Welcome, students, to Wheel Anatomy 201: Beads and Flanges. Today we will be reviewing the various structures located on the outer barrel of the wheel. These structures will include the drop center, beads, mounting humps and flanges. Please be sure that you have your wheel diagram available to refer to as we proceed. As always, you may find it easier to right-click on the link and open the diagram in a new tab.
The part of the wheel between the outboard face and inboard rim edge is called the barrel. The barrel is shaped to create the tire mounting structures such as the drop center and the flanges. When the tire is mounted, the outer surface of the barrel closes off the open end of the tire, enabling the tire to hold pressure.
Most wheels will have a portion of the barrel that is bent inward, creating a ringlike area around the barrel that is closer to the centerline of the wheel than the rest of the barrel. In order to mount a tire that has the same inner diameter as the wheel's outer diameter, one side of the tire must be placed into this depression on the wheel so that the tire can “slide over” just enough to let the other side of the tire slip over the rim edge. This “drop center” will be closer to one or the other edge of the wheel. When the drop center is closer to the face of the wheel, it is known as a “front-mount” wheel and can be placed on a tire mounter with the face up. The tire is then mounted over the outer face of the wheel. On many “deep-dish” wheels, however, it is not possible to place the drop center near the front face because of the dish, and so the drop center is placed closer to the inboard edge of the wheel. These wheels are known as “reverse-mount”, and must be carefully clamped onto the mounter with the face down.
What we call flanges are the flared edges of the barrel on both the inboard and outboard sides of the wheel. The metal of the barrel is flared 90 degrees outward on each side. This prevents the tire from slipping off the wheel. Of course, the outer edge of the outboard flange is also part of the cosmetic face of the wheel.
The beads of a wheel are the flat areas just inside of the flanges where the edges of the tire (which are also called the beads) seat onto the wheel. It is important that the beads be kept clean, as old rubber or corrosion on the beads can affect how the tires seal. The beads and flanges are also important as the “energy transfer points” of the wheel. Because the tire seats directly against the beads and flanges, any major imperfection of those points, such as a bend in the wheel or a damaged tire bead, will transfer vibration from the wheel/tire combination directly into the suspension and can make the whole car shake at speed
Mounting Humps are small ridges which circle the barrel on both the inboard and outboard sides. These ridges separate the bead surfaces from the rest of the barrel, and function as a block to keep the tire from slipping away from the edges of the wheel. Most mounting humps have a slanted surface, so that under dismounting pressure the tire beads will simply slip over the humps, allowing the tire to be removed. Some wheels for high-performance cars, particularly BMW M-series wheels, have what are called “asymmetrical humps” in which most of the hump area is built with a straight vertical surface, rather than slanted except in one small area next to the valve stem hole. This locks the tire into the beads, making it nearly impossible to remove the tire unless dismounting pressure is applied in that one specific place. This is a safety measure which absolutely ensures that the tires will not come off the beads even under the most extreme pressures involved in racing.
Thank you for your attention, ladies and gentlemen. Please do join us next week for the final installment of this course, Wheel Anatomy 301, in which we will discuss the rather complex concepts of offset and backspacing.
Previous Class – Wheel Anatomy 101: Structure.
Next Class – Wheel Anatomy 301: Offset and Backspacing.