Case Study: Mechanical Forensic Analysis – What Went Wrong?
A few years ago, there was a terrible accident involving a single motorcycle traveling on the freeway. The cause listed in the police report: Engine & rear wheel locked up.
Shortly after the accident, Synthesis was contacted and asked to perform a mechanical forensic analysis and determine what caused the rear wheel to lock up.
The case was not straightforward. Though the motorcycle was pretty new, it was equipped with some factory performance parts installed by a dealer. A lawsuit put the dealer at odds with the manufacturer in resolving blame. Was the accident a result of a manufacturing defect? Or was it an issue with installation of the performance parts?
Synthesis was contracted to be the unbiased voice, to perform a mechanical forensic analysis and to work with all the concerned parties in finding the root cause.
We had the bike delivered from the police impound to an uninvolved motorcycle dealer, where it would be torn down in an open meeting of all involved parties. I was impressed with the group that came. I’m not sure exactly what I expected, but a real tone of concern filled the conversation, and everyone was professional and polite. There were representatives from the manufacturer’s engineering and legal departments, from the involved dealership, and from the plaintiff. All were there to oversee the tear-down and failure analysis.
The tear-down proceeded by consent. We stated what we were going to do in each step, asked the group if they agreed, then proceeded. As each step unfolded, everyone photographed what they wanted, asked questions, et cetera. It was a meticulous process, but we needed to be very thorough. Surely it the slowest tear-down the mechanic had ever done.
The first steps verified that there was metal debris in the oil, both aluminum and steel. The clutch was still operational and did indeed disengage the driveline from the powertrain. (For some reason, no one had tried this. The police cut the drive link to move it, though simply pulling the clutch lever – as damaged as it was – would have released it.)
Side Note: I am an experienced rider. As such, I’ve experienced emergency situations on the bike, like having a rear wheel break traction at high speed in a corner. Never a lock-up, mind you (except deliberately in performance riding classes), but if the rider of this bike had simply pulled in the clutch, he would have coasted smoothly to a stop rather than spending weeks in the hospital. This doesn’t change the fact that something went terribly wrong inside the bike, but it does emphasize the need for education and riding skill. If you ride, please, learn how.
The next step in forensic analysis focused on the transmission and the engine top end. Still nothing, so we pulled the motor. As we began to take it apart, we found a gear that didn’t quite align, and a shaft that showed distress. We found catastrophic failures like a shaft key that sheered right in half (shown in one of the images), and bolts that had broken. Now we were getting to something. After several more steps, we finally split the case and found the pieces that locked the engine.
Of course, there was a bit of excitement in the air. Now we could see how it stopped, a violent, sudden stop. And the head scratching of “why” really began.
Through a process of analyzing the various witness marks, looking through the debris and deductive reasoning, we came to a thesis about what happened. A theory of unfortunate events, but the theory did not give an absolute answer to the root cause “Why” question. We needed some additional information.
Next stop, the metallurgical lab.
As a final step in determining the real root cause, we got a little more personal with some key parts. At the metallurgical lab we were able to section a couple key parts to see what was inside, and to measure metallurgical properties.
The failure analysis was fascinating — actually, forensic analysis in general is intriguing. I wish I could talk more about details and show lots of photos, unfortunately, that is not appropriate.
Suffice it to say we found the root cause. It certainly ended as a sudden failure, but it was some time in the making. The witness marks told a story of hundreds, maybe thousands of near misses before finally failing completely. As with many aspects of engineering, there were several factors that came together.
Our job in forensic analysis is to pull the tidbits of evidence together, and recreate the story of how the events unfolded, leading to the final tragic result. Failure analysis is going back to find the real root cause. It is to tell the story of what happened to start the chain of events.
In this case, like most cases, the evidence did not immediately point to the cause. This motorcycle had a lot of damage inside — most impressive occurring immediately before or as a direct result of the final engine seizure. It is important always to follow the chain in determining the cause. That takes a diagnostic skill, an attention to small details, and a mind to pull it all together. These are skills we offer.
Well, I can’t just leave the story hanging like that, so here’s what I can say. The engine stopped suddenly because the balance shaft interfered with the crankshaft — an event considered mechanically impossible. Direct impact brought the engine to an immediate and violent halt, which bent shafts, broke bearing races, sheared keys and much more. Yet, the cause was not in the sudden destruction, it was somewhere else.