About I-CAR - Current Events/News: e-newsletters: Advantage Online: February 18, 2002

February 18, 2002 -Flat tires were a frequent occurrence early in the last century. Today that frequency is relatively low. Many of these involve a slow loss of pressure. Sudden loss of pressure is usually the result of puncture by a foreign object. If a tire has a sudden loss of pressure while the vehicle is traveling at high speed, there's a safety concern. The development of run-flat tires is the tire companies’ answer to this safety concern. Run-flat tires are currently offered by the OEMs only on high-end vehicles. Further applications may be forthcoming. This article looks at the design of run-flat tires and how repair facilities can work with them.

Run-Flat Technology

Run-flat tire technology has been in development since the 1970s. The Calloway Corvette came with run-flat tires in the mid 1990s. 1994 Chevrolet Corvettes had an option for run-flat tires. Military Humvees also have a variation of this technology installed.

Figure 1-A run-flat tire (bottom) has thicker sidewalls and a larger bead than a conventional tire (top).

In normal conditions, the weight of the vehicle is supported by air pressure in the tire. For this to happen, the tire and wheel system must contain that air pressure. Run-flat technology provides support for the tread of the tire if air pressure is lost. The current design uses a large bead and very thick and stiff sidewalls (see Figure 1). This has the added benefit of being more resistant to puncture. The compromise is additional weight and a harsher ride.

Other designs, not yet on the road, include using an insert to support the tread in case of loss of air pressure. One company markets a device that could retrofit any tire. Two semicircular plastic inserts are installed with the tire to form a support under the tread. Another tire design features an internal support ring and a bead chamber to firmly hold the tire in place on the wheel. Preventing the tire bead from separating from the wheel greatly enhances stability in case of a sudden loss of air pressure. This design is being sold to some major tire manufacturers.

Checking Air Pressure Is Critical

Because run-flat systems support the tire when deflated, visual inspection will not reveal a low-pressure condition. So checking tire air pressure will need to become standard operating procedure for both the consumer and repair technician. Tire manufacturers require any vehicle with run-flat tires to also have a tire pressure monitoring system onboard. Air pressure sensors are mounted either on the wheel center or at the valve stem. They use long life batteries, usually with a motion sensor to turn them on when the vehicle is in motion. Tire air pressure is transmitted to a central display unit in the passenger compartment. An article on tire pressure monitors appeared in the August 13, 2001 edition of the I-CAR Advantage Online.

Run-flat tires currently available today have a warning imbedded directly on the sidewall that says the "vehicle must be equipped with a low tire inflation pressure warning system."

Tire Changing

fig 2 = #9757 on left and #9756 on right

Figure 2–The stiffness of the sidewalls on a run-flat tire can be seen when pushing down on a conventional tire (left), and a run-flat tire (right).

Each of these systems requires special handling during repairs. Today’s run-flat tires all have stiff sidewalls (see Figure 2). This means older conventional tire changing equipment may not be used. Equipment makers have special equipment and procedures to mount run-flat tires. The bead must be carefully pressed off the seat on the wheel. One company that makes wheel-changing equipment has produced a five-page manual specifically for mounting and demounting run-flat tires. The manual explains how mounting this type of tire can't be hurried. With run-flat tires, patience is required "to give the bead time to relax." Tire changing machines designed for use with run-flat tires use rollers to slowly press the bead off the wheel, and special tools to keep the bead unseated during the process. During mounting the bead must be well lubricated and air pressure kept below 40 psi.

Repairs

Whether a run-flat tire can be repaired depends on who made the tire. Firestone does not allow any repairs, so far. Any punctured or otherwise damaged Firestone or Bridgestone run-flat tire must be replaced, and the damaged tire sent directly to Firestone. Firestone prefers to directly evaluate the damage that can be incurred to run-flat tires.

Figure 3–This combination plug and patch is pushed through from the inside so that the patch is on the inner liner and the plug is through the hole. The plug is then cut flush with the outside tire tread.

Punctures in the tire crown on current Goodyear run-flat tires can be repaired using a combination plug and patch (see Figure 3). These can only be installed from the inside, which requires demounting the tire. The patch repairs the liner of the tire and the plug fills the hole. Plugging the hole is important because moisture can migrate into the cords of the tire if left exposed.

Figure 4–The initials "EMT" identify this is as a Goodyear Extended Mobility Tire.

Figure 5–It’s obvious what type of Goodyear tire this is.

Of particular importance on run- flat tires is inspection of the inner sidewall. If the tire has been driven for too long or at too fast a speed while deflated, heat may build up in the sidewall and cause deterioration of the structure. Heat buildup will be indicated by a gray-colored powder on the sidewall inner surface. Any wear or powdering of the inner sidewall will require replacement of the tire.

Identifying run-flat tires on vehicles is easy, so far. Goodyear run-flat tires are easily identified with "EMT" on the sidewall (see Figure 4). This stands for "Extended Mobility Tire." Some Goodyear run-flat tires are even more evident (see Figure 5). The Firestone run-flat tire has the initials "RFT" printed on the sidewall. Both tire manufacturers also have the warning imbedded on the sidewall about tire pressure monitors and a reminder to use the proper tire changing equipment.

Conclusion

Run-flat tires are a safety innovation by the tire industry to prevent a vehicle from losing control when a tire is punctured. Collision repair technicians and estimators need to be aware of these tires and of the tire manufacturer’s requirements for repair. It’s essential that conventional tire changing equipment NOT be used to demount and mount these tires. As other designs come into the market, new equipment and processes may be needed.

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