Automobile wheels are described by a designation like “15-6J.” The first number is the rim diameter, which is measured across the wheel inside the flange. Rim diameters are in whole inches, usually between 12 inches and 17 inches. (Some English minicars had 10-inch wheels.) The second number is the rim width in inches, which is measured between the inside surfaces of the flanges. Rim widths run from 5.5 to 12 inches, in half-inch steps in the smaller sizes and whole inch steps in the larger. The letter (or letters) refer to the shape of the rim. Rim contours are standardized by the Tire and Rim Association, so that tires will fit.
Another important characteristic of a wheel is its offset: the distance between the rim centerline (midway between the flanges, in the center of the tire tread) and the mounting surface (the part of the wheel that sits against the brake housing when the wheel is mounted). If the centerline is outboard of the mounting surface, the offset is negative, for example, “minus 1 inch”. If the rim centerline were 1 inch inboard of the mounting surface, the wheel would have an offset of plus 1 inch. Unfortunately, in some writing the designations are exactly reversed; inboard is negative and outboard is positive.
The wheel's bolt circle is described by a designation like “4-5½.” The first number is the number of lugbolts or studs. The second is the diameter in inches of the circle on which the centers of the bolts or studs lie.
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Last revised: 4 June 2004.