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Loaded Wheel Failure

Rear wheel from an Organic Engines "SUV" pedicab, carrying driver, two adults and two children on the bench seat, and two trailing bikes & riders on dummy axle mounts on the back.

  

The wheel "exploded", dumping everybody and the trike went on its side. There were only minor injuries.

The hub is a Heinzman electric motor; there were probably 36 spokes; the rim appears to be a Sun "Rhyno Lite" 20-inch size.

The cause of failure is unknown — there was no obvious road debris or damage and the wheel was not driving or braking hard at the time.

Sometimes when a tire casing comes off the rim suddenly, the sudden load can cause a wheel near collapse to "taco". This wheel was clearly holding a lot of load and the rim, while not a light or flimsy, had only modest support (36 spokes). It seems possible the wheel taco'd and the resulting loss of spoke support allowed the rim to collapse, and the resulting collision with the ground led to the rest of the damage shown here.

The tire is of fairly large section, which combined with high tire pressure may have contributed to an initial blow-off.

The spokes are also short and thus relatively stiff. Thus, local rim distortion can lead to rapid spoke dentensioning, and once the spokes are loose they provide no lateral support. This may have also been a contributing factor.

Alternatively, short spokes tend to be stiff, which accellerates fatigue of the rim spoke bed, the spokes, and the flanges. It is possible one of those failed, leading to loss of spoke tension, and then the wheel failed as above, or the loss of one spoke's tension led to a second failure, then a third, and so on to the failure shown here.