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The Case of the Fragile Fork
Kenneth Russell, Professor Emeritus, MIT -- Design News, October 5, 2008
Bicycling is fun, inexpensive, cheap, non-polluting and provides nice open-air exercise. Most mishaps are a nuisance rather than being life threatening, as may be the case with motor vehicles.
The Scene of the Crime
Serious accidents are possible. In the current case the front fork and wheel on a child's bike detached. The rider took a nasty header, flying over the handlebars and smashing his face into the pavement. The injuries were serious enough to inspire litigation and the use of experts.
The Investigation
The front fork assembly is a critical component in a bicycle. The fork straddles the wheel and is joined to a tube, known as a steering spoke, which extends through the frame and is connected to the handlebars. It is impractical to make the fork and spoke in one piece, as by forging or casting. The two must be made separately and somehow joined.
In this case the spoke was welded to the fork and thereby became subject to Pelloux's law that states, “It always breaks where it is welded.” This law was enunciated by MIT Professor R. M. N. Pelloux to twit a colleague who is a welding expert. But there is considerable truth in the law. The chances of a defect in the fork and spoke material are very small. Welds, and for that matter braze and solder joints, are all too subject to human error.
The plaintiff's lawyer first consulted a mechanical engineer who stated the welds were clearly inadequate but suggested a metallurgist also be contacted. This is where I entered the case.
The spoke and fork are of mild steel which is so easy to join it can even be arc welded in air using a piece of coat hanger as an electrode. I made such welds as a 12-year-old farm boy. My welds were not pretty, but they were strong.
The figure, above, is a view looking up at the top of the fork. The steering spoke should have been welded around the circumference of the hole seen in the figure. Instead, only the two tack welds were applied. These were easily broken, as the figure illustrates. A proper weld would never have failed, even during rough, off-road use. The weld metal was perfectly all right, there was just not enough of it.
The Smoking Gun
Was this an isolated case where the welder made tack welds to fix the two pieces into position and somehow neglected to follow up with the proper weld? Such was not the case. The manufacturer issued a recall notice for nearly 100,000 bikes made over a 10-month period. There was a major breakdown in both manufacturing practice and quality control.
The case was settled out of court. Two damning consultant reports and the recall notice gave the attorney adequate ammunition to extract a favorable settlement.
An earlier column, “The Case of the Beneficial Barrister,” (DN 08.18.03) asserted that the much-maligned plaintiff's bar had been a force for good in chasing shoddy aluminum extension ladders off the market. I praised the bar while fully recognizing its abuses, very notably in asbestos litigation where unscrupulous lawyers and experts have led to wholesale miscarriages of justice. See “The Case of the Fallible Forensics,” (DN 06.05.06).
In the absence of the threat of litigation the dealer would probably have explained to the boy's parents that such an accident never happened before and certainly would never happen again and could only have been caused by their son's horrific abuse of their fine bicycle. In other words, “get lost!” Threat of litigation led to a very different response.
