I just bought a BMW Z4 that comes with run flat tires as standard equipment. There is no spare, not even one of those tiny, compact spares that are used in most cars today. By eliminating the spare, it provides more trunk space especially in sports cars. BMW uses run flat tires on many of its models and they are found on Corvettes, Hondas, Toyotas, Minis and Acuras. The space not needed for a spare can be used for other purposes, such as being able to modify a front-wheel-drive minivan into all-wheel drive one.
There are three basic types of run flat tires - self-supporting, self-sealing and auxiliary-supported. Self-supporting run flat tires are the most widely used. Only about 3-percent of vehicles sold in North America are now fitted with run flat tires, and this expected to grow to only 4-percent by 2011.
The self-supporting design uses stiffer sideswalls so the tire can temporarily carry the weight of the vehicle, even if all pressure is lost. Typically, the side-walls are made of layers of rubber and a heat-resistant cord that prevent the side-walls from folding or creasing. They also have specialized beads around the edge of the tire so they more firmly grip the wheels so they don't come off the rims, even in the event of complete air loss.
Vehicles equipped with this type of run flat tires come with a tire pressure monitoring system (TPMS) to alert the driver if the tire loses pressure or completely fails. A TPMS is necessary, otherwise the driver probably wouldn't detect the change in the vehicle handling characteristics because of pressure loss and drive with under inflated tires damaging these more expensive tires, or lead to a dangerous driving situation. Typical run flat tires let you drive about 50 miles at speeds below 55 mph.
Self-sealing tires have an extra lining coated with a puncture sealant that self-seals the tires should they pick up a nail or screw. They are either permanently self-repairing or at least lose air very slowly. First they provide a seal around the object when the tire is punctured; then, fill the hole when it is removed. Because these tires seal the tire immediately, drivers may not even know there was a puncture. Since these tires are like standard tires, the loss-of-air symptoms of a flat tire still gives warning that a tire is severely damaged. Therefore, self-sealing tires do not require a TPMS.
Auxiliary-supported systems use unique wheels and tires so when the tire loses pressure, the flat tire's tread rests on a support ring attached to the wheel. This means the wheel does most of job of providing the run flat ability, minimizing the tire's role. Unlike the tires that wear out and must be replaced, wheels do not wear out. Also unlike the stiffer sidewalls with self-supporting designs that result in a harsher ride, auxiliary support systems give a better ride quality because the sidewall's stiffness can be like standard tires. The auxiliary supported systems do require unique, read more expensive, wheels that do not accept standard tires.
The biggest advantage of run flat tires is the safety they provide especially preventing the hazards of a blowout. They also mean you can usually drive to service facility to get a tire repaired rather than change a tire on the side of a busy Interstate or in pouring rain.
Run flat tires do have disadvantages. Typically, they are heavier. For example, self-supporting run flat tires can be 4 to 7 pounds heavier each compared to a conventional tire. This can be compensated by not having to carry the weight of a spare tire and jack. They also can have higher rolling resistance leading higher fuel consumption.
They are also more expensive. For example, the run flat tires on my Z4 cost about $200 each, approximately $80 more than an equivalent conventional tire. They can also have a shorter life, in some cases 10,000 miles compared to about 40,000 miles for a conventional tire. Because they are still quite rare, finding replacement tires and even tire shops with the equipment and trained technicians who make repairs may be difficult. This means required repairs could be farther away than the typical 50 miles recommended for driving on a failed tire.
