Run-flat tires cannot actually run flat.
Instead, because of special rubber compounds and reinforced
(stiffened) sidewalls, they do not go flat even when all air
pressure is lost. Run-flat tires are intended to be run at
speeds up to 55 mph for 50 to
150 miles (depends on car,
load, and tire make – check with your dealer for specifics)
with no noticeable alternation in performance – usually
enough to you get to a tire repair dealer.
Run-flat tires come in two technologies:
Self-supported and Auxiliary-Supported tires.
Self-supported Tires can be mounted on most
existing wheels because a special bead material in the rim-lip
continue to grip the wheel’s rim even after losing all
air pressure. Because these tires “look normal”
even if they lose all air pressure, a tire-pressure monitoring
system is required to alert the driver that a tire has lost
air pressure before the under-inflated tire is severely damaged.
Auxillary-supported
Tires are really a system
combining special wheels and tires so that when air pressure
is lost the tire rests on a support ring attached to the wheel.
This type of system has the advantage of providing run-flat
capability on the wheel (which doesn't wear out easily) instead
of the tire (which requires periodic replacement). Because
the tire’s sidewall does not need to be extra stiff,
auxiliary supported systems promise better ride quality. This
system, referred to as Michelin’s PAX System, is not
yet available in the U.S. One drawback, at least initially,
is that the unique wheels will not accept standard tires.
This will make them more expensive and require a complete
tire and wheel upgrade for the first set.
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