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FORENSIC ENGINEERING PRINCIPLES OF

MOTORCYCLE ACCIDENT ANALYSIS

By Jerry S. Ogden, MS, PE (NAFE 561F)

Ogden Engineering & Consulting, LLC, Littleton, Colorado

TOPIC OVERVIEW• Friction values and braking efficiencies

for motorcycle accidents.

• Motorcycle speed from rider/passenger ejection

• Motorcycle speed from rotational mechanics

• Motorcycle speed from fork deformation and vehicle damages

• Conventional speed analysis

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MOTORCYLE FRICTION

• BRAKING EFFICIENCY:

• REAR ONLY BRAKING = 40%-50% of full roadway friction (load dependent)

• FRONT ONLY BRAKING = 60% to 80% of full roadway friction (load dependent)

• FRONT/REAR BRAKING = 100% TO 130% of full roadway friction (motorcycle specific)

Total Friction % as a Function of Locked Brake Application

0

20

40

60

80

100

120

140

Rear Front Front/Rear

Baxter min

Baxter max

Limpert

Obenski

Hurt

NWTI

% o

f to

tal F

rict

ion

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Motorcycle Sliding on Side

Motorcycle Side SlidingMotorcycle Type Deceleration [g's]

Motorcycle speed 30 mph 50 mph 65 mph

Heavy, pavement 0.50 to 0.55 0.40 to 0.50 0.35 to 0.40

Moped, pavement 0.75 0.65 0.60

Bicycle, pavement 0.80 0.70 0.65

With fairing 0.30 to 0.40

Without fairing 0.35 to 0.50

On grass 0.90 to 1.10

Reference 3: Limpert

Motorcycle Side SlidingMotorcycle Deceleration [g's]

Asphalt 0.45 to 0.58

Gravel 0.68 to 0.79

Grass or hard soil 0.70

Sand 1.50 to 1.65Reference 13: Day, Smith

Motorcycle Side SlidingMotorcycle Deceleration [g's]

Asphalt, lightly scratching 0.30 to 0.40

Concrete 0.43 to 0.53

Crash bars only 0.20

Oil contamination 0.20James V. Ouellet, "Reconstruction of Motorcycle Accidents," Special Problems in Traffic Accident Investigation, IPTM April 1984.

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Motorcycle Side Sliding as aPercentage of Roadway FrictionMotorcycle moderate scratching/gouging into roadway

Speed [mph]

Surface f [g's]

Distance[ft]

Deceleration [g's]

25 0.82 36 0.54 (66%)

32 0.75 66 0.49 (65%)

39 0.79 96 0.51 (65%)

42 0.78 108 0.59 (76%)

48 0.82 135 0.62 (76%)

51Ave: 39.5

0.90Ave: 0.81

165Ave: 101

0.55 (61%)Ave: 0.52 (64%)

Reference 7: Baxter

Fatal rider errors• Rear-Brake-Only application during emergency

stopping.– Results in skid that is arced (short skid) or serpentine (long

skid mark).– Easily causes motorcycle to fall to side, potential for high-side

accident event.

• Hard Front-Brake-Only without experience.– Results in short, scalloped skid as bike falls to side.

• “Lay’er down, Charlie”.– Rider lays motorcycle onto side as an avoidance instead of

braking with front and rear brake.– In an attempt to avoid the accident, the rider “just created

one”!

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Stopping distance Comparisons

Speed Rear

Only(40%)

Front Only(60%)

Side

Slide(0.4 g)

Front

and Rear(0.80 g)

25 65 ft 43 ft 52 ft 26 ft

35 128 ft 85 ft 102 ft 51 ft

45 211 ft 140 ft 169 ft 84 ft

55 315 ft 210 ft 252 ft 126 ft

Conventional Methods• Conservation of Linear Momentum

– Determine post-impact velocities of vehicles.– Determine approach and departure angles.– Solve for pre-collision velocities.– Best for collisions producing less than 180 degrees

rotation of struck automobile.

• Conservation of Energy– Determine post-impact energy losses of vehicles to

rest locations.– Determine dissipated energy/work to damage auto

and motorcycle.– Determine pre-impact speed of one vehicle by other

means.– Best for collisions that are co-linear, or when the

struck auto is stopped or nearly stopped at impact.

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Rotational Mechanics

• Vehicle struck by motorcycle has accurate measurable rotation.

• Vehicle struck by motorcycle is traveling slow or stopped, or nearly perpendicular/ parallel (co-linear) at impact.

• Vehicle weights/masses, dimensions and yaw moment of inertia of automobile can be determined or are known.

YAW MOMENT OF INERTIA•Three separate methods for determining YMI. Two methods, Limpert and Garrott, only determine YMI for the vehicle at curb weight and without load. Garrott is the method used to determine YMI in Expert Autostats. Neptune is the only method that permits the user to add additional “load” to the vehicle, thus having a more accident specific value to YMI.

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Velocity Change from Rotational Mechanics

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Finally, for the automobile, the velocity change is:

•Method requires knowledge of following data:

•Mass/weight of motorcycle and auto•YMI of automobile (stats or calculated)•Angle of rotation of vehicle from impact•Normalized rotational friction for automobile post-collision motion (chart next slide)

Rotational friction is the full roadway friction adjusted by the “normalization” factor, n

Normalized Rotational friction

Reference 2: Keifer, Conte, Reckamp

40 deg

n

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40.5 deg

Reference 9: Adamson, Alexander, Anderson, Aronberg, Robinson, Johnson, Kinney, Burkhead, Sallmann, McManus.

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Motorcycle Fork and Vehicle Damage Analysis

•Impact results in damage to front forks of motorcycle and indentation to vehicle at contact area. Analysis approach:

•Motorcycle wheelbase reduction analysis methods.•Combined wheelbase reduction and vehicle damage depth analysis.•Work/Energy Theorem and Force Deflection analysis using Conservation of Energy to determine speed.•Work/Energy Theorem and Force Deflection analysis using Conservation of Momentum and Restitution to determine speed.

Fork DeformationARC-CSI 2008 Conference testing empirical formula

•Separates “hard zones” from “soft zones” at vehicle contact area•Accounts for wheelbase reduction and vehicle damage •Not effected by wheel type; i.e., spoke wheels versus cast wheels

Reference 16: Bartlett

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CAARS 2004 Conference testing empirical formula•Separates “hard zones” from “soft zones” at vehicle contact area•Accounts for wheelbase reduction and vehicle damage •Not effected by wheel type; i.e., spoke wheels versus cast wheels

Reference 16: Bartlett

Reference 9: Adamson, Alexander, Anderson, Aronberg, Robinson, Johnson, Kinney, Burkhead, Sallmann, McManus.

*Reported motorcycle impact speed was 46.0 mph from testing

Note: The Severy Empirical Formula of the early ’70’s was intentionally left out of this presentation. However, fork deformation calculations were completed using the Severy formula in the Math Appendix that is part of this presentation. Severy in every example dramatically under estimated the motorcycle impact speed and velocity change of all examples in the appendix.

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Work/Energy and Force Deflection Procedures

• Measure or approximate vehicle damage profile, and motorcycle WB reduction.

• Force deflection/damage analysis of vehicle dissipated energy by damage as well as impact force.

• Apply Work/Energy to motorcycle wheel base reduction (Fcar=Fmc by Newton 3).

• Calculate velocity changes and impact speed using Conservation of Energy, or Momentum/Restitution analysis

Reference 4: Ogden

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WORK/ENERGY THEOREM

NEWTON’S THIRD LAW OF MOTION

Where, L = the reduction in wheelbase to the motorcycle from the impact

Reference 4: Ogden

CONSERVATION OF ENERGY STATEMENT:

MOMENTUM/RESTITUTION STATEMENT:

FORCE DEFLECTION FOR VELOCITY CHANGE:

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EXAMPLE VARIABLES

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EXAMPLE SOLUTION SET:

COLLISION SPEED OF MOTORCYCLE FROM TEST #8 OF REF. 9, WAS 46.0 MPH

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Motorcycle Speed from Rider/Passenger Ejection

• Most models can be applied to bicycle and pedestrian analysis.

• Those based upon Vault and Uniform Projectile Motion (UPM) are most accurate.

• Speed determined by ejection to REST location of rider/passenger, not to first ground contact.

• Tend to underestimate motorcycle speed, or provide a “minimum” speed for motorcycles.

Vault Analysis:

Where,D= Distance rider/passenger thrown to rest location, as opposed

to distance to first ground contact by convention.h= distance traveled vertically from seat to ground at rest

(-h if landing is below seated height, +h if landing is above)= Ejection angle from motorcycle, ranges from 5-15 degrees

ANALYST MUST KNOW OR DETERMINE:•THROW DISTANCE•EJECTION ANGLE•CENTER OF MASS FALL HEIGHT

Reference 5: Fricke

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UPM/Limpert Analysis:

Reference 3: Limpert

Where,

Dmax= total horizontal distance traveled by rider/passengerhmax= maximum vertical height traveled by rider/passenger,

adjusted for takeoff and landing heights either up or down; +hmax if landing is below takeoff height, and hmax if landing is above takeoff height.

= ejection angle of rider/passenger; Limpert ranges 10 to 15 degrees

EXAMPLE VARIABLES

EXAMPLE SOLUTION (Vault)

COLLISION SPEED OF MOTORCYCLE FROM TEST #4 OF REF. 15, WAS 46.0 MPH

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EXAMPLE SOLUTION (UPM)

COLLISION SPEED OF MOTORCYCLE FROM TEST #4 OF REF. 15, WAS 46.0 MPH

General Trends for Analysis Methodologies

•Conventional methods such as Momentum and Energy produce reliable results for motorcycle collisions that fit the criteria. Range variables to increase confidence interval.•Rotational mechanics applied to motorcycle collisions produce reliable results for collision with rotation of the struck vehicle. Range angles of rotation and/or normalized friction for increased confidence interval.•Fork deformation along with vehicle intrusion produce reliable results for collisions where motorcycle wheel base reduction and maximum intrusion into the vehicle are known. Range damage depth and/or wheelbase reduction for increased confidence interval.

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General Trends for Analysis Methodologies

•Work/Energy and Force Deflection principles applied to motorcycle impacts where vehicle damage profile, stiffness data and motorcycle wheel base reduction are known, produce reliable results. Vary restitution for increased confidence interval.•Conservation of Energy and Momentum/Restitution methods produce reliable results. Range friction rates for motorcycle side sliding and vehicle post-collision motion for increased confidence interval.•Uniform Projectile Motion and Vault analysis of ejected rider and/or occupant must use total distance to rest of ejected, and produce minimum speed results when ejection angles range from 5 to 10 degrees.

Learning Assessment Question #1:

Name two types of analysis methods presented that produce reliable speed estimates for motorcycle accidents when the required data for that method is available.

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Learning Assessment Question #1:

•Conservation of Linear Momentum•Conservation of Energy•Rotational Mechanics•Fork Deformation/Vehicle Damage Analysis•Work/Energy and Force Deflection Analysis•Momentum/Restitution Analysis

Learning Assessment Question #2:

What analysis approach produces the lowest correlation to motorcycle speed of the methods presented, and should be considered a “minimum speed” analysis?

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Learning Assessment Answer #2:

•Analysis of rider/passenger ejection from the motorcycle.•Uniform Projectile Motion and Vault analysis of rider/passenger ejection.•Severy Fork Deformation Analysis