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COLLAPSING VEHICLE SEAT BACKS

With the advent of the three point seat belt harness, airbags, anti-lock brakes and other technological advancements providing drastic improvements in front, side and roll over occupant protection, attention has now focused on the vulnerability of front seat occupants in moderate rear impact collisions. The seriousness of this problem was sufficient to attract media attention, culminating in two recent feature articles on "60 minutes" and Marketplace".

It is estimated that approximately 50,000 rear impact injuries occur per year in the US alone. Eleven hundred of these are fatal. Another 1,600 are life threatening injuries such as brain damage, burns and paralysis. The problem occurs when a vehicle is struck from the rear at moderate speeds, often between 25 and 32 kilometres per hour, causing the seat back to collapse in front bucket and split back seats. The result creates an avenue which may allow occupants to be ejected even though they are using seat belt restraints. When a seat back collapses, current seat belt systems are rendered virtually useless. This allows the occupant to slide rearward under the seat belt then they are either ejected from the vehicle or forced head first into the rear seat back, exposing them to possible head or spinal injury.

In the event that the driver is not severely injured, a secondary, and often equally serious effect in the driver's case, is a loss of control of the vehicle since he/she is now in a horizontal position. This exposes the vehicle to a potential and otherwise unavoidable secondary impact.

The problem is magnified by the fact that seat backs virtually always collapse at impact speeds as low as 50 kilometres per hour, with reported serious incidents occurring at less than 30 kilometres per hour. This data was verified by North American and Japanese manufacturers' integrity testing. These rear vehicle barrier crash tests inadvertently revealed that in virtually 100% of the test cases involving speeds of 50 kilometres per hour, or greater the vehicle experienced seat back failures.

It would not be unfair to say that although the problem has only recently surfaced in the public eye, it has been an industry and regulatory concern for the last two decades.

Manufacturers defend their product by stating that it meets and often exceeds all federal motor vehicle safety standards (FMVSS). However, critics say that the FMVSS standards are inadequate.

Typically, the standard committee is made up of a variety of industry representatives with a bias towards their existing product. Compounding the problem is the fact that the standard has not been revised in about twenty-five years. Consequently, it could be argued that the standard and current technology are drastically underdesigned. Furthermore, in product liability cases, advocates will argue that simply meeting the standards is not good enough.

Some manufacturers defend their design with the "yielding seat theory". They take the position that the seat was designed to yield (or collapse). A yielding seat back in a rear end collision absorbs energy and reduces whiplash injuries because the head and upper torso are allowed to move together as a unit, reducing the possibility of extension of the head and neck. This defense is analogous to designing a seat belt to yield in frontal impacts to reduce head and neck flection. This theory is flawed. Good crash worthy vehicles will yield in areas exterior to the occupant compartment, absorbing the majority of the collision energy at the vehicle extremities (bumpers, fenders, etc.).

Indeed, there is even debate on the subject within the engineering community of the North American manufacturers. One design engineer employed by a major North American manufacturer admitted that he did not intend the seat backs to yield. In fact, an internal memo dated as far back as 1968 predicted the danger of seat back collapse.

Independent studies conducted in Canada for Transport Canada have concluded in general that seat backs are drastically underdesigned and penalize occupants in rear impacts.

Currently, the only major manufacturer seriously addressing this situation is Mercedes Benz. Mercedes test their vehicles with test dummies to simulate real world conditions. Their test results have revealed three critical areas in seat design.

  1. Pivot (connection)
  2. Seat (adjustable) track
  3. Floor fastening

Their result is an enhanced design with a wider, stiffer frame providing rigidity in both bending and torsion (i.e. twisting).

Federal agencies have been slow to respond with rule making. Politically imposed budgetary constraints appear to have stifled US regulatory advancement in this area. Unless movement is made in the National Highway Traffic Safety Administration (NHTSA) in the US, it is unlikely that Transport Canada will act on its own.

 

New generation seat belts that are an integral part of the seat and seat back may contribute to seat belt effectiveness with respect to this problem although they are not designed to protect against rear impact.

It is unclear exactly how many serious injuries occur from seat back collapse. Manufacturers in general appear to have taken the position that the problem is not severe enough to justify design improvements. Rather, the economics of the situation have inclined manufacturers towards settling claims out of court.

However, a recent precedent setting case, Cobb vs G.M., has led to generous awards to victims of this design deficiency with the result that it might soon ultimately be more cost effective to make a safer seat than to defend these cases in court.

Our best estimate is that it would cost less than $50.00 per car to strengthen the seats sufficiently to eliminate this problem.

 
The information contained in this web site is intended for marketing purposes only. It is not all-inclusive, and does not fully describe the many and varied services that the company provides, nor does it completely describe the education, training, skills, or expertise of our staff.

 
 
 

Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com