Finding the cause of an SUV-wheel separation

When a wheel came off an SUV, the Failure Analysis experts of MEA Forensic were called upon to investigate.

The Event:

An SUV traveling on an interstate highway suddenly lost its driver-side rear wheel. The vehicle lost control and rolled over, leading to serious injuries to the occupants. MEA was asked to determine why the wheel came off.

Our Analysis:

First, our experts closely examined the SUV and the wheel. The SUV had sustained extensive damage from the roll. There was no wheel on the left end of the rear axle, though the hub, brake rotor, and four of the five wheel studs remained. One of five wheel studs had bent before it broke off. The other four wheel studs had a deep layer of aluminum embedded in their steel threads. The wheel that separated from the left rear axle was recovered. The stud holes of the aluminum alloy wheel were badly elongated, and their bores had imprints from riding on the steel stud threads. These observations indicated that the wheel nuts had loosened and fallen off the studs, allowing the wheel to come off. Our research into wheel separations has shown that left side wheel nuts will spin off by themselves once they are loose and—once gone—this allows the wheel to migrate off the studs and separate from the vehicle.

But why did the nuts come loose in the first place? To gain insight, the lug nut tightness of the remaining nuts on the three wheels that stayed on the vehicle was carefully measured using computerized instruments. This torque audit revealed that some nuts were much tighter than the manufacturer’s recommended torque while some were much looser. We reviewed the SUV’s maintenance history and learned that the vehicle had been driven 1,600 km since its rear brakes were serviced 35 days prior to the accident. Testimony of the service provider indicated it was not their practice to use a calibrated torque wrench to install wheel nuts. At the time the vehicle was serviced, the owner was not advised to have the wheel nut torque checked after a short driving distance.

A diagram of a car, showing wheel nut torque

The erratic lug nut torques on the remaining wheels were consistent with the non-use of a calibrated torque wrench, which is contrary to industry best-practices. Based on numerous wheel separation investigations, the 1,600 km driving distance from wheel installation to separation was consistent with the wheel not being put on properly after the brake service.

The Results:

Our work helped our client understand the unusual phenomenon of a left-side wheel separation, and to relate the accident to a maintenance provider’s failure to follow industry best-practices. Furthermore, our research showed that if the wheel nuts had been installed properly, and if a re-torque advisory had been given and followed, then the wheel most likely would not have fallen off and the accident would have been prevented.

If you have any questions regarding a wheel separation case please do not hesitate to contact one of our failure analysis experts.

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