Random Flashcards
What does the “A” value in Crush represent?
The coefficient representing the beginning of plastic deformation.
A = (w)(b0)(b1)/gL
Units = Lbs/In
What does the “B” value in Crush represent?
The coefficient representing the stiffness of the vehicle structure.
B = (w)(b1)^2/gL
Units = Lbs/In
What does the “G” value in Crush represent?
The integration constant which represents the elastic work done to reach a force of “A.” Energy dissipated before permanent damage, aka Restitution
G= A^2/2B
Units = Pounds
What methodologies calculate delta-V?
Crush and Momentum
When must the momentum along both the “x” and “y” axis be considered?
When two vehicles whose angles of engagement exceed 10 degrees
In angled collisions (when using momentum calculations), what are V-1 and V-2 entry and exit signs?
—Based on Heading Angle—
V-1 Entry = 0 degrees
V-2 Entry = Psi (pitch fork looking symbol)
V-1 Exit = Theta (O with line looking symbol)
V-2 Exit = Alpha (A”ish” looking symbol)
When does separation occur?
Occurs when the collision force of the impulse between the vehicles has stopped.
What is the Big Mother Rule?
Delta-V1/Delta V2 = W-2/W-1
Name the different collision types?
Collinear
Central
Central Collinear
What are the five basic pedestrian trajectories in frontal collisions?
Wrap, Forward Projection, Fender Vault, Roof Vault, Somersault
Describe the Forward Projection (Pedestrian Trajectory).
- Usually occurs with a smaller Ped and a normal sized vehicle or an adult Ped and a full size pickup or van.
- PDOF above COM of Ped (Ped COM below the hood line)
- Speed match-up likely
- The second most common Ped trajectory
Describe a Wrap pedestrian trajectory.
- The Ped’s head and torso bend around the hood
- Ped’s COM is above the hood line
- Ped’s body contacts hood, windshield, A-pillar
- Usually a decelerating vehicle
- Close to speed match-up
- Most common Ped trajectory
Describe a Fender Vault pedestrian trajectory.
- Typically involves adults with normal sized vehicles
- With Braking Vehicles: Ped usually struck near corner of vehicle, wraps over fender, falls
- With Non-Braking Vehicles: Ped usually struck near corner of vehicle, slides up hood or fender to A-pillar. Vehicle then decels and causes Ped to fall. Rear wheels can hit head or legs
Describe a Roof Vault pedestrian trajectory.
- Usually high speeds
- Typically Ped’s COM is higher than hood edge line
- Ped is lofted in the air, over the hood/roof
- Ped lands behind the vehicle but may strike roof or trunk
- Usually no braking
Describe a Somersault pedestrian trajectory
- Usually high speeds or contact with lower portion of the Ped’s body
- Normally a decelerating vehicle
- Least common Ped trajectory
What are the three phases of impact for a Pedestrian collision?
- Impact - The Ped is accelerated
- Flight - W/Decel: Ped and vehicle may separate
W/No Decel: Ped may remain on vehicle - Sliding/Rolling/Tumbling: After impact and hitting ground, body tumbles, rolls, and slides to a stop
What does CRASH 3 stand for?
Calspan Reconstruction of Accident Speeds on Hihgways
When is Crush not validated (Hint: There are 9)?
- Roll Over
- Yielding fixed objects that break away
- Impacts other than a horizontal impacts (+,- 15 degrees)
- Severe over/under ride
- Undercarriage damage
- Large moving vehicles - Greater than 10,000 lbs
- Easily moveable objects
- Non-central collisions (i.e. sideswipe)
- Multiple impacts to the same body area
What are the problems with Small vs. Large vehicles in momentum calculations?
- Determination of entry and exit angles
- Small errors in post-collision velocities can have substantial impact on the calculated pre-collision velocity of the smaller vehicles.
- A very small variance in entry/exit angle of the large vehicle can have substantial effect on the calculated pre-collision velocity of the smaller vehicle
What has an affect of the rollover threshold ?
- Load distribution and mass
- Suspension loading/unloading
- Tires
- Multiple Axles
What can be expected with motorcycles both wheel braking?
- A straight rear wheel tire friction mark is normally associated with both front and rear wheel braking
- The application of the front brake provides more stability during braking and yields maximum braking efficiency
- The front tire does not generally leave a TFM
What can be expected with motorcycles both wheel braking?
- A straight rear wheel tire friction mark is normally associated with both front and rear wheel braking
- The application of the front brake provides more stability during braking and yields maximum braking efficiency
- The front tire does not generally leave a TFM
What can be expected with motorcycles rear wheel only braking?
- Will normally leave a wavy TFM
- The locked rear wheel tire has used up all the available friction longitudinally, therefore, the tire has 100% slip laterally and will easily be influenced by the roadway contour/slope, weight shifts, steering input, etc.
What can be expected with motorcycles front wheel only braking?
- TFM will be dark, straight, and short
- Steering geometry will cause the motorcycle to quickly go down.
Big Rig Dynamics when locking the truck’s front tires (steering axle)?
- Loss of steering
- Vehicle will remain directionally stable
- Jack-knife possible
Big Rig Dynamics when locking the truck’s rear tires (drive axle)?
- Jack-knife will occur rapidly
- Generally irreversible
Big Rig Dynamics when locking the trailers tires?
- Trailer slowly swings out
- Generally is reversible if brakes are released
What are the causes of a jack-knife?
- Locking driver wheel of tractor but not trailer wheels
- Sharp steering input puts the tractor but not the trailer into a yaw
- During evasive maneuvers, trailer pushes out the tractor from behind
- Brake lag time and poor timing on brake application from individual wheels
Explain Entry and Exit angles
Entry = From just prior to first contact to first contact Exit = From maximum engagement to separation
What is the Kinetic Energy equation?
KE = 1/2mv^2
What is translation?
Linear motion without rotation
- Must act through the COM
Where is the COM in a passenger vehicle normally located?
- Diagonally from LR wheel to RF wheel and LF wheel to RR wheel (where they cross X)
- 1/3 the height of the vehicle
P=mv
Momentum = Mass x Velocity
Explain Inelastic
- Some kinetic energy is lost (deformation)
- Ex: a “head on” collision between two pieces of soft clay…traffic collision are inelastic
Explain Elastic
- No kinetic energy is lost
- Ex: Two billiard balls in a head on collision
What force does not act on a body?
Cetrifugal Force
“The fake force”
Explain the difference of Mass and Weight
Mass = Amount of matter in a body
Weight = Gravities effect on that mass (weight = mass x gravity)
What is the best method for measuring distance of a straight locked wheel TFM (when calculating velocity)?
- The longest skid mark
What is the drag factor formula?
Drag Factor = acceleration / gravity (f=a/g)
Ex: Negative acceleration rate of 23fps (vehicle slowing), what is f?
-23/-32.2 = .714
What has significant effect on friction values?
- Slope
- Tire Compound (Commercial vs Passenger tires)
- Vehicle braking efficiency (BE)
- Loose material on the surface
What does NOT have an effect on friction values?
- Tire grade
- Tread depth
- Tire pressure
- Surface temperature
- Pavement texture
- Aerodynamic drag
Centripetal vs Centrifugal
Centripetal = “Central Seeking.” Component of force acting on a body in curvilinear motion that is directed towards the center
Centrifugal = The “apparent” force, equal and opposite, to centripetal force
When is the use of a drag sled not appropriate?
- Not appropriate on soft surfaces such as grass, sand, or gravel
“f” adjustments for speed
- A negative .1 adjustment for speeds over 30 mph
- At lower speeds, the vehicle spends a higher percentage of total stopping time in static friction. The coefficient of friction decreases with increased speed
“f” adjustment for slope
Slope = Rise / Run
- Add for uphill slope
- Subtract for downhill slope
- Due to gravity/normal forces
“f” adjustment for tires
- A negative .2 adjustment for commercial tires
- Commercial tires comprised of a rubber that is harder and stiffer than passenger vehicle tires.
- Commercial tires last longer but sacrifice some of the tires ability to adhere to the roadway
- Exposed to higher temps
- Higher psi
Time-G Graph
G on the Y axis
Time (seconds) on the X axis
Has first and second peaks with a trough in between
Piecewise vs CHP Method
Piecewise - Rotation with translation that is not straight
CHP Method - Straight, 0 degrees to 20 degrees equals free rolling, 20+ degrees equals locked wheel
When not using wheel by wheel method to calculate velocity, what do you use?
COM path of travel
What is the formula for velocity?
Delta-D / Delta-t
Change in distance over change in time
What is the formula for acceleration?
Delta-V / Delta-t
Change in velocity over change in time
Formula for weight
Weight = Mass x Gravity
What unit is velocity measured in?
fps
What unit is acceleration measured in?
fps^2
What unit is force measured in?
Pounds
What unit is weight measured in?
Pounds
What unit is mass measured in?
Slugs
What unit is work measured in?
Foot-Pounds
What unit is kinetic energy (KE) or potential energy (PE) measured in?
Foot-Pounds
What are Newton’s Laws?
First - Inertia (Object in motion will stay in motion / object at rest will stay at rest unless acted on by an outside force).
Second - F=ma
Third - Equal and Opposite (For every action, there is an equal and opposite reaction).
What is the basic speed concept?
- An energy equivalent to the work done over the distance under study that is solved for velocity
What are the four tire friction marks?
S.L.I.P S - Side Skid L - Lock wheel I - Impending C - Critical Speed Scuffmark (CSS)
Newton’s First Law
Inertia (i for inertia and i as in 1st)
- Body in motion / body at rest will stay in motion / at rest unless acted on by an outside force
- Example: A car moving in a certain direction, will take an equal or greater force to stop its motion, such as a car moving in the opposite direction with the same force.
Newton’s Second Law
Acceleration
F=ma
1/2m(2A) = 1m(1A) = 2M(1/2A)
Newton’s Third Law
Equal and Opposite
- For every action there is an equal and opposite reaction
Example: When you step off a boat onto a dock, the boat moves. The force that the person uses to push himself forward, ends up pushing the boat backwards in equal, yet opposite manner.
What are the Critical Speed Scuffmark indicators (Hint: there are 8)?
- Less than 20 degrees or 1/3 of wheelbase
- Start thin and widen
- Arced
- Decreasing radius
- Angled Striations
- Rotating Tires
- Outboard rear tire tracks outside of outboard front tire
- Outboard front tire leaves the darkest mark
What is the rule for adding speeds to Critical Speed Scuffmarks?
- Do NOT combine speeds determined from TFMs after the CSS - Can only compare
- Speeds determined from TFMs prior to the CSS can be added using Kinematic equations or combined speed equation
PDOF
Principle Direction of Force
- An indication of the magnitude and direction of the opposing forces acting on a vehicle
- It is the Vecctor sum of the forces acting during a collision
- The direction or line of action along which impulse occurs
Maximum Engagement
- Occurs when the greatest force or maximum penetration to the vehicles occurs
Collinear Collision
- Occurs when the entry angle between the involved vehicles is 10 degrees or less (either head on or rear end) based on COM
Central Collinear Collision
- When the COM of one vehicle acts through the COM of the other vehicle
- - Speed match-up occurs and vehicles separate (exit) at the same velocity
Impulse
- Occurs along the PDOF
- Impulse equals the change in momentum (Delta-P)
- Transfer of momentum between two bodies is described as impulse
- F(Delta-t) = M(Delta-V)
- F(Delta-t) represents Impulse
- M(Delta-V) represents Change in Momentum
What will cause a TFM?
Extreme Acceleration
Extreme Deceleration
Extreme Change of Direction
What is a tire abrasion with smear?
- A smear of tar/asphalt along with abrasions of tire material. They’re long lived and most common on Asphalt-Concrete (A/C) surface
What is the primary marking mechanism on Portland Cement Concrete (PCC)?
Tire Abrasions
- The texture of the roadway abrades material from the tire. Particles ground off the tire appear as rubber particles along the path of the tire.
- The roadway is acting as sandpaper
What is the best method for determining the distance the vehicle traveled if it is rotating during deceleration?
The COM path of travel
When does a Critical Speed Scuffmark transition to a Side Skid?
- When the heading direction of the vehicle exceeds 20 degrees from the COM path of travel of the vehicle (or 1/3 the wheelbase).
Velocity-Time graph
Represents Acceleration
- Velocity on the Y axis
- Time on the X axis
- Equals Velocity / Time = Acceleration
Position-Time graph
Represent Velocity
- Position on the Y axis
- Time on the X axis
- Equals Position / Time = Velocity
Force is measure in?
Pounds
Mass is measure in?
Slugs
Velocity is measured in?
FPS
