The earliest tires were solid-not air filled-rubber rings that surrounded wagon wheels, and although they did not improve the quality of the ride too much, they were most resilient to punctures and protected the wagon wheels from the rough terrain of the day for quite a long time. Much has changed since the early days of the tire, yet today's manufacturers are trying to recreate the tough enough to protect wheels without air concept, while also protecting the safety and comfort of the passengers as well as the vehicles chassis from damage, which has resulted in "run flat tires".
Still containing some air, the new run flat tires are available in two different designs and are much more complex than those rubber rings of old. One type of run flat tire is designed with stiff sidewalls that provide extra reinforcement and prevent the tire from deflecting and are rigid enough to support the vehicle's weight if there is a loss of pressure. Most of these tires are low profile due to the sidewalls' rigid design and would not be as effective with taller tires. Their low profile design makes them popular for small cars, though they are also designed for minivans and larger sedans. These tires can generally be driven with little to no air pressure at speeds of up to 55 MPH for approximately 50 miles. This type of run flat tire is offered by several tire manufacturers but it is important to note that these tires can not typically be repaired if punctured.
Run flat tires with an inner lining are no stiffer than an ordinary tire and prevent the sidewall from deflecting by utilizing a plastic or hard rubber ring inside. The Michelin Tire Company invented this design, which they have named the PAX system. At the current time, these tires are only available in the United States for the Honda Odyssey and Rolls Royce models, though they are expected to be available for other models in the near future.
As Michelin's PAX system designation might indicate, these run flat tires are more than just tires-the design represents a systematic combination of the tire, an inner support lining, the wheel and a tire inflation monitor. When air pressure is lost in a PAX system tire, only about half of the air is released before the underside of the tire tread is in position to rest upon the inner support liner that surrounds the wheel. Because the tire does not actually go completely flat, it is often hard for the driver to recognize the problem-thus the tire pressure inflation monitor alerts the driver inside the cabin through the instrument panel.
A beaded connection between the tire and the wheel helps attach the tire to the wheel securely when pressure is lost preventing further damage to the wheel and helping to ensure the safety of the passengers. The sidewalls do not have to support the weight of the vehicle as they do in other run flat tires, so they do not have to be as stiff and they can be taller. This makes the PAX run flat system better for larger passenger vehicles like SUV's. Once pressure is lost, PAX system tires can be driven for up to 125 miles at speeds of up to 55 MPH and can be repaired if the hole is less than 0.25 inch in diameter-which is the extent to which you could repair a typical or regular type of tire
Run flat tires are still quite expensive and the stiff sidewall versions tend to ride hard, and some manufacturers are bridging the gap by including TMK -Temporary Mobility Kits- that replace spare tires and therefore increase cargo space. The primary component of the kit is an aerosol can that contains a combination of compressed air and liquid rubber. The can is connected directly to the valve stem of the flat tire to inject the 'fix a flat' mixture which will seal the hole and refill the tire with air. However, this temporary fix is only intended to get you to the nearest gas station traveling at a speed of no more than 35 MPH. If the puncture was small and the tire was not driven long without adequate air pressure there is a chance the tire can be repaired, but there are no guarantees. In fact, sometimes the liquid rubber itself can cause additional harm to the wheel and tire if the tire is very flat when the TMK is inserted or if the tire is driven too many miles once the mixture has been injected.
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