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PASSENGER VEHICLE WHEEL SEPARATIONS

We have recently heard, read or seen media stories about commercial vehicle wheel separations (The Leading Edge Vol. 6, Spring ’97). Less well known are passenger vehicle wheel separations. Since passenger vehicle wheel separations are less documented, we do not know how often this phenomenon occurs. The fact remains that wheel separations do occur, whether on a truck, a light/SUV truck or a passenger vehicle.

The mechanics and causes of wheel separations on passenger vehicles may be similar to those of truck wheel separations: loose or overtightened wheel nuts, axle, bearing or disk/drum failures. Loose wheel nuts may eventually come off with normal use, or cause excessive shear forces on the studs as the wheel rocks back and forth. This may cause a sudden failure or an additional unbalanced load on the remaining nuts. If the nuts are overtightened (such as incorrect use of an impact wrench), the treaded studs may fail in tension due to the excessive load or the threads may break off the studs. An axle failure whether due to fatigue (cyclic loading) or impact with a foreign object will cause the entire wheel/brake assembly to separate from the vehicle. If the baring fails, the hub, disk/drum and wheel assembly will also likely separate from the vehicle.

The effect on the dynamics of an eighteen wheeler, after losing one set of wheels/tires is not as severe as losing one wheel out of four on a passenger vehicle. In the first case, the massive truck wheel becomes a hazard to other drivers, passengers and pedestrians, while in the second case the major hazard to the valuable human cargo becomes the vehicle itself. If the vehicle loses control, it may impact with other vehicles, fixed objects off the roadway, or pedestrians.

Symptoms prior to wheel separations range from nothing to significant noise, shaking and poor handling. Routine mechanical inspections may reduce the unlikely event of a wheel failure by identifying excessive wear and tear on mechanical items that can fail. These items must be repaired or replaced promptly in order to avoid any mishaps.

The direction of travel of the vehicle, after the loss of a wheel, is dependent on which wheel separates, the speed of the vehicle prior to failure, as well as acceleration/braking, or steering input by the driver. If a motorist is to lose a wheel, he/she should remain calm, maintain a firm grip on the steering wheel, and release the accelerator pedal. If there is a safe distance available, let the vehicle cruise to a stop without applying the brakes. If it is necessary to apply the brakes, they should be gently depressed, so as not to lock any of the remaining wheels.

A question which may arise in a litigation process, is whether or not the vehicle in question was controllable, in order to avoid an accident. In a recently settled case, the vehicle lost its rear left wheel/tire when the axle broke. The vehicle moved to the left, the driver let go of the steering wheel and the vehicle struck a tree on the side of the road. The right front passenger was ejected as the vehicle rolled over to its final rest position. Our testing on a similar vehicle would have still been steerable at the impact speed, and, if the driver had maintained a firm grip on the steering wheel, instead of letting go of it, the vehicle need not have impacted with the tree. The serious injuries sustained by the right front passenger could have been avoided through proper driver actions.

Each year, we examine a number of passenger vehicles which are involved in accidents. Many of these have wheels that separated from the vehicle during an accident, but in some cases, the wheel separation occurs due to a failure, and is the cause of the accident. It is always prudent to investigate wheel separations, to determine if it is the cause or result of an accident.

 
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Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com