Carl F. Thelin

ASSESSMENT OF COLLISION NECK INJURY

Frequently someone asks me how I evaluate a claim of neck or back injury due to a collision from the rear in an automobile. There are several factors that matter:

          THE EFFECTIVE CRASH SPEED

The crash energy must be absorbed in the plastic crush of vehicle components in order to limit the forces applied to the occupant. That energy is related to the closing speed and the mass of the vehicles. The simple equation for the exchange of momentum will suffice to estimate the change of speed within the subject vehicle. The change of speed, called Delta-V, is not by itself the only measure of injury potential. A Delta-V of 30 mph may take place within two feet if the car crashes head first into a fixed barrier -- and that will hurt. If you apply your brakes and stop your car within 150 feet that is a normal stop -- and it does not hurt. So, the real measure of injury potential is force. Force is proportional to the acceleration (or deceleration in the frontal crash). The acceleration is proportional to the quickness of the change of speed, inversely proportional to the duration of the collision.

          The stiffness of each vehicle affects the quickness of the crash event

The duration of the crash, (the quickness) is increased by the crush and override that occurs. This does not apply to rubbery (elastic) deformation because of the rebound effect.

The quickness of the crash is reduced when much crushing of the front of the striking vehicle and the rear of the struck vehicle occurs. When the striking vehicle over- or under rides the struck vehicle that also increases the distance over which the exchange of momentum takes place. 

However, it is NOT a good thing when the crash energy is absorbed by collapse of the front seatbacks. The force applied to the occupant comes from any contact with the vehicle. In the rear crash, this is primarily the force applied to the occupant

Average acceleration during a uniform crush pulse
		Closing speed (mph)
Crush – inches		3		5		10		15		30
3		2.40		6.68		26.71		60.09		240.37
6		1.20		3.34		13.35		30.05		120.19
9		0.80		2.23		8.90		20.03		80.12
12		0.60		1.67		6.68		15.02		60.09
24		0.30		0.83		3.34		7.51		30.05

  

THE VEHICLE STIFFNESS AFFECTS THE QUICKNESS OF THE CRASH EVENT

The duration of the crash, (the quickness) is increased by the crush and override that occurs. This does not apply to rubbery (elastic) deformation because of the rebound effect.

The quickness of the crash is reduced when much crushing of the front of the striking vehicle and the rear of the struck vehicle occurs. When the striking vehicle over- or under rides the struck vehicle that also increases the distance over which the exchange of momentum takes place. 

However, it is not a good thing when the crash energy is absorbed by collapse of the front seat backs. The force applied to the occupant comes from any contact with the vehicle. In the rear crash, this is primarily the force applied to the occupant’s backside from the car seat cushion and seat back.  See below.

Safety researchers have long discussed the notion of sharing the burden of crash energy management. Even if the front vehicle is a stiff pickup truck, a well-designed front structure with good crash energy management will benefit the occupants of both vehicles.

THE TYPE OF SEAT UPON WHICH THE OCCUPANT WAS SEATED

The seat may be the most important factor in evaluating how astronauts withstand very high accelerations and forces during the liftoff phase of their ride.

The forces are applied uniformly across their entire rear surface, including the back side of their legs.

The automobile seat cannot be shaped that well. The back of the seat should not deform so much that the occupant slides up and back during the crash. What deformation that does occur will be less harmful if the seat is well-cupped and the head restraint is wide and tall so that the occupants head will not ride above it and suffer next hyperextension. A typical sedan rear seat may be relatively stiff but few have tall head

restraints.

A good seatback will flex rearward in equally on both edges. Many small imported cars and most domestic cars with individual front seats lack this characteristic. The seatback is attached to the seat frame only on the outboard side, where the seat back angle adjuster is. The inboard edge of the seatback will deform more than the outboard side, producing a ramp-like effect that allows the occupant to slide toward the center of the car as he goes back and up along the seat back. This increases the likelihood that the head will miss the support of the head restraint.

There are many older passenger cars, vans and pickup trucks that have seats without any head restraint. Many also have very low seatbacks. Older large GM cars have the top of the front seatback below the shoulders of an average sized male driver. The older vans had very low seatbacks, as do most folding seatbacks in hatchback sedans and coupes, and station wagons.

WHERE WAS THE OCCUPANT LOOKING?

Yes, that innocent question does have a lot of meaning. If the occupant is looking to the side when the crash occurs there is a higher probability of a neck injury even in a low-energy crash. The reason is that the head will twist the neck. When viewed from the side, it is clear that the center of mass of the head is located ahead of the spine. If the occupant turns his head to look to the side, the center of mass appears to be laterally displaced relative to neck, increasing the twisting moment applied to the spine during a rear collision.

So -- ask the occupant:

Where you looking into a driveway or a cross street when you were hit from behind?

Or ask:

During a rear collision, the seatbacks act like a spring board. Many occupants recall striking the steering wheel or instrument panel. They think that is the only result of being struck from behind. The initial rearward pulse is so short in time that they cannot recall it. Never the less, the rebound can be injurious even though the second crash energy is low.

Many rear collisions are followed quickly by a frontal collision when the rear-impacted vehicle, without brakes applied, is propelled a few feet into the back of another vehicle ahead. Now the springboard effect is additive to the effect of the frontal low speed collision. The occupant may complain of many injuries, due to head impact with the instrument panel or steering wheel. An abdominal or lumbar spine injury may be caused by the lap belt, when the occupant slaps into it while rebounding.