Wheels that separate from a moving vehicle create two potentially deadly hazards; the first is a disabled vehicle with diminished braking ability and directional control/stability, the second is an uncontrolled wheel that is capable of striking people or objects that are a significant distance from the spot of origin. In this article, we discuss the common causes for wheel separation and the industry best practices for vehicle maintenance procedures.
Vehicle Wheel Separation: Causes & Consequences - Expert Article
From an engineering perspective, a wheel that separates from a vehicle can turn into a projectile, which creates a hazard. When other drivers are exposed to a separated wheel, a dangerous condition is created. The vehicle from which the wheel separated can also lose control due to the loss of the wheel, also creating a dangerous condition.
Following a series of fatal wheel separation incidents in the early 1990’s, the National Transportation Safety Board (NTSB) initiated a special investigation into medium/heavy truck wheel separations. The investigation found that there are a significant number of wheel separations that occur each year and identified the most common causes as:
- the loss or breakage of wheel fasteners
- wheel bearing failure
Vehicle Wheel Anatomy
Car and truck wheels are secured to the axles by a series of threaded fasteners, usually in the configuration of a “stud” and “nut.” The stud, which is affixed to and extends from the axle assembly, more specifically the hub, is designed to mate with a nut, a component that has a corresponding internal thread. This nut is referred to as the wheel nut or lug nut. The wheel has a matching pattern of holes and is positioned on the axle, over the studs, and the nuts are used to attach the wheel to the axle, creating a joint.
The design used to secure passenger car and light truck wheels is similar to the design used to attach most medium and large truck wheels. The size and number of fasteners used varies based on the type of vehicle; however, the governing engineering principles are the same.
HOW WHEEL FAILURES OCCUR
The strength of the wheel and axle joint relies on proper fastening of the individual stud and nut assemblies. If wheel nuts are not properly tightened, studs can break or nuts can spin off. If a wheel nut is too tight, it can damage and weaken the stud, causing it to fatigue, crack, and potentially fail. A properly secured wheel/axle joint distributes relatively even tension through the wheel studs, creating a compressive load in the joint that capitalizes on the material strength of the mating components. By maintaining a proper compressive load in the joint, the studs are isolated from varying loads allowing the system to perform for millions of load cycles without failing.
When the joint is compromised and a varying load is applied to a free stud (one that is not in a joint with a proper compressive load), the metal in the stud is at greater risk to fatigue and fail. The failure of any individual component of the joint greatly increases the likelihood of wheel separation.
PROPER MAINTENANCE PROCEDURES
Proper maintenance is essential to maintaining secure clamping (compressive) force/load in the joint between the wheel and axle. The clamping force must be sufficient to hold the wheels in place, but not so high as to over-stress the threads of the fasteners. In short, too much or too little torque can result in wheel separation.
Automotive manufacturers publish wheel nut torque specifications in vehicle owner’s manuals and also make this data available for service stations. Wheel tightening is not a “one size fits all” process. The standard of care for vehicle service is for the mechanic to tighten wheel nuts in accordance with manufacturer specifications. Part of this process should include the use of a proper torque limiting device or devices such as torque sticks and a torque wrench.
EXPERT WITNESS INVESTIGATION OF WHEEL SEPARATION CASES
While maintenance is the focus of most wheel separation cases, it is important to remember that a vehicle’s service history is not always straightforward. During the discovery process of these cases it is important to collect a comprehensive maintenance and incident history of the vehicle. Items to specifically consider include if the vehicle was serviced at multiple locations, whether or not a proper torque limiting device was used, and/or if any DIY maintenance was performed by non-professionals.
In cases where the cause of failure is unrelated to wheel nut tightening, our engineers follow a similar process to identify the mode of failure and then determine relevant details pertaining to causation and notice. Other potential causes of wheel separation include wheel bearing failures, advanced corrosion of the wheel, and improper assembly procedures. Most wheel separation cases are handled by an expert from our automotive engineering group; however, some cases call for the involvement of a metallurgist to understand the circumstances and timing of any failed components.
Tire Design & Failure Analysis Expert
Richard Sherman is an automotive engineer specializing in tire design and failure analysis. Prior to joining Robson Forensic, Richard worked for the Goodyear Tire & Rubber Company where he was engaged in the design, manufacture, and quality assurance of passenger vehicle tires. In a later role as Quality Team Leader, Richard performed root cause failure analyses related to product quality concerns. Richard’s experience related to tire engineering and failure analysis is well complemented by the experience he gained earlier in his career selling, mounting, and repairing tires at a quick service tire shop.