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How exactly does a runflat tire work?

Technology

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15/01/2019
 

Assistance systems help prevent accidents. In this series on VisionZeroWorld we explain how ABS, airbags and other safety systems work. This time it’s the turn of runflat tires.


It’s the middle of the night on a quiet country road and it’s starting to rain. ­All at once the car veers sharply to the right and you have your work cut out just keeping it on the asphalt. You pull over, climb out and look for the source of the trouble. The light from your cellphone reveals a flat front tire – how annoying is that? And in the middle of nowhere. ­ In circumstances like this, for many drivers a puncture can be a nightmare. But even in the center of town in broad daylight it’s at the very least an inconvenience – and of course when a tire suddenly loses pressure there is always the risk of an accident. That said, on average a driver will only encounter a burst tire every five to seven years − less frequently than a FIFA World Cup – or so the statistics for Germany tell us. And modern self-supporting runflat (SSR) tires eliminate this risk entirely.

When a conventional tire loses pressure, the sidewalls are pressed flat by the weight of the vehicle. As the tire flexes on the rim, the rubber heats up and soon the tire is damaged beyond repair and can even run off the rim. Photo: Continental

 

Runflat tires with handling properties that barely change when there is a loss of pressure have been around for some 20 years now. Today, almost all the major tire manufacturers have SSR tires in their portfolio. While these may be based on different technologies, they all deliver the same outcome: When a puncture occurs, they enable the driver to continue on their way in a safe and controlled manner.

 

In simplified terms, runflat tires are a combination of normal and spare tires. With a normal tire, if the tire pressure drops, the weight of the vehicle leads to the typical profile of a flat tire. The tire must be changed because driving on a flat tire makes the rubber heat up due to the repeated flexing of the material and the tire could even run off the rim. In runflat tires the insides of the sidewalls are reinforced, so that even if the tire pressure drops to zero the tires are not pressed flat by the vehicle weight but retain their form. As a result, the car can continue safely on its way at speeds of up to 80 km/h for a limited distance of 80 kilometers – enough to reach the nearest tire dealer, for example.

Our photo shows a cross-section through an SSR (Self-Supporting Runflat) tire from Continental. You can see the specially reinforced sidewalls which even in the event of a total loss of pressure ensure that the tire retains its shape and the car can be driven safely. Photo: Continental

 

Another way of providing a tire with runflat properties involved using a support ring on the rim. In the event of a loss of pressure, the ring would support the tread and prevent the tire being pressed entirely flat by the weight of the car. However, this system called for special rims, was more expensive and more complex, and as a result never became the first-choice solution. Instead, runflat tires with reinforced sidewalls are currently the best and safest technical means of enabling a car to drive on despite suffering a puncture. That said, if you fit self-supporting runflat tires you also need a tire pressure monitoring system. That’s because modern tires are so sophisticated that the driver might not even notice that a tire was gradually losing pressure. The tire pressure monitoring system issues a warning that enables the driver to adapt their style of driving accordingly.

An SSR (Self Supporting Runflat) tire from Continental with reinforced sidewalls. Thanks to this technology, the driver can keep the car safely under control even when there is a total loss of tire pressure. Photo: Continental

 

Continental has been manufacturing and marketing runflat tires for some 20 years now. At Continental they are marked SSR (for Self-Supporting Runflat tire) and are compatible with all standard rims. Along with the invaluable assistance that these tires provide in the event of a puncture, the manufacturer points up two more advantages that SSR tires deliver: By eliminating the need for a spare wheel, they reduce the overall vehicle weight by several kilos, which helps to cut fuel consumption. At the same time, the well where the spare wheel was previously stored can provide up to 80 liters more luggage space.