In commercial trucking applications, wheels may be properly aligned on a hub using either specially designed mounting hardware, or through the geometry of the hub itself. These two designs are respectively referred to as stud-piloted wheel mounting, and hub-piloted wheel mounting. In a stud-piloted wheel, the studs that are used to secure the wheel also serve to align the wheel on the hub. In this design, ball-seat cap nuts fit into chamfered ball-seats provided in the bolt-hole openings of the wheel to position the wheel. In a dual wheel hub, additional hardware is required to properly position both the inner and outer wheel. More specifically, a ball-seat inner barrel nut between the wheels serves as both a nut for the inner wheel, and a stud for the outer wheel.
Hub-piloted, also known as “unimount” disc wheels are designed to center on the hub at the center hole or bore of the wheel. The wheel center hole locates the wheel on pilots built into the hub. Hub-piloted wheels are used with flange nuts which flushly contact the flat wheel disc face around the bolt-hole. Only one nut on each stud is used to fasten single or dual wheels to a vehicle. Hub-piloted wheels have straight through bolt-holes with no ball seat.
Ensuring that a wheel is properly secured to the hub is of critical importance. This requires that each nut be properly torqued onto the stud within a narrow tolerance range. Over tightening the nut may result in damage to the stud, such as compromised threads, plastic deformation of the stud, or ultimate failure/sheering. Likewise, under tightening the nut may result in wheel movement or “indexing,” with the nut eventually backing off and the wheel becoming unsecured.