What are Run-Flat tires?

Run-flat tires are – like the name implies – tires that provide limited vehicle mobility in the event of a puncture and sometime even after tire pressure is completely lost.  Generally, flats happen at the most inconvenient times, and repairing a flat (or putting on a spare) can take precious time that you may not have at the moment.  Loss of pressure while driving at high speeds is not only inconvenient but can create real safety hazard for the vehicle occupants and other cars nearby. Whatever the situation, one thing is certain – having a flat tire is not a pleasant experience. This is why many people believe in the benefit and value of having a run-flat tire. Aside from the safety and convenience benefits, run-flat tires offer other advantages. They eliminate the need for a spare tire, providing a vehicle manufacturer with more undercarriage space for items such as folding seats. The elimination of spare tire and equipment reduces weight resulting in improved fuel economy.

The first run-flat tire was introduced by Goodyear in 1978, which named it a “self-sustaining” tire or SST.  Pirelli, Dunlop and Michelin followed in the early 1980s with their own run-flat tires primarily used in race cars. Chevrolet was the first to offer a run-flat tire as optional factory equipment on a stock vehicle (1994 Corvette) and by 1997 run-flat tires were being offered as standard original equipment on the Corvette. Today, vehicles from sports cars to minivans come equipped with factory run-flats.  They are most common on vehicles where the manufacturer was looking to eliminate the space required for a spare tire.

Technological advances and competition have led tire companies to create many versions of run-flat tires.  There are 3 basic types that people often call “run-flat”, although they do not all provide the same advantages nor work in the same way.  These types are self-supporting tires, self-sealing tires, and auxiliary support systems.

• Self supporting run-flat tires have a stiff internal construction – especially in the sidewall – which allows the tire to support the weight of the vehicle under limited conditions if the tire loses air pressure. These tires have a special bead that grips the wheel when there is no internal tire pressure to hold it on, and they have reinforced cords in the sidewall with rubber infused in layers to ensure that the sidewall does not disintegrate in the event of air loss. In general, self-supporting tires are only designed to operate at or below 55 mph, and not to exceed 50 miles without pressure.  They are intended to get you to a repair facility but do not provide the operating flexibility and longevity of a normal spare tire.  Self-supporting tires must be used in conjuction with a tire pressure monitoring system (TPMS). The TPMS system ensures that the driver is made aware of the loss in air pressure immediately and does not unknowingly damage it by continuing to drive on a flat tire. The following acronyms used by various tire manufacturers are all examples of self supporting run-flat tires: RFT, DSST, ROF, EMT, XRP, ZP, ZPS.  The disadvantage of these tires is their higher cost, and the fact that if you drive on them for any length of time without pressure they cannot usually be repaired (patched) like normal tires but instead must be completely replaced with a new tire.  This can make the cost of a puncture quite a bit more expensive than with a standard tire.  Due to the tires’ stiffness, self-supporting run-flats are also generally rougher in ride quality, with slightly higher road noise.
• Self-sealing tires are constructed to seal tread-area punctures of up-to 3/16” in diameter – and do so permanently. These tires are of standard construction but with an added inner layer of sealant that provides an instant seal when punctured and works to prevent loss of pressure. The sealant also permanently seals any small open space left behind due to the removal of the puncture-causing object. Examples of self-sealing tires include ContiSeal by Continental tires and NailGard by Uniroyal tires.  Technically, these are not run-flat tires in that they are designed only to help maintain tire pressure following a puncture;   if pressure is lost they cannot provide vehicle mobility like a self-supporting run-flat.  The advantage of self-sealing tires is that for many common punctures, they will keep the tire operable without any repair or intervention for the remaining tread life of the tire.  You may never learn that your tire was punctured.  These tires are somewhat more expensive than a standard tire, and may have reduced fuel efficiency due to a higher rolling resistance and rotational momentum.
• Auxiliary Support System tires employ a solid inner wheel insert which is connected to the rim and on which the flat tire rests in if pressure is lost. Michelin was the first to use this technology and called it PAV (Pneu accorage vertical) because the tire bead in this case seats vertically on the rim (obvious only if you speak French!). This name was later changed to PAX system (which sounds more meaningful to non-Francophones)  and this name is still in use on PAX tires today. The primary benefit of this run-flat system over normal self-supporting type run-flats is that there is no sacrifice in ride quality under normal pressured operation. However, since this system requires specialized wheels and is not compatible with everyday rims, it is more expensive to replace the tires at the end of their life. Another drawback of this system is that most automotive shops do not possess the specialized custom equipment to repair auxiliary support systems so service is limited only to vehicle dealerships – which makes repair and replacement of these tires cumbersome and more expensive.