Rim (wheel)

Characteristics

Diameter (effective): distance between the bead seats (for the tire), as measured in the plane of the rim and through the axis of the hub which is or will be attached, or which is integral with the rim.

Width (effective): separation distance between opposed rim flanges. The flange-to-flange width of a rim should be a minimum of three-quarters of the tire section width. And the maximum rim width should be equal to the width of the tire tread.

Type: Depends on the type of vehicle and tire. There are various rim profiles, as well as the number of rim components.

Vehicle performance: Because the rim is where the tire resides on the wheel and the rim supports the tire shape, the dimensions of the rims are a factor in the handling characteristics of an automobile. For example:

Overly wide rims in relation to the tire width for a particular car may result in more vibration and less comfortable ride because the sidewalls of the tire have insufficient curvature to flex properly over rough driving surfaces. Oversized rims may cause the tire to rub on the body or suspension components while turning.

Production

A standard automotive steel wheel rim is made from a rectangular sheet metal. The metal plate is bent to produce a cylindrical sleeve with the two free edges of the sleeve welded together. At least one cylindrical flow spinning operation is carried out to obtain a given thickness profile of the sleeve—in particular comprising in the zone intended to constitute the outer seat an angle of inclination relative to the axial direction. The sleeve is then shaped to obtain the rims on each side with a radially inner cylindrical wall in the zone of the outer seat and with a radially outer frustoconical wall inclined at an angle corresponding to the standard inclination of the rim seats. The rim is then calibrated.

To support the cylindrical rim structure, a disc is made by stamping a metal plate. It has to have appropriate holes for the center hub and lug nuts. The radial outer surface of the wheel disk has a cylindrical geometry to fit inside the rim. The rim and wheel disk are assembled by fitting together under the outer seat of the rim and the assembly welded together. The disk is welded in place such that the center of the wheel is equal to the center of the hub. The distance of the disc to the back edge of the rim is called 'offset', or ET, from German Einpresstiefe.

One-piece rim and wheel assemblies (see image) may be obtained by casting or forging.

Meaning

The terms wheel and rim are often used synonymously, as in decorative wheels being called rims. Used broadly, or used figuratively, the word rim can mean the outer edge of any circular object. Some authors are careful to use rim for only the outer portion of a wheel, where the tire mounts, just as the rim of a coffee cup or a meteor crater does not refer to the entire object. One engineering text says "alloy wheels [are] often incorrectly called aluminum rims". Others use rim to mean the entire metal part to which the tire mounts, because the rim and the wheel are often cast or stamped from a single piece of metal instead of being distinct as with wire wheels. At the same time, "wheel" may refer to the entire rotating assembly, including the tire.

Railroad usage

In railroad usage the conical running surface of the wheels may be called a rim, a wheel tread or a tyre.