There are several types of bumper systems including a honeycomb design, a foam core design, and an isolator/shock absorber/pistons designs, which moves in and out on impact. These bumper designs are intended to reduce damage to the body of the vehicle, not the bodies of the occupants. Bumper specifications are designed for barrier impact, I.e. a solid object, like a wall, which has different dynamics than a car-to-car impact. In actual circumstances, these bumpers may be able to survive a car-to-car impact of 8-9 mph with little or no vehicle damage. (See also Low Speed Impact)
In a rear end collision, the energy from the striking (rear) car is transferred to the front car, propelling it forward until it comes to rest, per Newton's first law of motion. The back of the car seat pushes the occupant forward, as well as lifting them up. The head, not being connected to any part of the seat, remains stationary, which gives the head the appearance of backward motion, relative to the rest of the body and the car seat. As the car comes to a stop, the occupant's body is generally thrown forward. If not wearing a seat belt and shoulder harness, the occupant may strike the steering wheel, dashboard or windshield. If the seat belt and shoulder harness are in use the head will be thrown forward over the torso, while the upper body is restrained to the seat, resulting in a risk of greater neck injury. This has been referred to as "acceleration-deceleration injury" and "hypertension-hyperflexion," but is most commonly known as whiplash.
Whiplash injury to the neck is generally thought to come from hyperextension. In terms of reference, imagine your head as a 10-12 lb bowling ball supported by the neck muscles. At the time of impact it is whipped backward and then forward in a brief time period, often as little as 1/4 of a second.
The body was not designed for the sudden unaware abnormal motions that occur at the time of a rear-end collision. A common myth that is promoted by the insurance industry is that if there is no vehicle damage there can be no occupant injuries. This is completely unfounded. As studies are becoming more sophisticated we are learning a great deal more about low speed injury to the spine. Another common myth is that being struck on a slippery surface (like ice, snow, or wet pavement) causes less occupant injury than being struck on a dry surface. The truth of the matter may be just the opposite. This is simply explained by the fact that road friction is much lower on wet and icy surfaces. When present, friction helps to keep the car from lurching forward when hit.
Accident Reconstructionists generally use a method known as "conservation of momentum" in determining the speed of vehicles at the time of impact. However, when surfaces and low speed are involved, the coefficient of restitution ("friction," for those of us who are not physicists or mechanical engineers) is a much more significant factor in the dynamics of the accident. For an example of this consider two sets of balls, one set is two ordinary billiard balls, the other is two identically sized and weighted balls made of soft clay. When the billiard balls strike each other, they spring apart as most of the original momentum is conserved. The clay balls, however, stop right next to each other, the kinetic energy (momentum) having been expended in the flattening of the shape of the balls where they hit. This is why the surface (in your car's case, usually the bumper) plays such an important role in the impact.
Injuries to occupants in these vehicles depend on many different factors.
- The position of the occupant at the time of impact
- The weights of the vehicles involved
- The road condition (wet, icy, dry) at the time of the accident
- The speed(s) at impact
- The head rest heights and position in the vehicle
- The rigidity of the occupants seat
- Prior awareness of the coming impact
- Seat belts and shoulder harnesses
View information about Whiplash Injuries
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