by Benjamin Yong
Almost as long as cars have had wheels, they’ve also had spare tires stashed somewhere onboard for the eventual inevitability of a flat tire. In some of today’s luxury vehicles, however, manufacturers have traded the donut spare for more cargo space by using run-flat tire technology. If you're not sure what these are, well, here's a quick, non-techy explanation to get you started.
What are run-flats used for?
Run-flat tires allow you to do exactly that: continue running on them even after you’ve experienced a flat. But it’s not meant to be a permanent solution. The idea is that you get to a service shop or garage as soon as you can to have the tire repaired* if possible, or replaced entirely.
Once punctured, run-flats have limitations on how fast and how far you can continue to drive. Every model is different, and it’s best to consult your owner’s manual or the tire maker’s specifications. For example, on the BMW 3 Series Sedan, drivers can continue on for up to another 150 kilometres.
How are they designed?
Not all run-flat tires are created equal —on the BMW 3 Series, the tires feature reinforced sidewalls. The thicker rubber provides the extra bit of support needed to allow you to get you safely to your destination.
Another type of common run-flat design utilizes what is called a support ring. Rather than thicker sidewalls, there is a ring comprised of a hard material underneath the tread capable of continuing to bear the weight of the vehicle even in a reduced air pressure situation.
Regardless of the design, all run-flat setups require a tire pressure monitoring system (TPMS) in place, which are a series of sensors that communicate with the vehicle’s computer to alert the driver of a puncture. This is critical so that you don’t continue driving without realizing a flat tire has taken place.
* Repair of a punctured run-flat tire should only be undertaken after inspected thoroughly by a professional and deemed safe to do so. Generally, repair is only possible with a puncture of less than six millimetres in depth, and on support ring equipped models, if the ring hasn’t sustained excessive damage.