AUTOMOTIVE WHEEL DETACHMENT
Charles C. Roberts, Jr., Ph.D., P.E.
Detachment of automotive wheels occur for a variety of reasons including impact from an accident, insufficient lug nut torque, insufficient lug nuts (some not installed), defective design, defective manufacture and misapplication. This article deals with the misapplication of lug-centric wheels mounted, without an adapter, on hubs designed for hub-centric wheels.
Figure 1A Figure 1B
Figure 1 is a representation of the hub-centric wheel compared to a lug-centric wheel. The hub-centric design is common in the industry and is characterized by the center bore of the wheel fitting snugly on the hub (Figure 1A). In the hub-centric design, the lug nut secures the wheel to the hub resisting primarily lateral forces on the wheel. The snug socket-like connection (arrow) helps support the weight of the vehicle along with vertical and fore and aft forces. Automakers design a specific hub-centric wheel to fit a specific hub, utilizing a variety of engineering design methodologies including the finite element stress calculation method.
Many aftermarket wheels are of the lug-centric design where the center bore of the wheel is larger than the hub so that they can fit a variety of vehicles (Figure 1B). This design requires the wheel lugs to locate the wheel on the hub without the added support of the pilot bore in the wheel. The arrow in Figure 1B points to the annular gap between the hub and the wheel flange, characteristic of the lug centered design. The lugs now support the vehicle loading in all directions without the aid of the sung fit of the center bore on the wheel. This can lead to wheel and lug bolt failure as a result of vertical and fore and aft loading from vehicles that were designed to accept a hub-centric wheel.
Figure 2 is a view of a lug-centric wheel that had been mounted on a hub designed for a hub-centric wheel. Over time, the lug holes became deformed as shown in Figure 3, resulting in the detachment of the wheel. As the lug holes became over stressed, the wheel was no longer concentric with the hub, causing further vibration and excessive loading to the wheel. Figure 4 is a cross-section of one of the wheel studs showing evidence of reverse bending fatigue as the fastener loosened as a result of the deformation to the wheel flange. It should be noted that a hub adapter was not used by the owner of the vehicle. Aftermarket wheel manufacturers provide spacer/adapters that fit the hub and the wheel center bore in the shape of the original equipment, attempting to convert the wheel from lug-centric to hub-centric. A wheel spacer/adapter should not be considered optional when mounting a lug-centric wheel on a hub designed for hub-centric. However, the use of an adapter may not solve the problem. Some wheel spacer/adapters are constructed of polymers which help center the wheel for tightening but may not serve the same supporting function as a metal adapter. If the wheel studs are not centered on the hub, then polymer spacer/adapters will probably not help the wheel run true. Some vehicles (early VW Beetles) used lug-centric designs with success. But installing a lug-centric wheel on a hub-centric hub on which it was not designed to be mounted, can lead to wheel damage and lug stud failure.