The hub refers to the central part of the wheel that attaches to the car's axle, consisting of the centerbore, the center disc, the lug holes, and the center cap. The hub's specifications are crucial for determining wheel fitment compatibility.
- Centerbore: The large hole in the center of the wheel which fits onto the axle hub assembly and carries the axle's load. This hole must be at least the same size as the axle mount or larger, or the wheel will not fit. If the centerbore is larger than the axle mount, hub rings can be used to fill the space around the mount to ensure the wheel is centered with the axle.
- Center Disc: The cylindrical part surrounding the centerbore which contains the lug holes. Also called the "plate", this is the part of the hub where the spokes meet. This part is responsible for receiving forces from the axle and braking systems. The back of the center disc, called the mounting pad, rests against the axle seat.
- Lug Holes: The holes around the centerbore, which are used in attaching the wheel to the axle. These holes are fitted to threaded studs on the axle seat, and lug nuts are fastened to the studs to secure the wheel on the axle. German cars typically use wheel bolts instead of lug nuts and studs.
- Bolt Circle: Measured as the diameter of the circle formed by the center of each lug hole. Bolt patterns are identified by the number of lugs and the bolt circle diameter.
- Center Cap: A removable piece that covers the centerbore on the outside. On many cars today, it may also serve to protect the lug nuts from damage.
Spokes are structures that connect the hub and the rim. They are the source of the wheel's structural integrity. Together with the hub, spokes form the wheel's outboard face. On multi-piece wheels, the wheel face is produced as a separate piece and bolted to the inside of the barrel.
Spokes also provide aesthetic value and are a major part of what makes a wheel unique. Custom wheels come in a wide variety of spoke styles for visual appeal, with spokes varying in quantity, width, shape, and pattern. Some truck or trailer wheels may have solid faces with no spokes at all!
Five-spoke wheels are a common wheel design due to engineering reasons: each spoke is supported by two opposing spokes that reduce torsional vibration. A low spoke count also equates to lower wheel weight. That said, the number of spokes on a car wheel has very little effect on weight or performance, especially compared to the materials and construction methods used, so just pick the wheel that you like most.
The rim is the cylindrical exterior of the wheel which supports the tire and gives the wheel its shape, also called the barrel. Wheels are sometimes informally referred to as rims. Technically speaking, however, the word "rim" only refers to the outer shell, not the entire wheel.
- Outer Lip: The part of the rim that faces outward, measured from the wheel face to the outboard edge. A deep outer lip is more vulnerable to impacts, as the spokes are further away and don't provide as much structural support. On three-piece wheels, the outer lip is produced as a separate piece from the rest of the barrel.
- Flanges: Flared edges along the inboard and outboard sides of the rim. These help to prevent the tire from slipping off the wheel.
- Bead Rests: The areas on the rim's outer surface where the tire's beads sit on the wheel. Before mounting a tire, the bead rests should be cleaned to eliminate any contaminants that might prevent the tire from forming an airtight seal.
- Mounting Humps: Small ridges that run along the inside of each bead rest. These help to prevent the tire beads from slipping and hold the beads in place against the wheel.
- Drop Center: The narrowest part of the barrel, used to mount the tire on the wheel without overstretching the bead wires.
- Valve Stem: The rim features a hole for inserting a valve stem when mounting a tire. This is a metal air valve used to inflate the tire and monitor its air pressure.