Motorcycle wheel valve stem failure

Figure 1. The motorcycle’s drive wheel valve stem is indicated by the arrow. (Photo: C. Roberts)

This case study deals with an accident caused by the sudden deflation of a motorcycle rear tire. According to a witness, the motorcyclist driving ahead suddenly began to twist due to a puncture in a rear tire. The motorcycle eventually pulled off the road and ended up in a ditch, causing serious injury to the rider. The rear wheel of the motorcycle is shown in Figure 1. The tire had been replaced approximately one month before the accident by a motorcycle store and was a new tire. The bead of the tire was intact and appeared to be correctly fitted. However, upon pressurization, a significant leak was observed at the valve stem indicated by the arrow.

Leaking valve stem. Figure 2. The crack in the valve stem where the air leak occurred. (Photo: C. Roberts)

Figure 2 is a view of the valve stem with the arrow indicating the fractured rubber portion of the valve stem. Figure 3 shows the rupture surface of the stem of the valve stem. The arrow points to the streaks indicating the growth of cracks over time.

Fractured valve stem. Figure 3. Fracture surface of the valve stem. (Photo: C. Roberts)

This is a failure mode of rubber that appears over time due to loss of elasticity due to exposure to oxygen (ozone). The crosslinking of polymer molecules by oxygen atoms over time causes the rubber to lose flexibility, making it hard, brittle, and prone to cracking. It is obvious that the valve stem was not replaced when the tire was replaced. It is common in motorcycle repair shops to replace the valve stem when changing a tire. This was not done in this case, resulting in an accident and injuries to the rider.

Charles C. Roberts, Jr., PE, ([email protected]) is President of C. Roberts Consulting Engineers, Inc., which provides professional engineering services in accident reconstruction, failure analysis, fire causation, explosion analysis and biomechanics .

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