with their respective

mathematical interpretations provide the basis to assess vehicle damage and

calculate injury loads sustained by the occupants.

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Vehicle damage photographs taken at ground level and with the aid of

an aerial boom are important in biomechanical analysis and accident

reconstruction. These photographs permit detailed analysis of crush and

principle direction of force. Post production photographic techniques can be

used to compare the damaged subject vehicle with an undamaged exemplar

vehicle.

An accident reconstructionist and biomechanical investigator should keep

in mind their job is to record facts necessary to mathematically analyze an

accident. Some data or clues are perishable and must be obtained or

photographed early. Each occupant has a unique level of injury and experiences

unique crash loads depending on their location and other accident factors.

The purpose of the biomechanical analysis is to determine means and rationale

for injuries and improve the chances of survival in future collisions.

REFERENCES

1. Daily, John, “Fundamentals of Traffic Accident Reconstruction”, Insti-

tute of Police Technology and Management, Jacksonville, FL, 1988.

2. Boddorff, Thomas C. and Ian S. Jones, “Simple Overhead Photography

Techniques for Vehicle Accident Reconstruction”, S.A.E. Paper No. 900370,

Society of Automotive Engineers, Warrendale, PA, 1991.

3. Baker, J.S., Traffic Accident Investigation Manual, Northwestern

University, Evanston, IL, 1975.

4. AI Tools, AR Software. Trantech Corporation, Redmond Washington.

5. McElroy, Robert C., “Aerial Crush Photography & Analysis For Acci-

dent Reconstruction” Special Problems in Traffic Accident Reconstruction,

IPTM, University of North Florida, 1994.

6. Fox, Roy G., “Helicopter Crash Survival Investigation”, Proceedings of

23rd International Seminar of the International Society of Air Safety Inves-

tigators, Dallas, TX, 1992.