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If you've ever been late for a date, appointment, or meeting because of a flat tire, you already know how frustrating it can be. If you've ever changed a flat tire in the rain, after dark, or on the shoulder of a busy highway, you already know how frightening it can be. So while we enjoy the freedom our vehicles provide, it's amazing how quickly that freedom vanishes when a flat tire strands us.
Since the early development of the automobile, tires have played an important role in determining a vehicle's overall comfort and safety. However, there are few consumer products placed in harms way more often than our tires, which encounter extremes in temperature, exposure to the elements, and attacks by debris on the road during their life. And while the tire manufacturers' continuous research and development efforts have improved tire durability and longevity, only recently have they developed tires that can temporarily maintain vehicle mobility using standard Original Equipment and aftermarket wheels. These run-flat tires provide the driver more flexibility when deciding where to have tire repairs made.
Tires don't typically carry the weight of our vehicles, the air inside them does. There are three basic elements which determine the load capacity of a tire: the size of the air chamber formed between the tire and wheel, the strength provided by the tire's construction to hold air pressure, and the amount of air pressure actually in the tire.
Most flat tires (and tire "blowouts") are the result of slow leaks that go unnoticed and allow the tire's air pressure to escape over time. Therefore, monitoring tire air pressure in real-time gets us half way there. If we had tires that could maintain temporary vehicle mobility even after air loss, we'd be just about invincible.
Today there are three technologies used as Original Equipment on vehicles to help maintain vehicle mobility when a tire is punctured. They are self-sealing tires, self-supporting tires and tires supported by an auxiliary system.
Self-sealing tires are designed to fix most tread-area punctures instantly and permanently. These tires feature standard tire construction with the exception of an extra lining inside the tire under the tread area that's coated with a puncture sealant that can permanently seal most punctures from nails, bolts or screws up to 3/16 of an inch in diameter. These tires first provide a seal around the object when the tire is punctured and then fill in the hole in the tread when the object is removed. Because these tires are designed to seal the tire immediately upon being punctured, most drivers will never even know that they just had a puncture. Also because these tires feature standard tire constructions, the traditional loss-of-air symptoms that accompany a flat tire remain to warn the driver if the tire is damaged beyond repair. Therefore, self-sealing tires do not require a low air pressure warning system.
Example: Continental ContiSeal.
Self-supporting tires feature a stiffer internal construction, which is capable of temporarily carrying the weight of the vehicle, even after the tire has lost all air pressure. To provide "self-supporting" capability, these tires typically attach rubber inserts next to or between layers of heat-resistant cord in their sidewalls to help prevent breaking the reinforcing cords in the event of loss of air pressure. They also feature specialized beads that allow the tire to firmly grip current Original Equipment and aftermarket wheels even in the event of air loss. Because self-supporting tires are so good at masking the traditional loss-of-air symptoms that accompany a flat tire, they require a tire pressure monitoring system to alert the driver that they have lost air pressure. Without such a system, the driver may not notice underinflation and may inadvertently cause additional tire damage by failing to inflate or repair the tire at the first opportunity. Typically, self-supporting tires maintain vehicle mobility for 50 miles at speeds up to 55 mph.
Examples: Bridgestone RFT (Run-Flat Tire), Dunlop DSST (Dunlop Self-Supporting Technology) and ROF (Run-On-Flat), Firestone RFT (Run-Flat Tire), Goodyear EMT (Extended Mobility Technology) and ROF (Run-On-Flat), Kumho XRP, Michelin ZP (Zero Pressure), Pirelli RFT (Run-Flat Technology) and Yokohama Run-Flat and ZPS (Zero Pressure System).
The International Organization for Standardization (ISO), a worldwide federation of national standards bodies, has adopted a run-flat system symbol for extended mobility systems featuring self-supporting run-flat tires.
Auxiliary Supported Run-Flat Systems
Auxiliary supported systems combine unique wheels and tires used for Original Equipment vehicle applications. In these systems, the flat tire's tread rests on a support ring attached to the wheel when the tire loses pressure. The advantage to this type of system is that it will place most of the mechanical task of providing run-flat capability on the wheel (which typically doesn't wear out or need to be replaced), and minimizes the responsibility of the tire (which does periodically wear out and requires replacement). Additionally, auxiliary support systems promise better ride quality because their sidewall's stiffness can be equivalent to today's standard tires. The disadvantage to auxiliary supported systems is that their unique wheels will not accept standard tires and that their lower volume will make this type of system more expensive.
Example: Michelin's PAX System wheels and tires
The International Organization for Standardization (ISO), a worldwide federation of national standards bodies, has adopted a run-flat system symbol for extended mobility systems featuring an internal support ring.
It is too early to confirm which system, if any, will be widely accepted by vehicle manufacturers and consumers in the future.
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