A bicycle failure

Sep 1, 2005 by

This month’s case relates to a bicycle failure. A gentleman went to a local department store and bought a 26 inch Men’s Mountain Bike. The bicycle was assembled by the department store. He had owned the bike for one week and went for a ride.

He was seated and pedaling on level ground. The gears suddenly “slipped” without warning. In an attempt to catch the bicycle, he was flipped over the handle bars and landed on his elbow. He sustained a non-displaced fracture of the olecranon process, with joint effusion, and an acute moderate lumbar sacral sprain and strain.

Subsequently he was treated by several doctors. The injured party then hired an attorney. The attorney contacted the manufacturer of the bicycle with a demand for $50,000. The manufacturer’s representatives then examined the bicycle at the attorney’s office. Since the alleged failure related to a sprocket problem, they were pleased to discover that the derailleur and sprocket assemblies were manufactured by a different company. They then passed this hot potato back to the attorney. The attorney then contacted the sprocket manufacturer. The insurance carrier for the sprocket manufacturer then contacted GEI. We were asked to examine the sprocket, and derailleur for any evidence of a design or manufacturing defect that would have caused the accident.

Our expert, a mechanical engineer, mounted the bicycle on a stand and then inspected it. The handlebar mounted thumb shifting control mechanisms operated properly. The front fork operated normally. The brakes were properly adjusted and operated normally. The seat was unremarkable. The cables to the shifter mechanisms were not solidly affixed to the down tubes and the cables leading to the shifter mechanisms were improperly adjusted, in that they were excessively loose. This would have lead to long-term problems, but was not the cause of the immediate incident.

The rear sprocket was an assembly of five sprockets of 28, 24, 20, 17, and 11 teeth. When the chain was on the 28 tooth sprocket, the operation was normal. When it moved down the line to the next sprocket, the operation was still normal. The next two sprockets gave similar results. When the chain was moved to engage the 11 tooth sprocket, our expert discovered that the 11 tooth sprocket then moved in relation to the other four sprockets. Normally the five sprockets are pinned together, to act as a single unit (some manufacturers fasten them together by a friction fit.) In this case, the cause of the accident was the failure of the last sprocket to stay fixed to the other sprockets. The rider had built up speed, starting in the lowest sprocket (28) and worked his way up the sprocket assembly as his speed increased. When he finally shifted the position of the chain onto the 11 tooth gear, he was traveling at a good clip (a technical term!) and leaned into the pedals to go even faster. At this point the 11 tooth sprocket slipped, creating a “freewheeling” situation. The rider lost control and crashed. The cause of the accident was a manufacturing defect in the sprocket assembly.

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