Run-flat Tires and TPMS
"My car doesn't have a spare?!" We hear this line MANY times during the year from frustrated customers a tire puncture and goes to install their trusty spare...only to realize the vehicle does not have one! For years now automakers have been shipping out new vehicles equipped with run-flat tires (and no spare tire) in the name of fuel economy gains and extra vehicle storage capacity, amongst other reasons. A Car and Driver article states that 36% of 2015 model vehicles sold in the USA were not equipped with a spare tire, this number WILL continue to grow. Vehicles equipped with run-flat tires will also have a tire pressure monitoring system installed to alert the driver when a tire start to lose pressure.
To help our customers understand how tire construction and pressure monitoring work we've compiled some brief explanations below.
Run-Flat Tire (RFT)
The most common type of Run-flat tire is designed with a reinforced sidewall that allows a vehicle to drive a certain distance (generally up to 80km) at a lower maximum speed (currently up to 80km/h) after a puncture before the tire loses its ability to support the vehicle. If driven beyond this point the tire will eventually disintegrate, possibly causing additional vehicle damage.
Some downsides of run-flat tires include decreased ride-quality and extra expense (we've seen up to a 40% difference in some cases) vs. conventional tires.
Continental Tire's SSR run-flat tire construction (pictured below) is typical of run-flat tire construction.
The TPMS monitors the inflation of your vehicle's tires and alerts the driver when pressures drop to a level that compromises vehicle safety. TPMS can be used on vehicles equipped with run-flat or conventional tires.
There are two varieties of TPMS sensor, direct and in-direct.
Direct TPMS uses transmitting pressure sensors in the vehicles tires to relay tire pressure to an on-board computer. This computer will display your pressures and provide an alert if pressure drops below an acceptable level.
Indirect TPMS uses the wheel speed sensors (part of the ABS system) to detect the speed that each wheel is rotating at. If tire pressure drops, the circumference of the affected tire will reduce as well causing that tire to run at a comparatively higher wheel speed compared to the three fully inflated tires. Once this difference in speed passes an acceptable limit, the driver will be alerted by the on-board computer
Most vehicles originally equipped with run-flat tires can be fitted with conventional tires if the owner desires as long as size, speed rating and load rating meet the vehicle manufacturers specification. To complement a conventional tire package a "mobility kit" is recommended and sometimes available directly from the vehicle manufacturer.
Mobility kits combine a compact air pump and tire sealing compound that allow a user to effect a temporary repair until they make it to a repair shop for a proper repair or replacement. The air pump and gauge can be used to top-up a low tire or combined with the sealant to seal minor punctures (up to 6mm diameter) in the contact patch of the tire. These kits cannot repair damage to the sidewall and it is possible the sealant can damage a TPMS sensor. It is important to read the supplied instructions with these kits as they differ between manufacturers.
Despite the possible complications, these kits offer a viable way to get a driver mobile again.
Hopefully this helped to lift some of the fog on these technologies. If you have questions or inquiries about these products don't hesitate to contact us!