Mounting Radial Tires on Classic-Vehicle Rims

Need to Know Info

	The terms bias and radial describe two different ways to structure a tire using similar, rubber-covered fabric cords of nylon, polyester, rayon or steel. A new white paper, now available at www.SEMA.org/WTC/ describes how bias and radial tires differ in working with the rim to provide a safe ride.

Over the past 100 years, tires and the wheels that support them have gone through significant changes as a result of technical innovations in design, technology and materials. No single factor affects the handling and safety of a car more than the tires and wheels it rides on and how the two work together as a unit.

One nagging question has been whether rims designed for bias-ply tires can handle the stresses placed on them by radial-ply tires. The answer depends on how the rim was originally designed and built as well as the number of cycles on it and how it has been used. Most importantly, it depends upon the construction of the tire and how it transmits the vehicle’s load. A new white paper, now available under the “WTC Resources” box, describes how bias and radial tires differ in working with the rim to provide a safe ride.

The terms radial and bias describe two different ways to structure a tire using similar, rubber-covered fabric cords of nylon, polyester, rayon or steel. The bodies of all tires are made from layers of such cords, and the layers are called plies. Within each body ply, tire cords lie parallel to each other encased in rubber, and each cord distributes stresses along its length to its ends, which are anchored around steel hoops called beads. Because the beads also hold the tire tightly to the rim, the bead areas transfer body tire cord stresses to and from the rim. Radial tires and bias-belted tires have additional belt plies located only beneath the tread area, and the cords of those plies are anchored only in rubber.

When radial tires began to be widely accepted, engineers and customers noted occasional rim-cracking failures that led to higher strength standards for light truck and large agricultural tire applications. Rim cracking was not prevalent in all wheels of that era (’70s), but wheels and rims of earlier years (with different design standards and many, many usage cycles) were not—indeed, could not—be fully tested for use with every possible radial. Therefore, it is likely that all old rims—and even some new rims made to old designs—may not perform satisfactorily with newer-technology tires, even if compatible sizing makes it very easy to install the radials.

Construction features of radial and bias tires make them bulge differently when deflected, and the radial’s localized bulge puts more stress on the rim flange than a bias tire does. Rim fatigue and cracking can happen to any wheel from normal flexing, cyclic loading and cornering, but radial stressing of the rim can accelerate metal fatigue and rim failure that is uncommon with bias tires.

Fitting radial tires to wheels and rims originally designed for bias tires may result in rim durability issues. Even same-sized bias and radial tires stress a rim differently, despite their nearly identical dimensions. Stresses that exceed a rim’s design limits can result in rim failure, with possible tire and vehicle consequences.

The white paper examines the construction of both tire types and factors such as inflation, load, tire bulge and weight transfer that allow or limit the use of radial or bias tires with older rim designs. It discusses the basic causes of uniquely radial rim stresses and identifies some likely results. The Wheel & Tire Council (WTC) encourages all member companies as well as other businesses involved with wheels and tires to access and review the document.

Supporting Best Practices Through J2530

Several years ago, SEMA and the WTC worked with the Society of Automotive Engineers (SAE) to develop an industry testing practice for aftermarket wheels. SAE J2530 (Aftermarket Wheels Performance Requirements and Testing Procedures) outlines performance and sampling guidelines, testing procedures and marking requirements for aftermarket wheels used on cars, light trucks and multipurpose vehicles. The document identifies three main areas of testing, including cornering and radial fatigue and impact strength. Manufacturers may purchase the J2530 specifications from SAE and test at their own internal labs or at testing facilities of their choosing, so long as the facility maintains the necessary equipment to perform
the tests.

SEMA and the WTC strongly support J2530 and urge manufacturers to adopt its recommended practices. For more information, visit here.