 |
 |
       |
    |
|
|
|
Shaw Chiropractic Accident Reconstruction and Delta V The staff physicians of Shaw Chiropractic have just completed the forty hour certification course in accident reconstruction (AR) offered by the Texas A&M University Engineering Extension. The program was sponsored by the American Academy of Manual and Physical Medicine and therefore designed specifically for physicians who need to be qualified as experts for legal testimony. The emphasis of the program was directed towards the low velocity impact since that mechanism represents the majority of patients seen in physician offices and because this seems to be the area of greatest controversy. This newsletter will very superficially identify what I perceive to be the major issues which the plaintiff and defendant attorney should understand to determine the applicability of the AR findings. 1. The topic of )V (Delta V) is central to collision assessment. In simple terms, )V is the difference between the velocities (measured as a vector quantity) of an object both before and after an external force acts upon it. For example, a car is in a stopped position and struck from the rear resulting in a post impact velocity of 10 mph. The )V is obviously 10 mph. Similarly, a vehicle struck while moving 10 mph which has a post impact velocity of 20 mph also has a )V of 10 mph. )V is the most significant AR finding when determining injury potential in a motor vehicle accident. To determine )V is simple if all the other components of the formula are present. Unfortunately, this is rarely the case. In fact, with the exception of well designed crash test research the best that can be determined is a range of )Vs. 2. As it relates to injury potential it is also important to determine the acceleration (") measured in gravitational forces (G)(32.2 ft/sec2 or 9.8 m/sec2). To determine the G force we use the formula " = )V/)T (T= Time). This means that the shorter the time the greater acceleration. To make this simple take a car accelerating from a stopped position to a speed of 10 mph in 10 seconds. If the same vehicle reaches 10 mph with a )T of one second the G force exerted on the vehicle will be 10 times greater. This means more injury potential. 3. In low velocity collisions it is important to look at restitution (e). Restitution is a measure of how objects react to one another. For example, compare two billiard balls striking each other and two clay ball striking each other. Even though the clay balls are the same size, shape and mass as the billiard balls the collisions are quite different. The billiard balls will "spring apart", while the clay balls will almost stick together. All bodies have a measure of restitution between zero and one. The more elastic (billiard balls) the closer to one. The more plastic (clay balls) the closer to zero. Higher speed impacts have low restitution values because of vehicle crush. Low speed impacts have high restitution due to lack of crush and other factors such as bumper systems. 4. BEV is the acronym for barrier equivalent velocity and used to test bumpers. Bumpers are made to withstand between 2.5 and 5 mph impacts into a solid barrier. This is much different than two vehicles of varying masses colliding. Now that these four concepts have been defined lets look how they are frequently misapplied. 1. Vc is known as the collision velocity. This is often inappropriately used in place of )V. To appreciate the difference consider the different effects of a school bus striking a mid size car at a Vc of 10 mph and a motor cycle striking the same car at the same Vc. The )V of the mid size vehicle in the bus collision will be high while the motor cycle collision would have a low )V. 2. No damage equals no impact. This defies the physics laws regarding restitution. In fact, no damage suggests greater exchange of energy in the form of )V. Of course, no damage can also indicate a sub-threshold Vc which is the argument used by defense council. 3. While the physics laws are absolute the application in real world situations are much less reliable. This is particularly the case with low velocity collisions in which the absence of property damage, skid marks, gauges and crush make the input data nothing more than an educated guess. 4. BEV values for different vehicles have minimum standards. Some vehicles like a Accord can withstand much higher BEVs. 5. Impact velocity and )V cannot be determined from BEV values although many AR reports suggest this relationship. 6. This is the most important point. Collision dynamics cannot be translated into bodily injury. Factors such as awareness, body type, gender, pre-existing illness, head and body position, position in vehicle, secondary collisions and more need to be taken into consideration on a case by case basis. All research thus far has been designed a properly position occupant. Therefore, the findings of the treating physician are independent of the collision dynamics although a familiarity will help guide them towards examining potentially injured bodily structures. If you want to read more about accident
reconstruction or biomechanics I want
to know |
|
