Vehicle Accident Reconstruction

An accident has happened. Something went wrong. Someone was injured or killed.

What is an accident? After a car accident, how does an expert witness give court evidence as to its causes and help identify who is responsible? As a very experienced forensic engineer, Dave can help you conduct your discovery process. Here is an example, drawn from an actual case, of how he works:

“Through a sample fault-tree analysis, I can provide an illustration of how forensics and expert testimony can shed light on one of the most notorious cases of product liability in recent times: a convergence of failures by two products from separate sources that killed or injured hundreds of people. As independent defendants, each source could potentially blame the other but the first instinct is to blame the product owner and operator. Years of litigation followed and the process and outcome almost bankrupted these two companies.

“I have read much of the commentary around this case including the plaintiff report that for a time was posted on the Internet that included a reenactment of the Tire Company failure on a Famous SUV. Coming at the problem from the perspective of a vehicle dynamics expert, I have yet to read an account that got it right. The car company may understand what actually happened to their Famous SUV (or they may not) but they have little incentive to tell the story.

accident scene: skidmarks on open road“The consequences of what the Car Company and the Tire Company put into commerce included the injury and death of hundreds of occupants. Once understood, the failure mechanism has implications for all rear-drive or all-wheel-drive cars and trucks and for tire makers to avoid at all cost this failure mechanism. Understanding also leads to the possibility of using yaw control as an ultimate solution.

“According to published documents, the Car Company had a vehicle control problem with its Famous SUV. Testing demonstrated that the driver with extreme steering input could roll it over on pavement. The Car Company’s engineers lowered the probability of the problem by reducing the tire pressure from 35 to 26 psi. This reduced the lateral acceleration the tire could sustain. It also increased tire flexing and the consequent heat build up, always a potential contributor toward tire failure.

“At the same time a rival Asian tire maker, came to the Car Company with a proposal for a reduced price/cost tire for their Famous SUV. The Car Company never bought the Asian tire company proposal, but the Car Company used it to pressure their qualified tire suppliers to reduce the price of their tires. The source of the reduction was the elimination of a circumferential fabric belt ply that covered the raw ply ends of the steel belt. The steel belt was in two layers. The steel wire was not circumferential but deployed at an angle and with the removal of the fabric circumferential belt ply the cut ends were uncapped and exposed within the tire.

“All of the Car Company’s tire suppliers except the Tire Company said, ‘No thanks.’ With the Tire Company the failure mechanism was now in place. The failure mechanism, which was quite consistent, was a complete separation between the inner and outer steel belt plies. Because the outer ply was not continuous (not circumferential), driving down the road, the centrifugal strain on the detached outer belt, which included the mass of the tread, was taken entirely by the rubber between the steel wires. The tread and outer belt, when it failed, had to go somewhere.

“By the way, this failure did not air out the tire, but it did lower the coefficient of friction at the tire road interface with either non-tread rubber or steel wire against the road surface.

“Analysis of the numerous real world tire failures revealed that a front tire failure or even a rear tire failure on the small pickup truck that also used this tire were relatively inconsequential. The problem was a rear tire failure on the SUV, and particularly a left rear failure.

“From a vehicle dynamics perspective a front tire belt separation may be a noisy event but it is comparatively benign. The vehicle is still easily controlled and wants to continue straight. The lack of a problem with a rear failure on the small pickup truck that also used this tire is an important clue. The pickup has no structure close to the shedding tire tread that can get in its way.

“On the other hand, the Famous SUV has the end of its structural rocker just ahead of the tire. If the belt breaks near the top of its rotation, its free end is driven forward and down the front of the wheelhouse. When it encounters the narrows at the rocker, if it balls up it can momentarily stop the tire’s normal rotation. The tire will rotate momentarily about this new fulcrum until it reaches full jounce (compression) where it stops.

“When you stop or lock up a rear drive axle tire, because it communicates with the other rear tire through the differential, the other tire speeds up. With one rear tire stopped and the other spinning above road speed, all rear directional control is lost. The engine and transmission are also involved but do not substantially change the scenario. With a knowledgeable expert forensic engineer on the scene, potentially trace evidence could have been collected to collaborate this accident reconstruction.

“The driver now has to deal with a severe roll transient and no rear axle directional control. This is virtually the classic “bootleg turn” perfected by Prohibition-era bootleggers trying to outrun the revenuers. At about 50 mph, lock up the rear tires with the handbrake, tweak the steering and the car will quickly rotate 180 degrees. Now pointed in the opposite direction but going 50 mph backwards, the driver applies power and the car comes to a stop and then accelerates off in the new direction. Rear tires locked or spun up is technically a meta-stable equilibrium. For all practical purposes it is unstable. It is analogous to balancing a broom handle on the end of your finger.

suv rollover“In the case of the Famous SUV, at the point that it is sideways it is most vulnerable to rolling over rather than continue toward pointing in the opposite direction. One of the plaintiff attorneys in the celebrated case hired a testing firm to simulate the tire tread separation and mounted this tire on the left rear of the Famous SUV. The vehicle was instrumented for yaw velocity and directional sideslip and a substantial outrigger was added to stop roll over.

“In the testing firm’s report the vehicle was accelerated in a straight line, on dry pavement, to 45 mph, at which point the tire tread came apart and the vehicle began to rotate. As shown in the data stream, it rotated to 75 degrees in 3 seconds at which point the data stream terminated. Turning the page of the report the Famous SUV was now pictured parked on its roof. The outriggers had failed to keep the vehicle from rolling over. Could even a trained driver stop the vehicle from rotating? Having done many bootleg turns in the days I was responsible for the Vehicle Dynamics Test Area at the General Motors Proving Ground, I doubt it.

“One can generalize that this tire failure mechanism can cause any rear drive vehicle to spin through 180 degrees. If there is vulnerability to rolling over or if a rear tire hits a curb or imbeds in soft earth the vehicle can roll over. This tire failure mechanism should be avoided at all cost.

“At about the same time as the Car Company was experiencing this tire failure mechanism, yaw control was coming into use in passenger cars and trucks. Within a few years the statistical data for rollover accident reduction with yaw control became so compelling that yaw control was made almost universal and the NHTSA ultimately made it mandatory.

“We don’t know if yaw control in its current form would eliminate the roll over consequences of this historic tire failure mechanism but it certainly could. Yaw control has enough control authority to lock any one of the four tires. If it sensed that one rear tire was locking and the other speeding and that the car was starting to rotate, it could by appropriate braking action with the front tires keep the vehicle on the driver’s intended heading. Yaw control would be relying on its much higher control frequency as compared with the rotation rate of the car. The driver might experience a brutal ride down for a few seconds but would continue unharmed.

“All of this could have been determined by a properly trained an experience forensic engineer and expert testimony could have been given that would, I believe, have influenced the duration and outcome of the case.”

– Dave McLellan

“Dave helped us steer the case into a better direction.”

“Your work on the case was invaluable. I had an incredible team of world-class experts but once I started talking to you and got back to them it was kind of “oh, yeah…wonder why we didn’t see that….” You very quickly sourced the problem, and helped us steer the case into a better direction.”

– Marcus Viles Esquire of Viles and Beckman LLC.

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