Hub-Bub

Hub-piloted wheels (sometimes called “hub-centred” or “WHD,” for wheel-hub-drum) are so common in over-the-road use that you might spec them without a second thought. But it’s important to learn how a hub-piloted system works, since your spec’ing decision will deeply affect your maintenance practices.

Hub-piloted wheels are different from the traditional stud-piloted ball-seat style (often generically called a “Budd wheel,” after one of the major manufacturers of the type). The ball-seat system has a ball-shaped bottom on the nut that fits into a spherical depression around the bolt-hole. A typical ball-seat wheel uses 10 studs; studs on the left side of the vehicle use left-hand threads, while studs on the right side take right-hand-threaded nuts.

In a dual-wheel installation, the stud-piloted design seats the wheels by the action of the ball-seat nuts in the ball-seat bolt-holes of the wheel. Stud-piloted duals require both inner and outer cap-nuts, since the outer nuts secure the outer wheel using the threaded ends of the inner cap-nuts. An 18-wheel rig will use 80 inner and 100 outer cap-nuts in order to centre the wheel on the studs and clamp the wheel to the hub.

A hub-piloted system uses machined surfaces on the disc wheel’s hub-called “hub pilots”-to centre the wheel on the hub. The bolt-holes go straight through, with no ball seat.

Available in eight- or 10-stud designs, the hub-piloted system uses one flange-nut per stud, and right-hand threads on both sides of the vehicle. The flange-nut consists of a flat washer permanently attached to the nut body. The body of the nut rotates against the washer, not against the wheel. The result is said to be less nut-to-wheel friction and up to three times more clamping force than traditional ball-seat systems. And the system uses fewer nuts overall: an 18-wheeler spec’d with a 10-hole hub-piloted system will require only 100 flange nuts-even fewer (80) with an eight-hole design.

VITAL HARDWARE

Hubs, studs, nuts, and wheels must all be specifically designed for hub-piloted mounting. Take care not to mix stud-piloted and hub-piloted components. Some ball-seat wheels have the same number of stud holes and bolt-circle diameter as hub-piloted wheels. If components from these two systems are mixed, the wheels won’t mate properly, and could cause the wheel to fail. Other hardware issues to deal with:

– Metric threads: The hub-piloted mounting system requires wheel studs and nuts with metric threads. Most frequently, these are of a size designated M22x1.5. Generally, the same-diameter stud and same nut are used to mount either single or dual wheels.

– Steel or aluminum?: A typical aluminum wheel in a 22.5-by-8.25-inch hub-piloted design weighs about 50 pounds, compared to 85 pounds for a comparable steel wheel. The aluminum unit is also thicker than the steel design, a point worth remembering. Wheel hubs designed for steel hub-piloted wheels may not have enough pilot length to properly centre dualed aluminum wheels.

Any pilot length that locates the wheel is satisfactory, but 1.06 inches (27 mm) for dualed wheels is best. Studs must be of sufficient length, too: proper stud standout for single wheels is at least two inches (50.8 mm); for dualed wheels, it should be 2-13/16 inches (71.4 mm) minimum.

– Wheel size: Big wheels-24.5-inch size-can deliver longer tire tread life and/or greater ground clearance. Small wheels-heavy-duty 19.5-inchers-have found a niche in low-profile applications. But the 22.5-inch size is now common on vans and reefers and this is the size where tire makers concentrate their research efforts, meaning gains in rolling resistance and tread life that you may not see with other sizes.

Regardless of which type and size you choose, and this is more true than most people think, wheels are not maintenance-free components. They all require care, especially in terms of the torque applied to fasteners at mounting time. The spec you choose will have a big effect on the maintenance you’ll be required to perform.