Bruce A. Currie, P.E., Tire Expert
The primary function of any engineer is to solve problems. Most people do not realize that tires are complex products made with various components and compounds. The compounds in tires are continuously changing with time. The engineering analysis of a failed tire in the “forensic world” differs from an analysis conducted by engineers in the “real world” tire manufacturer industry. In both “worlds,” the first step is to collect and analyze all the facts related to the tire failure. The next step that is taken in the “real world” is to cut up and take samples from the failed tire to analyze the materials and compounds, and then compare the test results to what would be expected behavior. However, in the “forensic world,” this step cannot be done since it is a destructive process that would hinder further analysis.
In the “real world,” the tire engineers and chemists will review the facts surrounding the tire failure with the test results from the samples taken from the tire. At this point, scenarios can be developed to explain the tire failure. In the “real-world,” project tires would be built and tested to duplicate the failed tire, a process that cannot be done in the “forensic world,” unless you have a tire manufacturing facility at your disposal.
In the “forensic world,” experience in tire failure analysis, construction, and compounding is vitally important. In most cases there are aspects that will support all parties. Only very few tire cases are either the result of a definite tire failure, such as a missing steel belt, or a tire failure due to a puncture. The tire failure analyst must be honest and express to the client what the case facts indicate and the strength of the case solely dependent upon the tire failure.
The following is an example of a case where the determination of the role of the tire itself was at issue.
Case Description: A passenger vehicle traveling at 60 miles per hour exits the highway to the right and strikes a concrete culvert. The right front tire is flat, and the wheel has a 6-inch dent in it. The right front tire does not have run flat damage but has a quarter inch hole in the inside shoulder.
Analysis: Since the right front tire was flat but did not have run flat damage, it means the tire lost air rapidly just prior to leaving the roadway. In most cases, if a tire has a slow leak and continues to run on the road at 60 miles per hour, it will have severe run flat damage within 1 mile. The right front tire going flat puts drag on the right front wheel position, causing the vehicle to pull to the right and run off the highway. Impact damage to a tire wheel assembly is a continuum of damage to the tire and/or wheel. A flat tire will flex under the impact and transmit the force to the wheel. The tire will have little damage, but the wheel will be severely damaged, as was the case with this accident. A fully inflated tire will absorb the impact, protecting the wheel. Many times, a radial passenger tire will have a split in the liner ply between the radial body ply cords resulting in a sidewall bulge or sidewall blowout, but the wheel will have little or no damage. Between these two end points, the impact to a tire/wheel assembly can cause damage of varying degree to the tire and wheel.
In this case, the quarter-inch hole in the inside shoulder of the tire was the cause of the rapid air loss which translated to drag on the right front wheel position, causing the vehicle to pull to the right, exit the roadway, and strike the culvert. The tire was not at fault, as the cause was a road hazard impact.