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Hummer Ball Joint Failure Analysis

An individual was driving a Hummer west bound on a State Highway and had pulled into the left hand turn lane to turn left (south) onto another street. According to the police report, the Hummer's steering locked up and the Hummer skid and hit an oncoming vehicle that was traveling eastbound.


Hummer Damage

At the time of the accident, the driver's side wheel assembly of the Hummer was not immediately found. It remained outdoors for about two days, during which time it is said to have rained. The wheel assembly was found north of the Highway and east of where the Hummer came to a final stop. Total distance from the Hummer to the wheel assembly was said to be about 20 feet.

Upon close examination the steering knuckle was found to have fractured. Chemaxx was asked to examine this fracture an to assess whether the fracture caused the accident or the accident caused the fracture.


Steering Knuckle in Relation to Other Parts
Steering knuckle is the part on the far left, #3


Close-Up of Lower Ball Joint Portion of Hummer Steering Knuckle
Information provided to the National Highway Traffic Safety Administration (NHTSA) indicates that there have been at least 11 previous incidents involving Hummer steering knuckles.

It did not go without notice that the Hummer's airbags did not deploy and that the collateral damage to other portions of the Hummer appeared relatively light. In other words, other parts of the Hummer might be expected to bend or break before the steering knuckle if the failure was in fact due solely to impact overload. Sometimes, what is not seen is as important as what is seen. The lack of significant collateral damage suggests that there was something going on that was not a simple impact overload. This is not to say that there was no collateral damage, just that it was not very heavy.

It was noted that the steering knuckle is made from alloy SAE J434 Grade D4512, which is a type of cast iron. According to the 9th Edition of the Metals Handbook, this alloy is suitable for "moderately stressed parts." Considering that all of the Hummer's front end weight is supported by the ball joints of the two steering knuckles, it is questionable that the Hummer's steering knuckle could be considered as a "moderately stressed part." The preferred alloy type for the steering knuckle would be forged steel. A second choice might be cast steel. Cast iron would not be a first choice.

The investigation is on-going.